ecology Archives » Botanish

A diatribe against landscape fabric

In an era of rising eco-consciousness, homeowners are gradually moving away from lawns and other heavily irrigated landscapes.

The trend is toward xeriscapes, landscapes that are designed for climate they are located in, with plants adapted to thrive on little more than rainfall, and gardens that use efficient irrigation systems like drip systems or, in some cases, no irrigation at all. That is definitely a laudable option in my book—when I install gardens, it’s practically the only thing I consider unless I’m asked to do otherwise. Not only is it better for the environment, a site-appropriate garden is more resilient and easier to maintain.

But one unfortunate detour in the path towards xeriscaping is a tendency for installers to plot out gardens with vast swaths of landscape fabric. The material, made of polypropylene plastic, is spun into threads or ribbons and woven into a cloth purported to allow air and water through, then topped with gravel or mulch. The stated purpose of the fabric is to prevent weeds from emerging between the plants, which, in a popular xeriscape style, are placed relatively far apart in holes punched strategically in the fabric.

There are a number of low-water garden plants that appear to do ok in landscape fabric, and you have seen them because they are the popular plants to install in road medians and around parking lots, or other rarely-maintained but busy public spaces, where landscape fabric is ubiquitous, or at least has been for a couple decades at the time I am writing this. And indeed, for the first year—or at least at the moment that the installation service collects payment from the property owner—it appears that the fabric is doing its job keeping plants comfortably spaced and blocking the weeds between them.

Many modern xeriscape styles, especially those I see around Denver, seem to exist on the idea of saving water by using a lack of plants. The few that are in place consist of some yucca or a small patch of daylilies, a fernleaf yarrow or two, some sedums, a couple stands of ‘Karl Forster’ ornamental grass, and a big patch of Russian sage, all arranged as islands in a sea of gravely rock. The absence of plant coverage means weeds would have ample opportunity to grow, but are kept at bay, at first, by the fabric beneath them.

That’s not the case the year after. Again, you have seen this before—how often do the gardens around the edges of parking lots keep looking great for long? There are always weeds, and probably a few patches where installed plants have died, and the opening is crammed with cheatgrass because the drip emitter is still trickling water in that spot. Even an untended irrigation ditch or rural highway shoulder, receiving no planting or maintenance whatsoever, probably has prettier vegetation than the berms around a suburban parking lot, middle school yard or boulevard median, where fabric is purportedly keeping things nice. Some lucky properties might get a second or third year out of their fabric, but ultimately, inevitably, the fabric will fail, and the garden will go to chaos unless it’s in an affluent area where it is frequently re-installed.

An example of landscape fabric deteriorating over time with abundant weedy growth on top of it and through it, but desirable garden plants are excluded.

Ironically, a garden bed covered in landscape fabric is like a curated space for noxious weeds

The types of weeds that we have learned to dread most as gardeners are species that have followed human inhabitation around the world—as invasive species, thriving on the peculiar alterations we repeatedly make to land.

Despite the wide variety of weed species we deal with, there are actually some pronounced ecological similarities between the most common garden weeds. They usually resent shade and do not thrive in competition from diverse neighboring plants, but spread eagerly in monocultures like lawns or crop fields or areas cleared by frequent mowing, herbicides, foot traffic or construction. Common weeds are resistant to chemicals that kill other plants, mainly because they have all been sprayed so many times and only the survivors, bearing genetic mutations for resistance, have passed on their seeds. Common weeds dominate dense compacted soils low in oxygen—humans are great at creating these compacted soils along streets, in lawns, in foot-trafficked areas and construction sites—and are tolerant of salt and other common pollutants found in developed areas.

The weeds germinate in two main phases depending on the species—late winter, when nothing else in the garden is active (we don’t often think of this time as meaningful for plant growth, though it is in the wild), or at the onset of hot temperatures in late spring when long days supercharge fast growth. The weeds reach maturity within a few weeks, faster than most other plants do, as a function of the frequently-disturbed settings they are adapted to. They quickly flower and drop more seed, or anchor their roots into deep soil before anything else can displace them.

These weeds—you know them as bindweed, dandelions, thistle, quackgrass, cheatgrass, sorrel and others—are good at overcoming the barrier weed fabric presents to them. They can germinate in the duff layer on top of the fabric and grow in soils that are extremely shallow (such as cheatgrass), or they send their taproots down through the fabric (examples include mallow or curly dock), or they manage to gain foothold under the barrier and spread on long runners that can grow several feet in all directions, searching for an opening. Bindweed and thistle are obvious examples of weeds that advance underground, exploit seams or holes in the fabric, and, left unchecked, can form dense mats on top of it with deep, unreachable roots burrowed in the low-oxygen soils beneath. Meanwhile, desirable plants—many of which can be quite hardy in the face of competition or drought—do not contend well with the surface barrier, and can’t fill in to displace the weeds. Groundcovers are limited to the size of the hole in the fabric that was cut for them, so they more resemble cute miniature plants than the sprawling spreaders they are selected to be.

The above are reasons why landscape fabric doesn’t work as well as we hope it would. But I’d also like to address some ways fabric could be actively harmful, at least in terms of missed opportunity to create a garden with ecological value.

How unnatural barriers block healthy processes in the garden

See, long-lived plants that colonize the soil actually improve it structurally and chemically, to make the soil layers much more porous, absorbent of water and efficient at recycling nutrients. This benefits the plants directly as the soil’s inhabitants, but it also has secondary benefits for the whole system. Plants’ deep roots periodically die and are replaced by new roots, leaving long-lasting channels in the earth, which water can trickle down and oxygen can move through. The leafy and woody debris plants drop on the soil surface feeds microbes that bind silt and clay particles to bits of organic material, forming granules that give soil a grainy, light texture even if the inorganic components are otherwise very fine and sticky. The worms and burrowing insects that feed off of dead leaves and roots stir the soil and make larger channels, crossing the soil profile in many directions.

All of this activity, in aggregate, drags organic material—often referred to in shorthand as carbon—deep into the soil, where it is sequestered for many years, and helps draw heavy rainfall into the deep layers, and also gradually wicks deep water back up towards the surface when the top layers are dry.

All of this further serves to capture heavy rain and hold it on site, or gradually feed water to aquifers, with minimal runoff. Limiting runoff has the obvious effect of reducing flooding, but also means our urban pollutants (mostly nitrates and sulfates) can be processed by soil bacteria and rendered harmless before they reach natural waterways. When land is covered by deep, porous, biologically active soil, streams are fed by rainwater moving through soil over weeks rather than runoff pouring over the land in hours, and thus streams will not swell as intensely in storms or dry to a trickle during brief droughts. Deep, uninterrupted soils also mean that water stays put in deep layers of soil for a long time, and plants can draw from the reserves between rains, so there is less need for irrigation, less loss of plant life during dry spells, and more biological productivity on the whole.

Landscape fabric interferes with all of this. When soil particles mesh with the fabric, it forms a sheet that is far less permeable to water and oxygen than the fabric initially was. Between holes punched in the fabric for plants, the soil surface is relatively lifeless and hard, although lifting a sheet of fabric might reveal the thin, white etiolated stolons of bindweed and Canada thistle corkscrewing in all directions just beneath it.

The benefits of open, mulched soil with no barriers extends to animal life as well. Beneficial insects—both carnivores, which eat other insects, and detritrivores, which eat dead leaves, stems and fungi—overwinter below ground and benefit from free movement between soil and the sky. Small birds, in turn, rely on an abundant supply of earthworms, beetles, grubs and arachnids in the mulch layer. This supply of protein is crucially important for them raise their young, as you will see when they arrive to pick through the garden and burrow into the duff layer with their heads. Soil creatures will also come up to eat dead plant material or to drag it into their burrows to moisten it so they can eat portions as they decay, cleaning up the garden and recycling the nutrients. All of this life and activity depends on a barrier-free continuum between the sky and the soil—two realms that are not really as separate on Earth as they seem.

Another example of weedy growth on landscape fabric. The fabric needs to be removed before the garden can be cleared and replanted, which is all the more difficult when it is buried under decayed organic material and grown over by plants, while the soil remains compact beneath it.

With this, I hope it’s clear that landscape fabric can cause more problems than it solves. A relatively recent invention, landscape fabric is a strategy that, though logically understandable, doesn’t hold up in practice—like so many of our attempts at micro-managing nature. Instead, using organic mulches like wood chips, or other natural materials such as large stones, can change the structure of a perennial garden favoring desirable plants over opportunistic weeds. In turn, it allows the plants themselves to control their habitat, feeding wildlife and forming a garden that does not deteriorate with time, but grows only more well-established and resilient.

Why snow cover is good for your garden

For those in northern latitudes, winter is a slow time in the garden. Plants are dormant, nursery stores are bare, and for most of the winter it’s still too soon to start your seed trays for the vegetable garden.

But one thing that keeps me active through the winter months is taking advantage of snow. In Denver, a semi-arid region in the rain shadow of the Rocky Mountains, we rarely if ever get deep accumulations. Six inches is a big storm, twelve is massive. It usually melts completely before the next snow. That’s to be expected in a climate with between 10 and 15 inches of precipitation in an entire year. But when the snow comes, I’m ready, heading out to scoop the treasure and pile it in the garden.

How snow protects plants from cold

It’s counter-intuitive that something that is, by definition, frozen, helps protect the garden from winter’s dramatic temperature swings. It comes down to the ways that plants protect their dormant tissues from the destructive power of ice.

Dehydration as protection

Plant cells are mostly water, and for non-adapted plants, ice crystals forming in cells pierce and rupture them. When the plant thaws, the pulverized tissues hang limp like cooked spinach, utterly destroyed.

Cold-adapted plants have a variety of defenses. First and foremost, they prepare for cold by allowing their tissues to dehydrate. They will often do this by moving cellular water to the spaces between cells, where it can freeze safely. Inside the cells, the water becomes thick and syrupy with sugar, electrolytes, peptides and enzymes that interfere with ice crystallization. Instead of freezing at 32 degrees Fahrenheit (0 degrees Celcius) as pure water does, ice in plant cells may not form until the temperature drops to 25 degrees, 20 degrees or lower. (The limit depends on the species, the variety, and how much time the plant has had to prepare for coming frost.) As temperatures continue to drop, more water will migrate outside the cell and freeze, leaving the inside even more dehydrated. Thus, freeze tolerance in many species goes hand in hand with the plant’s ability to cope with drying out.

Eventually, temperatures drop low enough that cell contents freeze, which is lethal to tissues on plants that die to the ground each winter. Those with perennial above-ground tissues may tolerate ice inside the cell. (Cell contents are by now so dehydrated that there’s only so much damage ice crystals can do.) Other kinds of plants retreat to the ground, where the thermal mass of the Earth keeps roots and storage organs within tolerable temperature ranges.

The burdens of sun and wind

Yet, even when plants are able to survive a frozen-solid state, they have additional issues to contend with. Drying winds and sun can loosen frozen water molecules and convert them directly to vapor in a process called sublimation. (Though very tough plants can survive down to 25 or 30 percent water in their tissues, they cannot survive drying out completely.) Oxidation reactions with air can degrade tissues when the frozen plants are unable to produce antioxidant compounds to protect themselves. Ultraviolet light from the sun damages cell organs and DNA. During the growing season, plants are constantly repairing damage from these assaults. But plants can’t heal while frozen, and damage accumulates. Thus it is not just the winter’s minimum temperature but also the length and frequency of hard freezes that determines whether plant tissues are injured beyond a point of no return before spring.

Snow creates a stable, protected environment

Fortunately, most of these issues are ameliorated by snow. Because a layer of snow is mostly air—around 95 percent air after a fresh, fluffy snowfall, and at least 80 percent on old, crusty snow or heavy wet snow—it is an excellent insulator. As temperatures crash below zero outside, they stabilize at around 25 degrees Fahrenheit when measured 4 inches into the snowbank.

Next, snow blocks ultraviolet light, and keeps stems and leaves from drying in a zone of 100 percent relative humidity. Yet despite the moisture, oxygen can diffuse through and most pathogens cannot grow below freezing. Thus, snow cover can keep even fairly tender plant tissues in a state of happy, mostly suspended animation for months on end.

A few bonus benefits of shoveling your sidewalk snow onto the garden

In the Western U.S. we’ve been contending with a gradual drying trend and droughts that get more severe with climate change. At the same time, though temperatures have gotten on average warmer, they also swing more fiercely now. Sudden spring cold snaps injure plants more badly when they’d let down their defenses during a warm spell.

Snow cover helps with both these problems—keeping plants properly chilled through winter, and supplying extra moisture that is happily taken up in spring.

An additional bonus for me is that it protects the environment. Runoff from streets in winter carries pollutants such as sidewalk salts (magnesium, calcium or sodium), fertilizer, oils, nitrates and sulfates. Often, stormwater systems move runoff directly into lakes, rivers and streams. When these substances contact soil, they’re quickly scooped up by soil microbes that convert them into nutrients. Those nutrients are in turn taken up by plants. But urban runoff systems don’t often exploit that step, sending water directly into streams. There, the pollutants are far more harmful, since they interfere with aquatic organisms’ ability to use dissolved oxygen. Thus, I’m helping clean the streets at the same time I protect and fertilize my garden.

Collecting wild plants for gardens: how to forage responsibly

If you like gardening and native plants like I do, you know there are some mixed messages about collecting wild plants. We hear growing native plants is good, but only if they come from reputable commercial nurseries. We’ve learned the harms of picking a wildflower, but give little thought to building with wood, meaning somebody harvested an entire tree from a forest. We understand hunting and fishing is legal with a permit, but what about plants?

Of course concerns about proper use of wild spaces are well-founded, but you’ll come across a confusing mix of sentiments. Since native plant enthusiast communities are typically pretty pro-environment, sometimes it seems like those who want to make the tiniest impact bear the most scrutiny.

The truth is, all cultivated plants came from the wild at some point. And there are many benefits to getting more diverse and more local plants in our gardens than what nurseries currently offer. To do that, we need to learn the legal, respectful and ethical ways to collect wild plants.

Your resources: the Bureau of Land Management, Forest Service and U.S. Department of Agriculture.

If you live in the United States, there are several federal agencies to help manage public resources and wild plants. They provide information and guidance for free, since working with the public is part of their mandate funded by taxes. BLM land is by far the easiest to collect on, and depending on what you’re going for, hobbyists may not even need a permit (though ethical principles still apply). National forests also exist for managed harvesting and allow regulated collecting by the public. Collection permits cost as little as $20.

If you plan on traveling large distances with plants you collect, such as from Hawaii to the mainland, you can seek guidance from the USDA.

Know what kind of plants you’re looking at

In some parts of the world, poaching rare plants is driving species to the brink of extinction. Some of the most common targets are carnivorous plants, certain cacti and succulents, orchids and bulbs. Their unique shapes and features make these plants stand out, and slow growth makes them hard to propagate, so nurseries can’t keep up with demand. Unfortunately, their slow growth also prevents wild populations from recovering from constant harvesting. Before you disturb wild plants, identify them well enough to be sure you’re not threatening any vulnerable populations.

On public lands where collecting wild plants is permitted, the National Forest Service or BLM will provide vulnerable species lists. But even if you are on private land with permission from the owner, it’s better to leave vulnerable species intact.

Harvest seeds or cuttings rather than whole plants

When a plant is producing seeds, the least disruptive collection option is to harvest a few seeds and leave the plant intact. If there are no seeds, a small twig or stem cutting lets the plant recover. If that’s not an option for the species, consider taking a basal cutting—a rooted stem or shoot from the base of a clump-forming perennial—without digging up the whole plant. All these methods reduce the need to deprive the area of a whole, healthy plant.

Artemisia ludoviciana cuttings, prepared from a single stem, cut into segments and prepared to be stuck into a rooting media.

Collect only the plants you can keep alive

Another reason to ID plants you want to harvest is to be sure it’s a species you can propagate. It’s wasteful to start digging at unidentified plants only to discover they have deep taproots you’ve broken off and doomed the plant. Trying to collect wild plants you don’t know how to transport or grow can lead to waste as well. Be sure you have a field guide, access to a digital database, and a cooler or container to keep cuttings cool, protected and humid for the rest of your trip.

Keep a light footprint when collecting wild plants

Evaluating a prospective collection is about more than that individual plant. Survey the other plants of that species in the area. If you see other signs of collection, move to a new area. Make sure there are other plants of the same species around. (The rule of thumb is to not collect more than 5 percent of the plants in a small area, or 1 in 20 plants.) Don’t take the largest or healthiest individual, and don’t take the only seedling if there aren’t several other seedlings nearby.

My own advice: don’t overlook common and non-showy plants

There’s no doubt that a cluster of native wildflowers is a lovely sight. Most wild plants taken into cultivation as ornamentals are the showiest species, then bred to be even showier still. But if your gardening goal is to widen the domain of the ecosystem and to invite birds and insects to make use of your land, the keystone plants—the ones that are so abundant and dominant that they define the ecosystem’s boundaries—are valuable to incorporate.

These wild-collected whiplash daisies, cool-season bunchgrasses and artemesias are from a private property with permission. None are rare or showy, but I think they are appealing in combination—something to keep in mind collecting wild plants.

Importantly, plants that don’t bloom brightly or tower above the others are underrepresented in native gardens. Depending on your region, that could refer to prairie grasses, sagebrush and other artemisias, rabbitbrush, milkweeds, sword ferns or others. It’s a challenge, but also a great benefit, to incorporate these plants into your garden—to evoke not only the floral color but also the texture and character of the wilderness.

A garden of Colorado native artemisias and grasses, with just a few native flowers scattered in. This will form a handsome naturalistic texture when it fills in.

Botanical tourism: bringing plants from Hawaii to the mainland

When I was a kid, my dad sometimes brought me bits of plant life from his work trips around the country. I was more interested in those kinds of souvenirs than anything from an airport gift shop.

They were nothing ecologically-sensitive or fancy. Just nuts or seed pods from trees in a public park, or sprigs of foliage from a buffer strip along a parking lot. I remember a husk of horse chestnuts from Washington DC, a handful of Agave bulbils from San Diego, and lots of pine cones.

It turns out I now travel the same way. But unlike my childhood gifts, which I usually dried and arranged on what I dubbed the “science shelf” in my bedroom, I’m more interested in things I can propagate. I find that having a story behind my houseplants gives me a deeper interest than just buying them at a garden store.

So, on a recent trip to the Big Island of Hawaii, I took the opportunity to collect some really interesting and beautiful plants, with the added challenge of following the stricter regulations of traveling over an ocean with cuttings and seeds.

Jeff (holding the camera) and me on a trail on the Big Island

Traveling eco-safe with plants

Over the years we’ve learned more about the risks of moving living things around the world. Every day, millions of tons of timber, grains, livestock, plants, flowers and produce are shipped across the oceans. Many have escaped into the wild, becoming invasive species that overrun native ecosystems. Others harbor hidden pests that could wipe out species or destroy agricultural industries. Consider the American Chestnut, a once-dominant North American tree that was wiped out a century ago by an exotic fungal disease, or the emerald ash borer, a bug from Asia that is steadily eradicating North American ash trees.

As much as I love collecting plants, it’s important to me to do it safely and legally. I would hate for my legacy of life on Earth to have been personally responsible for destroying a species. So I follow the regulations and take some additional precautions. That also means being above-board with the USDA and any other relevant agencies.

Safe vs unsafe plants

Living in Colorado, it’s unlikely a tropical plant brought from Hawaii would survive outdoors and become invasive. These are strictly to become houseplants, isolated from the local flora. But that’s not the case if you’re in a balmier state, like Florida, Texas, California, or even marginally similar places like Arizona or Oregon, so there will be stricter considerations than what I describe here.

A few categories of plants contain species that are remarkably adaptable and widespread, and the U.S. Department of Agriculture prohibits their transport entirely. They can also harbor viruses, fungi or insect pests that can more easily jump from one climate to another. That list includes grasses, sedges and cacti, as well as most commercially-important vegetables and fruit. In the case of cacti, an extremely destructive cactus moth was brought to Hawaii to eradicate an invasive Opuntia species, burrowing deep into the flesh and eating it from the inside. The moth could put several North American prickly pear species at risk.

Plants must be soil-free

Moving soil is a concern unto itself. A handful of wild soil can contain hundreds or thousands of species bacteria, fungi, viruses, insects, worms, nematodes, slugs and snails or various pest eggs. Some of these species may not even be documented by science yet, and it’s likely there are organisms there that can live in both tropical and cold climates. Due to the incalculable risks, moving soil into the U.S. mainland from overseas is banned entirely. Plants need to be bare-rooted and washed, or better yet, restricted to dry seeds and soil-free cuttings. Potting media, which is an artificial product made of pasteurized dead plant material and fillers, is not soil, and that’s what nurseries cleared for shipping to the U.S. will use.

My experience collecting plants in Hawaii

Hawaii has an impressive variety of landscapes and microclimates. In the rainforest biomes, you can really get a sense of the risks posed by invasive species when you realize that almost all the trees and vines growing there came from someplace else. From the dominant eucalyptus trees to the ubiquitous pothos vines, the wettest zones are basically a hodgepodge of aggressive plants from jungles around the world. The ecosystem there is basically a brand new assortment of organisms, and we’re yet to see how it develops over coming generations.

This jungle on the Big Island’s northwestern coast is captivating, but most of its plants come from elsewhere and have overwhelmed the native species.

The resorts and villages, meanwhile, are manicured collections of what I basically think of as houseplants. The giant banyan trees, with curtains of aerial roots dangling from the branches, come from a group of Ficus tree species that grow in the wild by sprouting from seed on branches of mature trees and eventually smothering them with their roots (strangler figs). They include the familiar “weeping fig” and “rubber tree” which we know of as houseplants. There are hedges of hibiscus, beds of crotons and bromeliads, Monstera vines climbing the trees. There are Sansevierias and Heliconias. Aloes and Agaves. Everything you’ve seen in your local nursery’s tropical section is now here, giant-sized.

And that’s where I got almost all of my cache. The gardeners coming through to trim the plants on practically a daily basis provided lots of cuttings, and the seeds scattered around the ground made good finds. Tropical trees tend to be heavy seed producers and nearby sidewalks and streets are littered with them.

Whenever I found a cache of seeds or downed branches that looked appealing, I looked the species up to make sure I thought I could grow it at home. Before the week was up I compiled a cooler’s worth of seeds and cuttings, bagged, labeled and ready to go.

Going through the USDA checkpoint

Getting live plants to the mainland is an intimidating thought, given that you can’t even bring an orange for lunch onto an airplane headed to North America. But the regulations are there to prevent specific pests from getting across, and due to the millions of pounds of commercial food crops shipped around the world each year, some of them have unfortunately already gotten to Hawaii—but some have not yet reached the continental U.S. Others have reached the continental U.S. but aren’t yet in Hawaii, so restrictions go both ways. Additionally, there are a lot of pests that hide in fruit in particular. If you want seeds, which are less hospitable to stowaways, just make sure to scrub the flesh off and put clean, dry seeds in a package for inspection.

Cuttings and seeds are cleaned, dried, bagged, labeled and ready to go through USDA inspection to bring to the mainland.

Cuttings are, similarly, surprisingly easy to bring. As long as they don’t come from a list of prohibited plants (which includes plants in the citrus family, cacti and grass), and they’re not endangered (which they wouldn’t be because I only collected cultivated plants), they’re likely permitted.

In this case, you want to remove any leaves with bruises or spots that could potentially harbor disease. Luckily, many tropical plants are vigorous enough that they can grow from a segment of stem without any leaves at all.

It’s probably important that you are able to identify the species you are working with, in case the USDA agent doesn’t recognize it and wants to exclude it. Or, worse, the USDA agent could accidentally miss a prohibited species and allow it through. Make sure you read and can understand the guidelines. If you have any doubt, you can call the Hawaiian USDA office, which I did, and get connected to someone who can offer guidance.

I went through the checkpoint at the Kona airport on the Big Island with 27 types of plants and cuttings. The agent only blocked one item from getting through—a bag of Crinum asiatica seeds that were still very green and fleshy. She picked out a couple other seeds here and there that looked damaged, letting me keep the rest. She took a close look at some agave bubils to make sure they weren’t some type of cactus, and that was that. After inspection, the plants went into a cooler that was tagged with a USDA sticker, and arrived in Denver for me to process and propagate.

Additional safeguards

After getting my cuttings home, I’m still not going to take any chances that some unusual organism slips through. They are being nursed and propagated indoors, in most cases under a plastic humidity dome. That way I’ll be able to observe them for some time as they get growing. You can read a bit about general plant propagation techniques here. But in my next post, I’ll write more about bringing these particular little stems and seeds to life.

Wealth for worms: what a gardener should understand about soil

Soil is one of the most important concerns for gardeners, so I want to take a moment to address a few basics. It’s the medium into which we add our plants, it’s where non-woody plants retreat when dormant, it’s where water is stored, nutrients are recycled and it is the home for countless bacteria, fungi, insects and other organisms that play an important role in the garden ecosystem that we usually don’t get to see.

Defining soil

For most of us, the question “what is soil?” seems too obvious to ask. Maybe we don’t have an encyclopedia definition in our heads, but we know dirt when we see it: it’s that workable, wetable brown stuff that covers the Earth’s surface naturally, that you find wherever you dig, that must be cleansed from clothes and scraped out from under our fingernails, that tracks into the house and needs to be swept. In the garden, it’s the stuff plants use to anchor their roots.

But this is a good opportunity to draw a line and explain something that turns out to be exceedingly important to us gardeners: what soil is not. Specifically, potting mix is not soil. That stuff is better described as a “potting media” or “soil replacement.”

Native soils

Soil in any natural ecosystem on Earth is a blend of decaying organic particles and living and dead things, water, dissolved gas and small open pores of air suspended between very small particles of rock—a lot of rock.

By dry weight, native soil is almost invariably going to be between 90 and 95 percent rock, in grains that range from small (sand) to very small (silt) to microscopic (clay). The proportion of particle sizes determines much of your soil’s character. Whether your native soil is fluffy, powdery, sandy, hard as porcelain, dark and moist, gritty and dry or easily waterlogged, it’s still around 90-95 percent rock. The remaining proportion of composted organic material makes the rest of the difference in soil’s potential water and nutrient retention capabilities and texture. (I say “potential” because another important aspect to soil is its structure, which is something I’ll return to later).

Being mostly rock, soil from the ground is heavy—between 75 and 125 pounds per cubic foot. And that is one way you know the mixture of blended materials you call “potting soil” is really not soil at all. Your potted plants would be really difficult to lift and move if they were in soil. Additionally, every time you water, the container’s drainage holes would leak muddy brown water containing microscopic clay particles and dark-staining dissolved substances. When it comes to larger containers, root systems planted in native soil would mostly be concentrated in top 8-12″ of the soil volume where oxygen is most available, or wrapped around the outer edge of the container and clustered around the drainage holes were additional pockets of highly-oxygenated soil are available. In other words, dense native soil in a container would not oxygenate evenly, causing root systems to form distorted structures and waste the space in the center of the container.

Potting blends

Potting mix manufacturers make potting blends out of non-decomposed and decay-resistant shredded plant material, such as peat moss, or most commonly, shredded bark and wood pulp that comes from the byproducts of logging operations. Light, fluffy types of rock such as pumice could be added in to help reduce compaction and slow the breakdown of organic components. More commonly, perlite or vermiculite serves that function. Because the the types of organic material used are mostly nutrient-poor and there are no mineral particles to supply minerals to plants, slow-release fertilizers or compost blends will be mixed in to provide them. This sort of mix resists compaction and is mostly air, so dry potting media can weigh as little as 8 pounds per cubic foot. The abundant air space also helps the roots penetrate deeply and use the entire volume of the container. Generally, plants in containers eventually need to be fertilized, while plants in real soil generally do not.

The fact that containerized plants are not growing in soil turns out to be important when it comes to planting. That’s because a plant transplanted from a nursery container into the ground exists with its root system confined to a plug of non-soil in a soil environment. Ideally, the first “watering in” that is always recommended after planting a will help some of the native soil dissolve and infiltrate the potting mix plug, getting the roots in contact with the denser particles that tend to be better at releasing nutrients and carrying water. But that’s not always the case, and the failure of roots to integrate into the native soil bed in time for the next drought is a major reason newly-planted plants die. It’s why I advocate breaking up the rootball as much as possible when you’re planting a new plant in the ground, and, in some cases, shaking off or flushing out the potting media as much as possible to get roots in contact with native soil.

Where the nutrients are

Most gardeners are well aware that organic material releases a burst of nutrients into the soil as it decays. Additionally, small organic particles in soil are useful for storing nutrients in a way that makes them available to plants, and the bacteria and fungi that live on these organic particles provide a host of benefits to plants.

(As a side note, this understanding gives us some insight into the popularized practice of fertilizing gardens with “compost tea.” That’s the practice of running water through compost and capturing its tea-colored extrudate to water a garden as a form of organic fertilizer. It’s true that compost tea contains some dissolved nutrients and free-floating microorganisms, but it doesn’t contain the bulk organic particles that hold them in place and provide the most benefit to soil biodiversity. Why go through the extra effort to separate dissolved nutrients from the substrate that puts them to work? As clever as the idea may seem, there’s no scientific support for the idea that supplying nutrients in tea form improves plant health compared to a top-dressing of compost on soil.)

But a lot of us take for granted the fact that the rock particles themselves contribute to nutrition for plants. Rocks naturally release trace amounts of nutrients, or bind excess nutrient molecules that can be re-released later. That’s a reason why volcanic regions, where soils are full of fine mineral ash have some of the most fertile soils on Earth.

Understanding soil types

As a gardener, it’s good to understand the soil type you’re working with. Above I mentioned the three main particle sizes—sand, silt and clay—which lend to different soil qualities. Most soil will be a mix of all three particle sizes in various concentrations.

Sandy soil, with large grains and pore spaces, drains and dries out quickly. It is beneficial in that it is less likely to become waterlogged, but is also more likely to dry out. Since larger particles have a smaller surface area, they don’t exchange nutrients easily, and can become nutritionally poor.

Clay soil is made of microscopic grains that fight tightly together. It holds large amounts of water and resists drying. Oxygen has a harder time penetrating clay soils deeply, so trees growing in clay will have shallower root systems that stretch out farther. Clay is particularly vulnerable to compaction and in urban areas it can be very degraded. However, clay, with its high surface area (more than 1,000 times that of an equal volume of sand) is excellent at storing and exchanging nutrients with plants.

Silty soils, as might be intuitive, combine the qualities of sand and clay.

Loam is an optimum mix of sand, silt and clay. Many farmers and gardeners consider it the best: it carries the benefits of sand and clay but avoids the drawbacks. It holds both oxygen and water, and stores and releases nutrients. Additionally, loam can be divided into sandy loam or clay loam based on the dominant particle size. A lot of gardeners seek to create a loamy soil for their garden, but…

You can’t change your soil type

The mass of soil in the ground is huge. Since each cubic foot of soil weighs close to 100 pounds, a small back yard of 40 by 40 feet has 160,000 pounds of soil in the top 12 inches alone. And, while most root system activity occurs in the top 8 to 12 inches of the ground, deep anchor roots and the roots of drought-tolerant plants and grasses can extend several feet!

So imagine you’re trying to change your soil type because your current soil is heavy clay, and you envision creating an ideal, loamy soil for your vegetable patch by adding sand. To tip the soil past the threshold where it really behaves differently, you’d have to add enough sand that that there aren’t enough clay particles to completely fill the gaps between grains of sand, providing empty space That means your soil has to be more than 50 percent sand, and to amend a 10-by-10 foot area you’d have to truck in 5,000 pounds of sand and till it thoroughly to a depth of 1 foot. That’s a lot of work, and over time, living organisms and water are going to spread that sand deeper into the soil and out into the surrounding areas so you’re going to have to add even more sand.

I think it makes a lot more sense to just work with your existing soil. In truth, even the heaviest of clays can become excellent soil with proper management. Plants can be selected to work with the existing pH. Generally, plants can weave their roots around rocks and adapt to different soil depths.

Summary

Potting mix is a lightweight replacement for natural soil that is made of shredded plant material and added fillers and nutrients. It weighs about a tenth as much as natural soil, lets oxygen in more easily, and is ideal for plants in containers.

Natural soil is 90-95 percent small to microscopic rock particles, with the rest being composed of organic material. It weighs 70 to 100 pounds per cubic foot.

Soil is composed of sand (large particles), silt (medium particles) and clay (small particles). Most soil has a mix of all three, but one or two types may dominate. The balance between particle sizes affects the soil’s characteristics—for example, sandy soil dries fast and encourages deeper roots, whereas clay soil stores more water and nutrients, but is more prone to compaction. A well-balanced soil is called loam.

Organic material helps water and oxygen move through soil, provides food for beneficial microorganisms, and stores nutrients. But microscopic rock particles in soil also supply nutrients to plants, and gives soil weight to help stabilize trees and shrubs. Ideal soil has a balance of about 90 percent mineral particles to 10 percent organic material, which is close to the natural ratio.

Adding compost to soil will temporarily increase the soil’s organic component, but it will gradually break down until the soil reaches equilibrium.

Soil has so much mass that it is very difficult to add enough material to change the composition of a garden. For example, a 10 by 10 foot garden would require 5,000 pounds of sand to change the soil type from clay to loam. It’s easier for gardeners to add mulch to optimize soil and choose plants wisely to work with the existing soil type.