Growing a pineapple plant from a pineapple top is a fun and simple gardening project to do with kids, or an easy way to get a free, low-maintenance houseplant that actually looks pretty good in your home.
A pineapple top rooted a few months ago producing a new set of leaves on top.
For an industrious plant-enthusiast there is a litany of ways to turn grocery store produce or kitchen scraps into live plants—sprouting mango and avocado seeds, planting ginger root or replanting the base of your onions. They’ll all get you something, but out of all the possible scavenged garden projects, pineapple plants grown from pineapple tops make for the most decorative and tidy houseplants that look just as nice as a plant you buy from a department store or nursery. They have an attractive symmetrical shape and interesting spiky leaves, they don’t shed or make a mess and they are easy to keep alive on a windowsill, tolerating both overwatering and underwatering without much fuss.
You need:
A pineapple with a nice, green healthy top
A container or flower pot that is at least 6 inches wide (ideally 8 inches), and has drainage holes in the bottom
A tray to go under the container (unless it’s built into the pot itself)
Potting mix
A knife/cutting board
Scissors (optional)
A bright windowsill that gets direct sun—ideally one that faces south
Putting it together
First, cut the pineapple top off of the pineapple as close as you can get to the spot wher where the foliage meets the fruit. Inside the cluster of leaves there is a short, thick, somewhat fibrous stem that can be difficult to cut through, so if you’re with young kids you might want to leave that part to the adults. Make sure to remove all the fruit from your cutting; it just rots and it’s not where the roots emerge.
First: cut the pineapple top off of the fruit and cut off all residues of fruit.
Next, strip off some of the lower leaves to expose at least a half an inch of the bulky stem. At the base of each leaf you may see a tiny bud or node—those are small roots, ready to grow on contact with soil.
Pull off some of the lower leaves to expose at least half an inch of stem. New roots will emerge from the exposed area after the cutting is planted.
You can trim off dead portions of the leaves or leaf tips as desired to make the cutting more attractive; it doesn’t make a huge difference for the plant unless there was so much brown that it was shading out the green leaves, but it will be more enjoyable to grow if it looks healthy.
I like to use scissors to clean up dead sections of leaves at the tips. It’s not crucial, but makes for a more attractive plant.
Fill your container with potting soil up to about 1″ from the top. Dig a small hole in the center for the pineapple top.
Plant the cutting deep enough to cover all of the exposed stem and the bases of the lowest leaves.
Press the pineapple top into the soil so that the bare stem section is covered and so are the bases of the lowest remaining leaves. It’s OK if some soil gets into the crevices between the leaves. Compress the soil lightly around the plant.
A newly-planted pineapple cutting at the ideal depth.
Water the plant thoroughly, and place it directly onto your bright sunny windowsill. Pineapples need a lot of sun to root!
Be sure to place the plant close to a bright window to root. An established plant is more tolerant of receiving less light, but cuttings prefer some direct sun.
That’s it!
Caring for the plant
Keep your pineapple cutting’s soil moist by watering it regularly until a some water drains through and collects in the tray. That’s how you know the soil is saturated. It’s OK to leave a little standing water in the tray; it will help keep the soil moist as some of the water is drawn back in. Leaf tips may brown a bit as the plant struggles to grow roots, and you can trim them back with scissors. But the rooting process does not take very long and the plants aren’t temperamental.
Resist the temptation to lift the plant to check for roots; you could break the roots and force the plant to start over. The first sign that roots have formed is that the leaves will suddenly begin growing, especially the young ones in the center of the plant. They’ll soon stand tall above the others. You can safely cut back on water when that happens, and even add a dilute fertilizer every now and then for faster growth.
Collect different types of pineapple plants
Commercially-available pineapples are grown from 5 main classes with several varieties or cultivars in each class. All pineapples of a particular cultivar are clones (propagated by cuttings). Pineapples can’t self-pollinate, and since they are only grown among their own clones they won’t produce seeds since the plant treats pollen from a clone as being from itself.
The varieties of pineapples in grocery stores lead to slightly different-looking pineapple plants; some form a taller and spikier plant, some form a stouter plant, some plants have a glaucus coating that gives them a nice blue tint and some are deep green. Some have serrated or barbed leaves and some are smooth. You can tell the difference by looking at the pineapple top even before you buy it. The plants may have slightly different light and water needs, but for your purposes at home you can treat them pretty much the same.
Will your pineapple houseplant produce fruit?
A note: you probably won’t ever get a pineapple from an indoor plant; in tropical areas they can form fruit in a year, but indoors the plant will tend to reach a certain size and plateau. It can live indefinitely. However, you can eventually force your plant to fruit by covering it in a plastic bag with a ripe banana inside; the ethylene gas released by the banana will trigger a hormone reaction that forces a small pineapple to emerge in a few months. If it fruits, the main portion of the plant will stop growing and 2-4 shoots will emerge from the sides.
A note on starting your pineapple top in water
Don’t. I know people are used to starting cuttings in water and for many plants it’s a very easy way to propagate them, so it’s the default strategy when trying a new plant. You can find dozens of guides online that tell you to root your pineapple top in a dish with water, but there’s really no reason for it. Pineapples are not tolerant of being submerged; they rot easily in water, especially if there’s any fruit left attached to the stem. Meanwhile, they root with a high success rate in soil. So skip the unnecessary step!
Most of us got our first trial at plant propagation by sticking a sprig of mint or philodendron vine in a glass of water. After a few weeks, we had roots! Then they were ready to be planted in soil.
Plants that root in water
There are a number of plants which can be propagated in this exceptionally easy way: coleus, dieffenbachias, begonias, brugmansias, tomato plants, bee balm, many types of salvia, dracenas, african violets and more. These plants resist rotting, absorb water directly into a freshly-cut surface, and may have latent root buds along the stem. Many come from the mint or nightshade families, or are tropical houseplants, selected from species that tolerate wetness and are adapted to rooting quickly to spread in their highly-competitive native habitats.
Cacti and succulents
On the other end of the spectrum are succulents, many of which would rot in water but carry enough moisture in their fleshy leaves or stems that they can root in soil without special care. Most sedums, aloes, aeoniums, jade plants or cacti will easily root after air-drying for a day or two and being inserted directly into soil. They can last a long time on stored water so the cuttings can live for weeks or months without roots. Some types of succulents will generate new plants from a single fallen leaf.
Everything else
The largest number of plants fall into a difficult middle category: they’re both vulnerable to rot in pure water and unable to store enough to live without their roots unless they have some sort of life support. Some species seal wounds quickly and won’t take in water through a cut stem in a vase. Others are adapted to dry climates and haven’t developed defenses against bacteria and fungi that live in water, or can’t transport oxygen to submerged tissues so they suffocate and die below the water line. To get these cuttings to root, you need to control their environment more carefully.
Industrial nurseries use elaborate systems for this: tight temperature controls, chemical formulas, sterilized equipment and sprayers that mist the plants every couple minutes to keep leaves consistently wet. They’ll even create new plants by tissue culture, an advanced propagation process starting with small clusters of cells added to a sterile petri dish. For the home gardener, this is too elaborate to be practical, so some of the most challenging plants are beyond our reach. But we can still propagate a long list of plants at a moderate skill level with a fairly simple setup.
A simple nursery setup for cuttings
I propagate most of my cuttings in a basic “greenhouse” made with clear plastic containers and plastic wrap. The clear container lets me see the young roots forming, and the plastic wrap will trap humidity to achieve a 95-100 percent humidity level that prevents cuttings from losing moisture. I fill it part way with a rooting media (potting media, sand, perlite or vermiculite). Most plants are OK with potting media but some of the plants that rot more easily need an inorganic substrate.
Artemisia ludoviciana cuttings, prepared from a single stem, cut into segments and prepared to be stuck into a rooting media.
There’s no easy rule to determine whether a plant is easy to propagate by cuttings or how to select the best part of the plant to use; plants are too diverse. I recommend looking up the species or variety to learn its propagation methods. Government databases, universities or botanical societies have straightforward online databases to search. You should find whether or not propagation from cuttings is possible, the best time of year to take them, whether the best tissue comes from the stem tip or the base of the stem, whether to select young or old stems, you’ll see how long the cuttings should be, whether the rooting media should be kept very moist or on the dry side, and other variables that will impact whether your cuttings succeed.
Taking the cuttings
Generally, if I’m lacking more detailed info about a species, I will take a full stem from the donor plant if that’s available without doing too much damage to it. It’s best to do this in the morning when the stems are hydrated after a cool night.
I’ll take the single stem and cut it lengthwise into several individual cuttings. That way, one cutting will be made of old basal stem, which is hormonally prepared to root in many species, one cutting will be a young stem tip, which in some species forms the most vigorous roots, and gradations between them. I make sure each segment has at least three nodes (the place where a stem connects to a leaf and new shoots can emerge), and with each segment I’ll remove the lowest leaf and keep the top two.
Inserting the cuttings in the rooting media
It can be tricky coming up with the right moisture level for rooting media, since different plants prefer different amounts of moisture. A good middle ground is to wet the media thoroughly and let it drain so it is saturated but not super-saturated.
I stick the cuttings so that the node corresponding to the lowest leaf is right on the surface of the rooting media. That places one node at the bottom of the cutting under the media, one internode is buried the media, and one node is partially covered. Two leaves are able to photosynthesize, but the top leaf is completely clear of moist rooting media in case the one resting on the media rots. The batch contains variety of tissue types and positions from which roots may grow.
Artemisia ludoviciana cuttings have been stuck in the rooting media, in this case vermiculite that has been wetted just enough that no water drips out when tipped on its side.
Rooting hormone powder or gel is optional. It certainly helps plants that are harder to root from cuttings, while fast-rooting plants produce a lot of the same hormone internally and won’t derive much benefit from what you add.
If the stem is rigid, you can stick the cutting directly into the media, but in most cases it helps to dig a hole with a pencil or toothpick, insert the cutting and pat down the media around it.
The Artemisia ludoviciana cuttings are wrapped in plastic wrap to keep the air inside humid. That way, the cuttings will not wilt despite lacking roots.
Maintaining the cuttings
After the cuttings are struck, I cover the container with plastic and poke small holes in it to enable a little bit of gas exchange. It can be placed on a windowsill that doesn’t get too much direct sun, or, more successfully, under bright white fluorescent lights.
A note on plant lights: Artificial light for plants doesn’t need to be special, but it has to produce light without too much heat, so do not use incandescent bulbs. The closer the light appears to natural sunlight, the better. Newer LED setups might combine red and blue light, which should work. Otherwise, plain white light is best. (DO NOT use blacklights or UV lights; plants don’t use ultraviolet light to photosynthesize. They use the same wavelenghts produced in the largest amount by the sun, which are also what your eyes use to see and it will appear white.)
Cuttings under artificial lights need to be on a timer: plants require a period of darkness every day to undergo a full metabolic cycle, but will root faster if there is more light than darkness. I use a 16-8 photoperiod, or 16 hours on, 8 hours off, unless I learn otherwise about a particular species.
Several sets of cuttings are wrapped up and placed under lights for rooting.
Moving to the next phase
Different plant species take varying lengths of time to root, so you’ll have to keep an eye on your cuttings by looking through the container. Sometimes I lift a cutting or two out to check, although I do this with caution because it can break small roots off or delay their growth. I usually move the plants to the next phase when the roots are a centimeter to an inch long.
After the cutting has formed small roots, it no longer needs the high-humidity environment under plastic wrap. It can be removed from the container transferred to soil to continue to develop there.
When a cutting is mature enough to transfer, it’s better to do it as soon as possible because the very high humidity environment under plastic wrap can leave plants vulnerable to forming mold or developing diseases. But the plant will be shocked and die if it is moved directly into dry air or hot sun, and needs some intermediate steps. Initially, you can transfer it into small pots in potting mix and left in the same environment with the same light cycle that it rooted in.
After the young plants form roots, they can be transferred into soil trays and returned to the same environment they were in before to continue to develop. After further growth, you can start moving them into brighter light or the more variable weather conditions outside.
From there, you can gradually harden your plants off. Watch closely for wilting or signs of severe drought stress such as leaves crisping up around the edges—if that happens, you’ll want to slow down the hardening-off process or cover the plants with a clear plastic bag to raise the humidity again.
If all goes well, root systems will quickly grow. When you see roots coming out from the bottom of the container, you can transfer the plants outdoors in shade on a mild day. Avoid hot sun! Over time, you can move the plants to a spot that gets early morning sun. You may want to pot them up into larger pots if you plan on leaving them in hot sun or overwintering them in containers. Otherwise, continue to introduce them to brighter light and eventually you’ll be able to plant them out in your garden!
Bearded iris, Iris germanica, is one of the most easy and rewarding perennials you can grow. They can survive in xeric gardens with as little as 10 inches of rainfall a year, and will grow and bloom without care as long as they get plenty of sun. Multiplying perpetually, there is no limit on a bearded iris’ lifespan. They rarely if ever need fertilizer. They tolerate a wide range of soil types. In dry-summer climates they’re highly resistant to disease. They’re able to push back and hold their own against aggressive weeds and groundcovers, without ever becoming invasive themselves, since a clump of irises only creeps outward by about 3-4 inches per year.
Bearded iris are one of the most carefree and reliable garden perennials
They’re also rapid multipliers, which is both a blessing and a curse—a few new plants will become a cluster and a cluster will become a dense, thick mat of roots and leaves that eventually choke each other off and stop blooming. Like many clump-forming perennials, they’re prone to developing the “crowded doughnut” shape, with weak, depleted plants in the center of a cluster and relatively vigorous plants around the edge where there is still access to fresh soil and sun. To avoid this situation (or fix it), it’s best to dig and divide bearded iris every 3-4 years.
Timing
In truth, bearded iris is tough enough to be divided any time of year. But since it takes some time for roots to re-establish, the most often-recommended time to transplant is right after they bloom: in June for Northern Hemisphere gardens. The reason for that is not so much about the health of the plant (the least-stressful time to dig them, from the plant’s perspective, would probably be early fall) but so that the plants are completely recovered by the following spring when you want them to bloom.
It’s notable that June and July are very stressful months for plants to be transplanted, forcing them to re-establish roots in the midst of long, hot days when demands for water are at their peak. June is also the most productive time of year for photosynthesis, an opportunity missed if plants are dealing with their re-establishment phase when they’ll have fewer leaves. But this is the best time of year to do it if you want to maximize blooming the year after. Since bearded iris is such an easy and vigorous plant, one that can survive being unrooted for a long time and multiplies fast anyway, you can go ahead and dig it in summer without much harm. Why not? Spend the spring and fall transplanting seasons attending to your more temperamental plants.
How bearded iris grow
Irises grow from a rhizome, a form of modified stem that creeps along the ground just beneath the surface. Irises have a fat, fleshy rhizome for storing water and starch, which contributes to their durability. (Other rhizomatous species have simple, spindly rhizomes that help them spread rapidly but won’t enable them to live long outside soil). The top of the rhizome is usually exposed, revealing ribs where now-dead leaves used to be attached. The underside of the rhizome is covered with stringy, strong, fibrous roots that eventually die and slough off, leave small superficial holes in the rhizome.
A keen gardener who looks at the structure of different plants eventually comes the realization that all vascular land plants, from 350-foot-tall redwoods to dainty daffodils, are composed of the same three basic parts: leaves, stems and roots. Leaves and roots always attach to the stem, which, for most plants, is usually the only type of tissue that generates the other two types. Stems always grow new tissue from specialized cells at the tip, and produce new branches at nodes that lie dormant at the base of a leaf or leaf scar until it’s time to grow. Cells at the growing tip of a stem divide rapidly to form the basic structure, then stop dividing and go through an elongation phase to reach the full size, then harden and stop growing as they mature.
Whether plants grow from bulbs, tubers, crowns, corms or woody trunks, the dramatic differences in the way plants grow are simply a matter of the way these three main parts are arranged. In an onion, for example, the stem is a small mass in the bottom of the bulb called the basal plate, where the onions layer’s (which are modified leaves) are attached. The roots emerge from the bottom of the basal plate. Only this basal plate tissue can perpetuate the life of the onion; you can replant the base of an onion bulb to generate a new cluster of leaves, but you can’t plant leaves or chopped onions to grow new roots, nor can you generate onion plants from the spindly roots. In a potato, the entire edible tuber is made of stem tissue and each of the eyes is able to sprout new potato plants. If you look closely at a carrot, which is a tuberous root, the stem is a small area at the top of the root where a cluster of leaves emerges. Neither the root, nor the leaves, are viable on their own.
So the bearded iris’s rhizome is the plant’s main stem. The new cells are created inside a small, actively-growing region inside the fan, which is located at the young growing tip of the rhizome. New leaves are always emerging from the center of the fan, and old leaves are always sloughing off the outer edges, exposing new sections of rhizome. New roots emerge from just behind the spent leaves. Iris leaves are constantly being renewed, and roots are not long-lived either; they live about a year, and after they die, the older portion of the rhizome simply sits on top of the ground, anchored by the actively-growing regions at the tips where new roots are always forming.
Irises multiply in two phases. The most prolific is related to blooming. When an iris prepares itself to bloom, the actively-growing tip of the rhizome—in other words the growing tip of the plant’s stem—curves itself upwards and hoists itself into the air as a flower scape. There, the stem terminates, and the fan that it originated from will die.
Like so many other plants, pinching or terminating the growing tip of a stem forces it to branch. This is also true when a rhizome terminates in a flower scape. In fact, in early spring, before the flower buds even appear, the emergence of offsets that pop up just ahead of the main fan is a strong indication that this particular iris will bloom. Weak or shaded irises will produce 1 or 2 offsets per flower scape. Moderately-vigorous plants will produce 2 to 3 offsets, and highly vigorous plants will produce 4-5 new offsets to replace the portion that bloomed. That gives you a sense of how quickly irises multiply.
The second source of new offsets on a bearded iris comes from older nodes in the aging portion of the rhizome. These auxiliary shoots are usually small and will require multiple growing seasons to reach blooming size. Any old chunk of iris rhizome, whether it has leaves or not, will produce one or two of these types of offsets if it is separated from the rest of the plant. If you want rapid propagation of your irises there’s no reason you can’t keep every bit of living rhizome to create lots of new plants, but many gardeners will discard older portions of the rhizome when they divide their iris since the offsets from old portions take 2 years to bloom.
Digging and dividing
It’s fairly easy to dig, lift or yank bearded irises out of the ground. There’s not a lot you can do that harms the plant much, even if you lose some of the leaves in the process. When the cluster is out, you can shake or hose off the dirt and separate the cluster into individual plants.
Rhizomes will initially be connected together into large branching structures, tracing back to the original parent portion, which may or may not still be living tissue. To divide iris, you’ll need to break the branches apart into manageable bits. Some guides suggest using a sharp, clean knife but tearing is also fine. The exact point of breakage is not that important; separate the offsets into 2-6 inch sections each containing 1-3 fans. Try to maintain at least two inches of rhizome and at least one highly-vigorous fan on each segment, since it leaves you more likely to get blooms next year. But even smaller sections will grow stronger and produce blooms in the second year.
These rhizomes are still connected because they grew from the same original plants. The red lines are reasonable places to cut or break off portions for re-planting. If iris are divided and replanted in early summer, these divisions would be able to bloom next spring.
You’ll be left with a lot of nice-looking healthy green vegetative fans connected to rhizome, and a lot of leafless chunks of older rhizome or rhizomes with only small, weak vegetative shoots. You can feel free to plant these other chunks in a separate part of your garden to propagate more iris, or mix them in with the cluster you intend to keep, or throw them out. They will bloom in 2 years if grown in good conditions. They’ll also root in your compost pile.
Dividing iris will produce a lot of older portions of still-living rhizome. Some gardeners will discard them, but if you plant them they’ll produce shoots that bloom in 2 years.
Should you trim the leaves off of bearded iris?
Some gardeners consider it good practice to trim the leaves back when transplanting iris. There are good reasons for it, but they might not apply to every circumstance. First of all, if the plants are left out of the ground for a while to be shipped or stored, the leaves will dry up and die back. Irises never go fully dormant, but it is a quasi-dormancy that allows the plants to live a long time without water. Trimming the leaves keeps them looking cleaner and saves a lot of space.
Second, in some climates, particularly those with rainy or humid summers, bearded iris are more prone to disease. The plants will be weakened temporarily by transplanting, giving diseases like iris leaf spot fungus an opportunity to spread. So if you see a significant number of streaks or brown spots on the leaves even before the plants are transplanted, cut them back halfway or more to remove some of that diseased tissue. In climates with dry, warm summers, this is not likely to be a concern.
In all other circumstances, I would make this your standard: after you transplant iris, you want the fans to stand up. Iris are adapted to hold their leaves perpendicular to the sun at midday to keep cool and avoid water loss, and fans that tip or flop over will scorch and brown. If they’re tipping over, cut them halfway down or low enough that they stand up. If they’re standing up already, it’s not necessary.
Planting
To plant your iris, dig a wide, shallow basin of the desired with and 2-4 inches deep. Pile the soil to the side. Place your prepped iris divisions in the basin about 6 inches apart at the tips, with the fans pointed outward—this is the direction they’ll creep along the ground as they resume growth, and this way they’ll spread away from each other rather than getting too dense. After you have them placed, you can invert a couple plants with the fans pointed towards the center of the circle so there won’t be a blank space there.
Replanting divided iris Is very easy. Dig a wide basin 2-4 inches deep, place the divisions pointing outward in the same position they were growing in before, and re-cover with soil. Make sure the leaves are vertical to prevent them from scorching in the midday sun. Water thoroughly to settle the soil and hold the plants in place.
Use the soil you removed and cover the rhizomes. Iris like to creep along the surface of the ground, but if they end up being buried 1-2 inches deep, they’ll be fine. You might find it’s easier to keep them stable with a little soil covering, and subsequent growth will stair-step up to the surface so the rhizomes are growing at the depth that they want. Gently pack the soil with your hands enough to keep the plants from tipping over. Then, you can replace mulch on top.
Should you fertilize or amend the soil?
Bearded iris are tough plants that do well even in poor soil. That’s one of the things we like about them! Additionally, their roots leave channels in the soil when they die, allowing oxygen and nutrients to circulate deep into the garden. I prefer not to collapse those channels by tiling the soil unnecessarily.
However, if it looks like the clump of iris really depleted the soil (indicated by very weak plants at the center of the clump), you can consider sprinkling a little compost on top of the soil to gradually break down and fertilize the iris over time. You should do this after the iris are planted and covered by native soil. It really doesn’t take much compost to replenish soil nutrients and may not be necessary at all. Then spread mulch over the top, and water them thoroughly to settle the soil around the rhizomes and initiate new root growth.
Summary
Bearded iris multiply quickly and the number of fans can triple every year. Clumps that become too dense can start to decline in the middle, and should be dug and divided every 3-4 years.
Bearded iris is hardy enough to be dug or and divided throughout the year, but dividing clumps in June—just after they bloom—gives plants the longest time to re-establish before the next blooming season.
Rhizomes creep along the ground at or just below the surface and form branching shapes that can be split into 2 to 6 inch segments, each containing 1 to 3 fans.
Divisions with large green fans can bloom the next year. Older portions of rhizome that are still firm but don’t have much foliage may take two years to grow established enough to bloom.
Growers in humid-summer climates may want to trim the foliage during transplanting to limit the growth of fungal diseases. Foliage that flops over will scorch in the sun and can be trimmed halfway. Otherwise, if the foliage looks healthy and can stand up straight after the plants are transplanted, it can be left intact, or trimmed part way down to clean it up.
Bearded irises should be planted in clusters in a wide, shallow hole just 2 to 4 inches deep (not including mulch). Arrange rhizomes around the hole, several inches apart, with the rhizomes laying horizontally and the fans pointing upwards. Point the rhizomes in all directions with the majority pointed outward, then cover with soil.
Irises typically do not need fertilizer and can grow in poor, compacted soil. Optionally, you can sprinkle a thin layer of compost on the soil to supply a nutritional boost.
After transplanting, reapply mulch and water the plants in to get them established.
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.
