Monday, June 1, 2015

An ecological look at vegetable gardening systems

I’ve examined some different systems of growing vegetables in earlier posts, viewing them primarily from the standpoints of yield (pounds produced per unit area) and inputs required. Now I want to view them from another perspective: that of ecology. What does the science of ecology suggest about how we might best grow vegetable plants, and how do different growing systems support ecological insights or work against them? Fighting against the ecological tendencies of a plant makes extra work for the gardener and causes the plants to grow less well than they otherwise would. If we understand the ecological needs of the vegetables we want to grow, we can create a garden habitat that is better able to meet their needs. That might lead to a better yield of the vegetables that we grow or if not a better yield, at least a better use of our limited time as folks with lives outside the garden.

Caveat: I am not a trained ecologist. While I have studied aspects of ecology that relate to gardening, I cannot guarantee that I have applied them correctly. I think this topic deserves more study, especially by people who know a lot more about ecology than I do.

The Big Cover-up

To understand how ecology relates to growing vegetables, let’s examine what happens to a bit of ground that is stripped clean of plants. For instance, you might decide to fill in a depression in your backyard with a couple of wheelbarrow loads of soil. You intended to toss some grass seed on top of the bare soil after you dumped it and smoothed it out, but one of your children suddenly needed your attention, and then work called, and then ... pretty soon weeks have gone by, in which every time you remembered the bare soil that you should have covered with grass seed, you found some excuse to ignore it. (Doesn’t this sort of thing happen to you sometimes? Right now I am ignoring a patch of my yard where poison ivy is taking up residence even though I know it would be better to deal with it.) Finally, about the time when spring is turning into summer, you decide to take a good look at that spot. The bare soil is no longer bare (you knew that, of course; that’s why you’ve pretended not to notice that spot for weeks). Instead many plants have taken up residence, in varying stages of growth. You sigh, realizing you have a lot of work ahead of you if you mean to return that patch to grass.

What you may not have noticed is what kinds of plants are growing on the formerly bare spot. You might find a few seedling trees, especially if maple tree seeds fell between when you spread the soil and when you finally gather enough courage to tackle that layer of plants. But almost all the plants growing on your patch of soil will be what we call weeds. You may hate them (companies that sell herbicides spend lots of money training you to hate them), but Nature needs them. Those plants you call weeds are Nature’s repair crew.

When you dumped that load of soil in your backyard, you created what ecologists call a disturbance: an area in which the previous plant cover no longer exists. Many naturally occurring events create disturbances, such as fires, windstorms, and floods. Human activities such as plowing and its smaller-scale relatives, tilling and digging, also disturb existing plant cover. Whenever you dig or till to create a vegetable garden, you are causing a disturbance, leaving bare soil behind.

Ecologists tell us that what happens next, in a process they call succession, depends on several factors. Broadly speaking, whether the end result over a period of many years is a forest, shrubland, grassland, or desert depends on details of yearly rainfall and its patterns, yearly temperature patterns and extremes, the nature and thickness of the soil, and the chemical character of the underlying bedrock. On the time scale of several weeks or months during which our vegetable garden grows, however, the more critical factors are whatever seeds already exist in the soil, what seeds are brought there after the disturbance happens, the day-to-day weather, and any other disturbances that might take place after the initial tilling or digging.

Lifestyles of the Green and Rooted

Nature’s response to a disturbance is to cover the soil with a set of plants that stabilize the soil and re-establish water and nutrient cycles that were disrupted by the disturbance. Most of these plants grow, set and release seed, and die within the space of a few weeks or months. Plants that adopt this lifestyle are called annuals, meaning growth, maturity, and death take place within a year or less. Annual plants tend to set lots of smallish seeds that can live for a comparatively long time, and they grow best in full sun. As they die, their bodies slowly enrich the soil, allowing larger, slower-growing plants to gradually move in, shading out the annuals. The annual plants thus create conditions that eventually lead to their own demise (though not before they have left a legacy in the seed bank of the soil to re-establish plant cover in the event of future disturbances).

Next in line are two other plant lifestyles: biennials and perennials. Biennials usually sprout and grow a little more slowly than annuals do during the first growing season after the disturbance. However, the next growing season, they spring into life earlier from already-established root systems than annual seeds can sprout and start growing, thus shading out some of the potential annual seedlings. Later on in the spring or summer the biennials mature their own seeds and then die. Like the annuals, they tend to prefer considerable sun and create conditions that allow for the next lifestyle of plants, the perennials that live for a few to many years, to become established. The perennials might be even smaller than the biennials at first, and some of them cannot sprout and grow unless the annuals and biennials have already prepared the ground by re-establishing the water and nutrient cycles and increasing the levels of nutrients. The perennial seeds may also need to be brought to the site by wind, water, or animals if they are not present in the seed bank or if the perennial seed is short-lived. But as the years pass, more perennials take root and those already present grow taller, shading the soil and making it less hospitable for annuals and biennials. As perennial woody plants become established, they also appear to change the microorganisms in the soil from the bacterially-dominated soils of grassland ecosystems like prairies to the fungally-dominated soils of forests and woodlands, according to the discussion in Edible Forest Gardens. I suspect that recently disturbed soil, covered mostly with annual or biennial plants, is bacterially-dominated since it takes time for fungal networks to re-grow following disturbance.

Home Green Home

Let us inquire into the lifestyles of the vegetable plants that we grow. Are they annuals or biennials that thrive on disturbance, or perennials that require some previous soil preparation and a lack of disturbance to do their best? As it happens, most of the vegetables that we gardeners grow are annuals or biennials. Among the annuals are lettuce, beans, peas, sunflowers, sweet corn, melons, cucumbers, and squashes. Onions, leeks, cabbages, broccoli, kale, collards, mustard greens, turnips, beets, carrots, parsnips, and potatoes are all biennials. The only commonly-grown vegetables that are perennial are tomatoes, peppers, and eggplants (not that those of us in frosty climates would know that if we weren’t told), asparagus, and rhubarb. Many if not most of the vegetables we grow are derived from a weedy wild ancestor that we’ve improved to a greater or lesser degree by selective breeding. Is it any wonder, then, that traditional methods of gardening and farming start with plowing, tilling, or digging to create bare soil and then add the plants or seeds that we want to grow there? It’s the genetic expectation of these refined cousins of annual and biennial weeds to grow in bare soil and to have potential competitors removed or reduced in order to express themselves fully.

It’s become fashionable to revile bare-soil gardening and farming, not without reason. Soil left bare for too long erodes away in rain and blows away in the wind. Without plant cover, nutrients such as calcium cannot cycle properly and leach away with the rain as it soaks down through the soil. But to balance this, most of the vegetables, grains, and root crops that we humans living in temperate climates have come to depend on grow best on bare soil. Agricultural techniques such as cover cropping (the growing of a non-edible crop that covers the soil) have been developed to minimize the time that soil is left bare and to keep some of the nutrients in place between food crops. Most home gardeners do not use cover crops because few home gardening resources tell us how to use them effectively. Instead, mulching as a means to reduce weed competition has been popularized by garden writers like Ruth Stout. In Stout’s case, deep mulching allowed her to garden well into her 90s without having to expend energy weeding that she no longer had. Some gardeners, especially those in dry climates, use mulch to reduce the water lost from bare soil. Others use it because it makes the garden look better to them. However, mulch can offer living space to certain plant-eaters such as slugs. I found it difficult to move aside mulch to plant tiny seeds and to mulch tiny seedlings. In my opinion mulching is most useful to gardeners who grow vegetables in large containers or small backyard beds that are not suited to digging or tilling.

Permaculture designers advocate growing perennial plants, including perennial vegetables, fruits, and nuts, as they attempt to create self-sustaining yard-scale ecosystems with a higher proportion of edible plants than occurs in unmanaged ecosystems in temperate climates. Mulching to reduce the weed load and to conserve water is an essential part of their toolkit. I have been working on a permaculture design in the treed portions of my yard, where I grow tree fruits and nuts, a few other edible and herbal plants, and a variety of native and introduced plants that attract wildlife and beneficial insects. In the semi-tropical and tropical climates in which permaculture design developed, many good green vegetable crops and some calorie crops as well as fruits are perennials. Some green vegetables, like asparagus, grow as perennials in temperate climates; more could be grown and used, as described in the books How To Grow Perennial Vegetables and Perennial Vegetables, or could be developed by backyard or professional plant breeders. I grow a few of them and plan to try others, as they bring diversity to our diets as well as to our gardens. They grow well in the mulched ground typical of a permaculture design. Tomatoes, peppers, and eggplants, all perennials in frost-free climates, should integrate well into the sunnier spots of such a design. However, it bears noticing that most of the plants which most of us in temperate climates eat most of the time, the grain, root, and vegetable crops that provide most of our calories and nutrients, are plants which are adapted to grow in sunny conditions on disturbed soil like your old-fashioned tilled or dug vegetable garden or plowed farm field. That’s why I have a large treeless space in part of the backyard where I’ve established my vegetable gardens, why I dig each bed before I plant it, and why I don’t mulch the vegetable garden. That’s why farmers plow before planting. Digging, tilling, and plowing aren’t evil, though the last two do require the use of substantial amounts of petroleum and thus are not sustainable over the long haul (unless and until draft animals make a serious comeback in agriculture). Digging, tilling, or plowing provides the disturbance that the disturbance-adapted crops we depend on require for good growth. And while these techniques do harm some of the soil life, primarily the fungal life, it also bears remembering that the disturbed soils in which the ancestors of our annual and biennial crops grew would not have had much fungal life in them because of the disturbance that created them. Further, our crops have adapted to life in frequently disturbed farm fields. My suspicion is that a yard like my own in which a vegetable garden that is disturbed frequently is bordered by areas in which localized infrequent disturbances occur and that grows a variety of annual and biennial plants as well as perennial trees, shrubs, and non-woody plants, provides a similar kind of animal, plant, and soil diversity as does a natural area that undergoes occasional disturbances at various scales.

Rooting for the Best System

Having said that, can we use these arguments to look more closely at and perhaps choose between the two techniques of bare-soil gardening that I’ve been examining in my blog? In order to do that, we need to look more closely at how the plants that colonize bare soil compete for nutrients, and how to slant the odds more in favor of the plants we want (our vegetable crops) and against the plants we don’t want (everything else).

As plants grow, they send their roots out in whatever pattern is typical for each plant as they search for the nutrients that they need to produce their bodies, and they grow their bodies upwards and outwards in their typical pattern to obtain the sunlight they require. If they are fortunate, they will find all the nutrients they need within the soil that their roots occupy and sufficient air and ground space for their bodies to get the sunlight they need to photosynthesize. Most often, however, before they can access everything they need, their roots will bump up against the roots of another plant trying to do the same, or another plant’s body will intrude on the space the first plant requires for its body. Then those plants have to compete for the nutrient, sunlight, and water supply within the root and air space that they share. 

I’ve already examined in a previous blog post the contrasting ways that two systems of vegetable growing that I have worked with supply what each considers to be sufficient amounts of nutrition. Now let’s look at another aspect of competition: reducing the numbers of competing plants sufficiently for our desired plants to utilize all the nutrition the soil provides. The way John Jeavons does this in How to Grow More Vegetables differs from the way Steve Solomon does it in Gardening When It Counts. Let’s see if we can use ecology to argue for one over the other.

The methods differ in how they pack the plants you want and how they remove the plants you don’t want.  Jeavons suggests growing almost all vegetables as seedlings in flats and planting out the seedlings on a hexagonal grid in order to pack the plants just close enough together near or at maturity to shade the soil. If the bed is prepared just before the seedlings are planted, then when the seedlings are added, they will start using the nutrients right away, getting off to good growth before the seeds in the soil seed bank can germinate and grow large. Minimal to no weeding should be required before the vegetable plants grow large enough to out-compete the weeds. Jeavons claims further that his spacings allow the vegetable plants to grow large enough to provide the best yield (pounds of the desired part per unit of area); even though the plants are crowded closer together than is usual in home gardening and thus the pounds of desired part per plant is lower, the higher number of plants per unit area makes up for that.

That’s in theory. My experience, after more than a decade of using the method imperfectly (according to Jeavons’ criteria), is that for most crops Jeavons’ method does not work as well as he suggests it should, at least not under the conditions in which I garden. The crops for which his close spacings work well for me are large, fast-growing summer crops, especially tomatoes and peppers. These crops out-compete most weeds even if I don’t weed regularly. However, the other crops I grow as seedlings (eggplants, onions, lettuce, cabbage, and broccoli among them) do not yield more per unit area when crowded together at Jeavons’ spacings compared to the more-generous spacing that Solomon suggests for them. They seem to need less competition, even from their own kind, than Jeavons’ spacing allows to do their best in my climate. Or perhaps it’s because I’ve done a better job of balancing the soil nutrient levels using Solomon’s methods than using Jeavons’, or because I’m weeding more frequently. Furthermore, with Jeavons’ hexagonal packing a goodly number of plants are located near or at each edge of the bed, hanging half or more of their bodies out into the 12 inch wide paths I maintain between beds. Inevitably those plants get stepped on or crushed as I walk on or kneel in the paths to weed or harvest, reducing or eliminating their yields and making my work more difficult. Sometimes I have to stop weeding earlier to avoid harming those plants, which may increase weed competition and lower yield. I could compensate for protruding plants by using 24 inch wide paths, but that would widen the area needed for the entire garden by about 20%, effectively reducing the yield measured using the size of the entire garden as a basis. I find it is much easier to weed and harvest from 12 inch wide paths when the plants are spaced according to Solomon’s larger spacings, which allow for the plants to grow almost entirely within the bed and away from the paths.

Seedlings of tiny plants that I want a lot of, like carrots, beets, or turnips, take a long time to plant because of their delicacy and large numbers (I know because I’ve tried it) and the roots are misshapen compared to direct-seeded roots. It takes less space in the cold frame or on the porch and less time to direct-seed into short rows in the garden and thin the resulting plants as they grow than it does to start these seeds in a flat, care for the seedlings, and painstakingly transplant the tiny seedlings to their final spacing. These seedlings are slow-growing and need a lot of weeding in spring under my conditions, a process much easier for plants in rows spaced 12 inches apart than for plants on a hexagonal grid that might be only 3 or 4 inches apart, a spacing which is too close for any hoe on the market. True, I found I needed to hand-weed the row-planted seedlings twice while they were still tiny, but the third weeding was done standing up with a hoe as will any other needed the rest of the growing season.

Jeavons suggests that only some easy hand-pulling of weeds should be necessary before the planted seedlings crowd out the weeds. Again, that isn’t what has occurred in my garden. I’m sure the weed-seed-infested compost I use and the occasional neglect of weeding to the point that the weeds go to seed in my garden has a lot to do with my garden’s weediness. But I don’t think that is all that is going on. I think that in my climate, with the weed seed bank in place, weeds can get the upper hand more quickly than most transplanted vegetable plants can grow large enough to shade them out. Far better to set up my garden to make it easy to hoe from a standing position most of the time than to spend more time hand weeding plants that are too close together for a hoe.

Solomon’s answer to competition is to set each plant far enough apart for root systems to not compete, and hoe or thin out all unwanted plants. I thought this would require more weeding work and reduce the yield per unit area too low for even my rather large (1500 square feet) garden to provide a significant fraction of our vegetables. But it turns out that it doesn’t. As I mentioned above, I did have to spend several hours hand-weeding the rows of direct-seeded root crops and the transplanted onion seedings (the latter because onion plants are small, don’t grow fast or compete well in spring, and are easy to cut with a sharp hoe). I also need to start thinning the rows of direct-seeded plants, which will be hand work. But the carrots, beets, parsnips, and onion plants are all much larger than I have seen them in past seasons at this time. By now it’s easy to hoe between rows of these seedlings, and it has always been easy to hoe around the large, well-spaced leaf crops, tomatoes, peppers, and eggplants. In addition, the cabbage, broccoli, bok choy, and lettuce plants are larger than usual at this point in the season, and the bok choy is yielding near to or over the best I’ve obtained in spring. The potato vines have been hilled twice, have very few weeds, and are much bigger than I’ve seen them at this point (about 6 weeks after planting, each hill is 9 to 10 inches high and the vines on the first-planted variety are about 30 inches tall, with flower buds in evidence).

Managing soil water levels also differs among the two systems. In Jeavons’ method, the bed is dug and then planted with seedlings. As each seedling is planted, the gardener firms the soil around it, re-establishing capillarity, the means by which water travels through the soil to get to the plants that need it. Years ago, I tried direct-seeding small seeds like turnips and carrots in Jeavons’ hexagonal pattern but got poor germination compared to direct-seeding large seeds like corn and beans. I’d wondered why that was the case. Now I suspect it was because I didn’t press down hard enough on small buried seeds to re-establish capillarity. With the larger seeds I tended to press down harder on a larger area, which seemed to do the trick. Solomon’s method of pressing a hoe handle horizontally into the soil where you intend your rows to be re-establishes capillarity following digging, and in my experience it works very well. Even small seeds like carrots, sown thickly, make a good stand of seedlings (and is why thinning is now required).

Solomon mentions the creation of a dust mulch when weeding with a hoe. I’ve noticed that weeding with a hoe stirs up the surface of the soil, reducing its capillarity to the point where most weed seeds find it difficult to germinate. As long as I hoe while the soil is dry and the weed seedlings that are dislodged dry out before the next irrigation or rain, well-rooted crops get the water they need and weeds do not get well enough established to compete for water. Jeavons’ method, by contrast, requires the soil be moist enough when weeding to pull out the entire weed, root and all, when weeding the more closely spaced crops, and the weeds must be collected and composted instead of being left on the bed as I do when I am hoeing. With the soil moist, more weed seeds can continue to germinate, competing with the crops for water.

One last ecological issue remains: nutrition and how it is provided. One of the reasons that Jeavons’ system has gotten widespread good press for many years is his claim that his system is sustainable: that in principle, a properly designed garden produces enough compostable materials to produce enough compost to maintain its fertility indefinitely without any outside inputs. Those of us, myself included, who are concerned about sustainability find this a very attractive feature. But a careful reading of Jeavons’ and Ecology Action’s books and booklets, as well as my own experience, leads me to doubt that his system actually requires fewer outside inputs than Solomon’s does.

In the later editions of his book, the ones published since 2000, Jeavons says that to produce enough compostable material within the garden’s confines to make enough compost for sustainability, about 60% of the garden space should be devoted to grain crops and another 30% to high-calorie root crops (potatoes and/or sweet potatoes) with only 10% of the area for all the other vegetable crops people want to grow (see pages 39 to 43 of the 8th edition for details). I don’t know of any gardener, myself included, who wants to allocate this much space to grain or root crops. Very few city or suburban gardeners want to grow any grain crop other than perhaps sweet corn. Most grain crops require considerable effort to liberate the ripe seeds (corn the least such) and all except for popcorn require a grain mill, a tool few people have or want to buy, to convert the seed into the product that most people use in the kitchen. And while potatoes and sweet potatoes are used as they come out of the soil, they take up a lot of space in our living quarters and that space has particular requirements that may not be possible for many people to provide. Most people with backyard vegetable gardens grow salad crops such as lettuce and tomatoes, greens such as kale, collards, and mustard greens, green beans and peas, herbs such as basil and parsley, root crops like carrots and beets, zucchini, cucumbers, melons, and maybe some sweet corn, potatoes, or sweet potatoes if they have enough space. I grow more grains, mostly popcorn, than most, yet I can make enough compost for the garden only by adding lots of autumn leaves from the rest of the yard to the compostables from the garden. Then, looking further into the argument (pages 37 and 38 of the 8th edition), we learn that even with the 60-30-10 design, we still have to compost all of our urine and humanure along with the compostable materials from the garden to recycle all the nutrients lost from the garden when we eat the produce. And that doesn’t address any nutrient deficiencies the soil began with, hence the need for a soil test and the addition of inputs at least the first time the garden is planted, as Jeavons discusses in his chapter on fertilization. He suggests that each gardener “strive to use less and less fertilizer brought in from outside his or her own garden area” (page 72). To me, this is an admission that it will be necessary, for a considerable period of time, to bring in amendments to balance the nutrient levels of the garden’s soil. I think Solomon’s approach better reflects the underlying ecological reality of gardening in small spaces. While my own garden requires less of certain nutrients than it did when I began working with Solomon’s approach, I still add others and expect to do so for the foreseeable future.

The upshot of this discussion? In my opinion, Solomon’s approach to organic vegetable gardening is based on a better understanding and application of ecology than is Jeavons’. And while I think much can be done through the use of permaculture design for perennial crops, including perennial vegetables (and we need to do a better job of this in our own yard and our kitchen), Solomon’s version of organic vegetable gardening is the soundest way to raise the standard vegetable crops at the level of a decent-sized backyard garden in the St. Louis region.