Botanish

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.

Plant profile: Allium aflatunense ‘Purple Sensation’

Ornamental alliums are by no means rare or exotic garden plants. Even big box stores and grocery stores often sell the bulbs in their garden sections in fall, and the most common cultivar is Allium aflatunense, ‘Purple Sensation.’

Allum aflatunense is one of the most eager naturalizers among alliums. It blooms in mid-spring at roughly 2 feet tall.

But its ease of cultivation and showstopping quality warrants some special attention here. When I’m out tending the garden in spring and people pass by on a morning stroll, many will ask, “What are those big purple things?”

So let it be known: they are alliums. Allium is the genus that contains onions, garlic, leeks, chives and similar food plants, and the entire genus of some 500-plus species spanning the Northern Hemisphere and tropics is technically edible to humans, although not all of the species are palatable or have the spicy, pungent taste and aroma we associate with onions and garlic. Most ornamental alliums aren’t useful as food crops and ‘Purple Sensation’ certainly isn’t, but it is very showy and easy to grow.

‘Purple Sensation’ foliage emerges in early spring with the daffodils, but won’t bloom right away. In the Denver area the flowers bloom in at the beginning of May during the waning days of Darwin tulips. The tennis-ball-to-baseball-sized spherical globes containing hundreds of small violet-purple flowers stand around 2 feet tall and each one lasts about 3 weeks. Since a cluster of bulbs will produce plants that bloom at slightly different times, the display will last a month; they make a good bridge to tie in the end of the spring bulbs and the beginning of the season for bearded iris, oriental poppy and early summer plants.

When the flowers fade, the seed pods of Allum ‘Purple Sensation’ are ornamental. Foliage dies back while the flower is blooming so there is no lingering eyesore to worry about; foliage could be clipped off at the base with no detriment to the plant. However, I like to leave some of the stems and seed pods around for structural interest. As they dry, they continue to resemble fireworks, which is a festive addition to the garden or a dried flower arrangement on the Fourth of July.

After the flowers fade, the seed pods of Allium “Purple Sensation” continue to provide textural interest.

Allium aflateunense is native to central Asia, the same region that hosts many species of wild tulips, and they have similar growth habits. ‘Purple sensation,’ however, is more likely than most tulips to multiply and spread. Bulbs grown in optimum conditions the bulbs can roughly double in quantity every year by division and seeds can become plants large enough to bloom on the third year after they fall. If you cut off the ripe seed pods and toss them into various gardens, it’s likely that you’ll see blooming alliums there eventually. Small bulbs produce smaller flower stalks, while large bulbs produce large flower stalks or possibly 2-3 medium-sized stalks emerging from a single rosette of leaves. After plants go to seed, they tend to gain some genetic variation and produce plants of slightly different heights and sizes.

Allium aflateunense thrives in Colorado and the Mountain West, and is slightly more forgiving than tulips when it comes to the winter cold period needed to provoke spring blooms. I’ve heard anecdotal reports that it blooms sometimes (but skips years) Northern California, but in the southern part of the state it doesn’t get cold enough and skips blooming more often than not. Where they do grow, they are drought-tolerant, hardy and forgiving plants, that provide a large colorful flower on a very small footprint that makes them a worthwhile addition to many different kinds of garden plots.

How to naturalize tulips for a carefree spring garden that returns and multiplies

Few plants evoke the joy and nostalgia associated with tulips, the quintessential spring flower with big showy blooms and a color palette broader than almost any other category of bulbs.

For many gardeners and designers, tulips are treated as a short-lived perennial or even an annual—one that requires extra planning since the bulbs must be purchased and planted in fall, leading to a show of spring color that is all too short compared to box-store annuals that bloom from May until frost.

That’s because many tulip varieties don’t perennialize well in common garden conditions. Though rugged wild populations of Tulipa gesneriana, the first tulip species dug up from the mountains of Iran and Turkey to be cultivated, are vigorous even in dry rocky soil, centuries of breeding and coddling in northwestern Europe have made them fussy. Many of these popular commercial varieties like carefully-controlled temperature treatments and soil components, resent summer rain, and in the garden they are prone to “shattering” (dividing excessively) in their second year into countless small bulbs that are each too small to bloom. The next year’s growth is mostly small spikes of foliage, and things continue to devolve from there. If that’s what growing tulips really entailed, I’d say why bother and put my focus on a different selection of plants.

But that scenario doesn’t have to be the case. The high-maintenance tulip cultivars, albeit popular, don’t need to be representative of a truly rugged and resilient genus of plants. With a little bit of discrimination and planning, tulips can be an excellent and carefree long-term investment in the garden in any region that has a cold winter and mild spring. Particularly in the semi-arid American West, tulips can be some of your most reliable and consistent low-maintenance perennials.

Tulips that return year after year: Getting your bulbs to naturalize

First, a definition. When it comes to bulbs, the word “perennialize” generally means flowers return for at least 3 to 5 years or, at best, they can return indefinitely but typically don’t increase. Since we can’t control nature, a bad year in which uncommonly destructive weather or an invasion of hungry rabbits and deer lops off the tops of the plants could permanently knock the population of perennializers down. Because it can’t return quickly, a simple perennializer will eventually have to be replenished. Meanwhile, the word “naturalize” means the plants multiply and spread like wildflowers, rebounding from almost any temporary setback.

The genus Tulipa is composed of dozens of species native to central Asia, cool mountainous parts of the Middle East, and southeastern Europe, in various climate regions that range from dry alpine to Mediterranean. In other words, tulips are native to climates that are a lot like western North America. These different climates share the presence of a cold winter, then a brief but mild spring with at least some precipitation or residual moisture from melting snow, followed by a dry summer. They often grow at high altitudes where temperatures can swing unexpectedly and snowstorms can occur well into spring. It’s why the ephemeral tulip evolved to be exceptionally tolerant of growing and blooming amidst occasional hard frosts and cold, and can emerge from the soil, grow quickly, bloom and return to dormancy within a span of two to three months before summer’s heat. Wild tulips tend to be tolerant of a broad range of climatic and cultural conditions, while domesticated tulips can be more particular, often resenting a wet summer or soil that is too rich or dense.

With that in mind, the most important step in achieving a tulip garden that naturalizes is choosing the right kinds of tulip to begin with.

For a perennial tulip garden that doesn’t need to be constantly restocked, right off the bat we have to give up on some of the most popular classes of commercialized tulips. That means to avoid building your garden out of things like towering single late tulips, showstopping double tulips, exotic fringed, viridiflora and rembrandt tulips, trumpet tulips, and the formal cup-shaped single early and triumph tulips. Sure, there are a few of these varieties contain cultivars that perennialize decently, and some might multiply well if you are fortunate enough to have the perfect soil and microclimate in some corner of your garden. But, in general, I recommend saving these short-lived varieties for an accent, planted here and there in a contrasting color for a striking standout effect that only requires a few bulbs. Meanwhile, I recommend sticking to tried and true multipliers en masse for the tulip garden’s backbone.

Fortunately, there are so many kinds of tulips in so many shapes and colors that the remaining classes—the naturalizers—give us a lot to work with.

Tulips varieties that naturalize

Greigii tulips

Greigii tulip ‘Calypso’ is a vigorous multiplier even in adverse conditions, with a tropical-hued flower that is hard to put into words. It blooms in early to mid spring before most other tulips, standing less than a foot tall. A cluster of 2-3 bulbs will eventually expand into a dome-shaped bunch. This image shows bulbs I planted with great success in a neglected corner of the yard.

My favorite class of tulips are hardy Greigii tulips, with big midseason blooms, sturdy stems and interesting patterned foliage that is attractive enough to appeal even when it’s not in bloom. Some varieties of Greigii tulip are vigorous enough that a small offset bulb can grow in early spring to produce a standard-sized plant and flower that is nearly as big as one from a commercially-bought bulb. Additionally, Greigii tulips tolerate wear and tear and grazing from herbivores at a level that would be fatal to other tulip classes.

Darwin hybrids

Darwin tulips are the quintessential tulip, coming in the familiar reds, yellows and oranges many of us remember from childhood. Newer Darwins are available in many other colors. They return and multiply well. This cultivar is ‘Golden Apeldoorn.’

Darwin tulips are the most popular and familiar of the classes of tulips that can multiply, and are most likely to be seen blooming in an old garden that hasn’t been restocked in a very long time. You might even see one or two of these tulips peeking out in an abandoned lot. Created by crossing vigorous fosteriana tulips with showy single late tulips, Darwin hybrids have a very “classic” and recognizable tulip shape, and bloom mid-spring. They exist somewhere along the threshold between perennializers and naturalizers—although they can multiply in many conditions and are easy to bring back year after year, they won’t usually form expansive clumps and could eventually lose the battle in a garden that becomes crowded out by more vigorous perennials or weeds. Still, in a garden that is even moderately tended, you can get them to maintain their numbers and even gradually increase your stock.

Emperor tulips

Fosteriana tulips have elongated flowers and bloom earlier than most other varieties. This unique cultivar, fosteriana ‘Purissima blonde,’ has variegated leaves.

Fosteriana tulips, also known as emperor tulips, are half of the Darwin hybrids’ genetic heritage and the side that gives them their perennializing ability. Fosterianas are big and showy early-spring bloomers that flower with a peculiar hourglass-shaped bloom, and spread their petals wide on a sunny day to resemble an oriental poppy as much as a tulip. They are solid naturalizers that will multiply in most circumstances.

Kaufmaniana tulips

Kaufmanniana tulips are the earliest-blooming tulips, and have distinct star-shaped flowers with striking patterns in the center. When the flowers close in the evening, they are reddish-pink.

Kaufmaniana tulips have an un-tulip-like “waterlily” shape, spreading their petals wide to reveal striking bullseye patterns in the center of a starlike flower. On a cold or cloudy day, their tightly-closed flowers appear slender and often show different colors on the outsides. The plants are short—five to eight inches tall—exceptionally tolerant of snow and cold, and bloom the earliest of any type of tulip, sometimes even before the daffodils do.

Species tulips

Finally, there are species tulips, a class that is really a catch-all for anything left over from all other tulip categories. While the other classes are, generally, different blends of a few closely-related species within the Tulip genus, each type of species tulip is a unique species, many of which are too diverse to hybridize—so there’s more genetic diversity in this class of tulips than in all the others put together. They are sometimes referred to as “wild tulips” since most varieties were only recently brought into cultivation. This category tends to contain lots of smaller plants—some as dainty as two inches tall—and holds some of the most aggressive and resilient multipliers (although not all of them will multiply as quickly). Many of them produce multiple flowers per stem or send up successive flowers from the same bulb. They can spread through your garden not only by offsets but by seed, and are diverse and unusual enough that visitors might be surprised to learn that some of your species tulips are tulips at all.

The best sites for naturalizing tulips

With the right varieties in tow, you have a wide range of options for planting tulips to return year after year. Although tulips will generally suffer in waterlogged soil, and tend to prefer a sandy soil, naturalizers will still naturalize even in poor clay, and don’t really require fertilizer as long as mulch or compost is periodically used as a top-dressing to recycle nutrients in your garden.

A basic rule will be that tulips prefer full sun to part shade—I’d say at least 4 hours of direct sunlight per day will work if you live in a mostly cloud-free climate like New Mexico or Colorado, but 6-10 hours of direct sun with a bit of afternoon shade is ideal. In the Pacific Northwest or Eastern U.S., you should aim for at least 6 hours of direct sun.

Remember that tulips emerge and bloom in early spring before deciduous trees have leafed out, meaning that most areas in your garden, even under trees, will count as full sun until the tulip season is on the wane. Planting around trees can actually be a great benefit to your tulips because the dappled shade that develops as the trees leaf out later in the spring can protect fading tulip foliage from heat and keep it green longer. That means the plant will have a chance to grow more and bigger bulbs for next year’s blooms.

On the other hand, the South-facing side of a home, fence or building may not be a great site for tulips in sunnier regions. The problems with this setting are twofold: the heat of a southerly exposure tends to break tulips’ dormancy too early in spring, leading them to bloom when hard frosts can still nip the petals. Meanwhile, midday heat dehydrates the petals, leaving them tattered and faded and shortening the lifespan of the flower. If unseasonably warm spring weather raises the temperature past 80, full sun at midday will zap the blooms of all but the species tulips.

One place I love using for tulips is a neglected area near a sidewalk or driveway where shoveled snow gets piled over the winter. Provided that it’s not loaded with salt, tulips appreciate the winter moisture and the insulation the additional snowpack offers. When the snow melts, the soil’s water storage capacity is often great enough to get tulips through the growing season with no supplemental irrigation at all. Then the garden bed dries out in summer when the plants are dormant—conditions that tulips love. In fact, any dry, unirrigated area could be ideal for tulips, which do not lose much moisture to evaporation when temperatures are cool in spring.

Spacing

One important aspect in getting tulips to naturalize well is ensuring that the planting bed is not too dense. While drifts or clusters are always attractive ways to arrange bulbs, the clusters should be loose—the bulbs of large tulip varieties should be started out 8 to 10 inches apart (or clusters of 3-4 bulbs spaced at least a foot from the next cluster), and smaller varieties should be planted 6 inches apart (or in clusters surrounded by a wider space).

The issue is that, after a couple years, each bulb will itself have developed into a small cluster, and too much competition can stunt them. Meanwhile, you’ll find that tulips are somewhat difficult to dig and divide without unearthing the entire garden; right after they die back into dormancy the bulbs are easy to locate, but their skins are still soft and digging can easily puncture them. Later in the summer or fall they’re firm and come out easily, but it’s more likely that the dead foliage showing you where the bulb is buried has already rotted away. Although digging and dividing bulbs is a good way to keep and propagate your collection, it can be difficult to be thorough enough to get all the bulbs out if your goal is to relocate an entire planting or radically thin your stock.

Care and cultivation

Beyond the proper plant selection and planting, tulips grow best with little to no interference. That means, most importantly, leaving the foliage intact after the blooms have faded. Planting tulips in and around other flowers and perennials helps distract from the boring green foliage and reduce the temptation to clip, but this is merely aesthetic. In truth, you don’t have to do anything except occasionally replenish the mulch.

After tulip flowers fade, it’s important to leave the foliage undisturbed so the plants can photosynthesize feed bulbs for next year’s blooms. The foliage sticks around for 1-2 months.

Some varieties of tulips are sterile or reluctant to go to seed, but sometimes you may see the seedpod begin to swell. Since most tulips take several years to grow from seed to flower and that’s under careful conditions, it’s appropriate to snap those off to promote energy flows down to the developing bulb, rather than up to developing seeds. The only exception I would make is for species tulips, which can sometimes go from seed to flower in 2-3 years and don’t seem to loose much momentum by making seed. Still, I like to remove all but a few pods if I have the time, just to give those bulbs and their offsets a little more oomph.

And that’s it. With a little investment upfront, you can create a dramatic and colorful spring garden of tulips, rewarding you, your neighbors and populations of pollinators for years or decades to come.

Summary:

Perennializing tulips bloom for years to come, but may eventually dwindle. Naturalizing tulips multiply and spread.

You can create a permanent, naturalizing tulip garden by choosing varieties that return and multiply. Species tulips, greigii tulips, fosteriana tulips, Darwin tulips and kaufmaniana tulips are most likely to naturalize.

To naturalize well, tulips prefer well-drained soil in dryish areas that receive part sun or full sun in early spring. A spot under a deciduous tree that doesn’t leaf out until tulips are done blooming might be an ideal garden.

To naturalize tulips, plant them in drifts spaced farther apart than the label usually calls for, so that the plants don’t compete with each other and have room to multiply.

Don’t trim or mow foliage after tulips are done blooming. Let bulbs recharge for next year’s blooms by leaving the leaves intact until they die on their own.

On larger tulips, cut off the seed pods after the blooms fade to direct the plant’s energy to next year’s bulbs. Small species tulips may be aggressive enough to spread and multiply by seed and by offsets at the same time.