Chapter 2a: the Shallow Bed Garden - Urban Gardening

Urban Gardening

Cool Climate Crops

Chapter 2a: the Shallow Bed Garden

Chapter 2a

Three Basic Gardening Systems

Part I: The Shallow Bed Garden

The Shallow Bed Garden on a roof is usually a bed of perhaps 3-6 inches. If the roof is quite limited in the weight it can bear, then either no soil is used, some lightweight material is mixed with the soil, or the bed is very shallow. If sufficient compost is not available (a common situation), plants can be successfully grown in fresh organic matter of many kinds. Such beds are fertilized and covered with at least a thin covering of compost or soil. Almost any vegetable can be grown in shallow beds. Once the beds are established, they are like regular gardens except in their need for more frequent watering.

We have had success growing a wide variety of vegetables in shallow beds. Some examples include amaranth, broccoli, cabbage, cow peas, corn, eggplant, cucumber, green beans, herbs (rosemary, tarragon, basil, sage, mints, chives, garlic chives), cabbage, collards, broccoli, kale, kohlrabi, lettuce, okra, onions, Lagos spinach, radishes, edible podded peas, tomatoes, winged beans, sweet corn, yard-long beans and a variety of flowers.

It is easier to say what crops may give problems. We stay away from large vines that have such a large leaf area that they quickly deplete the reserve of water in the shallow bed, such as tropical pumpkins, watermelon, jicama or sweet potatoes. The shallow pool or "hybrid" methods described later may be better for these large plants; however, with sufficient volume (with either a deeper bed or fewer plants in a bed) or more frequent watering, there should not be a problem growing these larger leafed plants, letting them flow over a rooftop, down the side of the building, or over rocky soil.

Root crops require deeper beds. We have grown acceptable carrots in grass clippings, but had to make the bed about eight inches deep. The bed shrank so much during the growing season as the grass clipping decayed that the carrots stuck out of the top by an inch. Carrots grown in a 3-inch deep bed had L‑shaped roots because the bed was too shallow. Carrots grown in wood chips were distorted because of the twists and turns the taproot made to avoid wood chips become the permanent shape of the carrot. This does not seem to be a problem with radishes though.

How deep should the "shallow" bed be?

If there were no other considerations, an above-ground bed that was perhaps 9-12 inches would be about ideal. That is more than sufficient to support even tall plants like sweet corn and to encourage plenty of root growth for even the most demanding of plants. For most rooftop applications, however, the bed will probably need to be shallower than this.

A bed half that deep would have the advantages of weighing half as much and requiring half as much material to be located and moved to the roof to make the bed, then removed from the roof if plans later change. The biggest disadvantage is that the maximum amount of water the bed can store is half as much as in the one twice that deep. This means that it will need to be watered twice as often. A bed a tenth as deep would hold only one tenth as much water and need to be watered ten times as often, and so on. Ultimately one could end up with a bed of perhaps an inch or less that might need to be watered several times on hot, sunny, windy days if there was a big leaf area through which a lot of water was being transpired into the atmosphere.

Choosing the Material for the shallow bed garden

Fortunately in constructing shallow beds we can consider almost any growing medium because the weight is not much of a factor if the bed is not very deep.

At ECHO we tried beds made from whatever we could think of to see what would happen, e.g. regular soil, compost, woodchips, sand, gravel, grass clippings from lawns, corncobs. Had we been in a tropical country we would also have tried things like rice hulls, shredded coconut husks, sugar cane bagasse (what is left after the juice is squeezed from the cane), coffee pulp, etc. We found that we could grow plants in any of these, but not all were equally good. I was especially surprised at how well many vegetables did in a shallow bed of grass clippings.

Professor B. D. Cotton, an ECHO volunteer who spends most of his time in his native England, tells me that in England and other European countries it is very common to grow vegetables in plastic "grow bags" filled with well-rotted manure, peat, compost or some commercial potting mix. These would fit into the category that I would call shallow beds.

You should experiment with mixtures. When possible, if compost is not available, we like to use a mixture of different kinds of organic matter (dead plants) and perhaps inorganic materials. Mixtures are especially good because you have more flexibility to create the kind of environment that roots like. It is also likely that if a needed nutrient is not released as one component begins to decay, it will be by another. You can include many common garbage items.

A bed made from grass clippings and cola cans turned out to have some special advantages. Such a bed can be several inches deep but still not too heavy, enabling larger vegetable plants to grow without special support. This bed is constructed using approximately 40% by volume cola cans (with slits cut into the sides so roots can enter that well-aerated and hopefully humid interior). The other 60% is grass clippings mixed between and placed on top of the cans. Ordinary garden fertilizers may be added if necessary. We added the cola cans when we found that making a grass clipping bed deep enough to support taller vegetables heated up for good plant growth due to the rapid decay process. Also due to rapid decay, the beds shrank into a rather shallow bed that became so dense that the roots could not get enough air. A variety of okra that grows to perhaps four feet tall produced well in the grass clipping and cola can bed without any special means of support. We have not tried this again. If you experiment with this technique, let us know what happens. An alternative to cola cans might be pieces of coconut husk placed positioned so that there were air spaces under each piece.

A bed made from a 2-5 inch layer of weeds packed closely together and covered with perhaps a couple inches of grass clippings or (better) compost from a previous bed works well. A 6-8 inch deep bed for corn/maize was made in this way. A benefit to placing weeds on the bottom (rather than grass clippings, for example) is that there are more air spaces between the weeds, which is better for roots.

Once you have settled on the materials for garden construction, the available fertilizers, and the vegetables to be grown, you will be able to develop straightforward, detailed instructions for your unique system of shallow bed gardening. That is what you will pass on to new gardeners in your community.

Constructing the Shallow Bed.

No Container is Necessary. An important factor that makes these beds inexpensive is that no container is necessary. Depending on the material used, sides may not even be needed, especially if mulch is placed on top or at least on the edges of the bed. Sides are only necessary if the garden is placed on a platform or table of some sort where the garden extends right to the edge of the structure, or where appearance is important. We have had a lot of heavy rains and strong winds over the years, but the only bed that gave us a serious erosion problem was one in which we used a large amount of silt from the bottom of a fishpond.

The Shape of the Bed. The shape of shallow beds is determined by the same considerations that one uses in making raised beds. They can be of any length, but a break for a path every 8-12 feet is helpful. They should be just wide enough (4-5 feet) so that a person can reach to the middle of the bed from either side. Thought should also be given to maximum use of space. A path down the length of the rooftop with beds and aisles going off to either side might be the most efficient.

Begin with a sheet of plastic. If a sheet of plastic is available, we like to place it on the roof surface and then build the garden on top of the plastic. People are understandably hesitant to place the garden bed directly on the roof surface because of the danger of damage to the roof. Roots might grow into any cracks that might be in the cement and eventually make them larger. Placing the garden directly onto a cement rooftop might cause minor discoloration. The plastic should eliminate both problems. If there are substantial cracks already in the cement roof, water might seep through to the ceiling below. The plastic should minimize but not eliminate this possibility. No doubt it will always be moist under the plastic, but that is less of a worry than a considerable supply of water and roots in direct contact with the roof.

Pat Lahr found that the roof surface stayed constantly wet under the plastic, though there was not a lot of water. Without plastic the roots would occasionally dry the beds and hence the roof surface under the beds. Whether there is any benefit to having occasional dry surfaces is doubtful and the risk of roots growing into cracks would seem to be more of a problem.

Starting with a sheet of plastic where the garden is on top of the ground keeps the growing medium and roots completely isolated from the soil and any diseases or pests that inhabit the soil. It let's you avoid any chemical imbalances in the soil, e.g. highly acidic or alkaline soils, heavy clay, aluminum toxicity etc. It keeps out invading tree roots as long as the garden bed itself stops just short of the edge of the plastic sheet.

Choose materials to make the bed. Look for materials that are light weight and easily obtained at little or no cost, such as wood chips, rice hulls, sugar cane bagasse, or grass clippings that have spent several weeks in a pile (more on grass clippings below). You can place pulled weeds in the bed as long as they are covered with enough soil that they will not germinate. Avoid plants such as Napier grass (elephant grass) because the stems readily sprout and grow. Arrange the materials to form the bed, then thoroughly wet the bed. If you notice that water doesn't tend to adhere to the surfaces of the material you are using, as often happens if the materials are quite dried out, add a tablespoon or so of dishwashing or laundry detergent (any variety) to the watering can and pour evenly over the surface. Detergents fall under a class of compounds known scientifically as wetting agents or surfactants (surface-active-agents). They help water adhere to surfaces. This will help keep the materials constantly moist, and so hasten decay. It is also important that the particles composing the bed be easily wetted because the roots that will begin growing in the bed will get much of their moisture from the surfaces of these particles.

You can transplant seedlings directly into these beds. However, you may find that seeds do not absorb enough moisture to germinate, or tend to try out between waterings. See the section below on "Planting in the new bed."

Some special techniques for using grass clippings to make a shallow bed garden. We place the grass clippings in large piles (perhaps a meter across and a meter high) until needed. Within a few weeks the pile will have heated up and considerable decomposition will have begun. Also we believe (no data) that most pesticides that might have been on or in the grass clippings are likely to be destroyed during this time of elevated temperatures caused by high biological activity.

If the pile of grass clippings is not too old, the contents will be fluffy and moldy. When you dig into the pile spores from the molds growing in the pile may move into the air almost like smoke. (Be careful breathing these spores. I have developed quite an allergy to the mold, though only one other staff member at ECHO has reacted to it.) If you leave it too long after it reaches this fluffy stage, the pile shrinks and becomes dense and wet and difficult to work with.

Make the initial bed of grass clippings deeper than what you want to end up with, to allow for shrinkage during the initial preparation and continuing as the bed decays. It might take a foot of fluffy grass clippings to end up with one to two inches after the first cropping season.

Thoroughly wet the pile. Often the clippings do not want to absorb water--even after adding a lot of water, the clippings half an inch below the surface may be dry. If that happens, wet the pile with detergent in the watering can as previously described. While adding water, walk over the bed to compress the fluffy grass as much as possible. We want to end up with plenty of air space, but we also need the bed to be sufficiently dense to have ample moisture in the vicinity of the seed and roots.

Add fertilizer. We add an ordinary garden fertilizer and dolomitic limestone. If it is 10/10/10 fertilizer, we add 5 pounds per 100 square feet. We did not arrive at this amount by careful experiments. It works, but you can do you own experimentation to see if less would work or if more would be better for the materials you are using to make the bed.

The numbers refer to the percent of nitrogen, phosphorous and potassium, respectively. If it is 5/5/5, we add twice that amount, etc. Neither the exact numbers nor the exact amounts are that important. There are many other fertilizer formulations on the market. You might only be able to get something like 8/6/10, for example. Don't worry about it. Just avoid extremes like 36/10/10, a very high nitrogen fertilizer used for lawns, or something like 10/0/10 which would be a special purpose formulation completely lacking in phosphorous.

We always use fertilizer with micronutrients (that is elements needed only in minor amounts). If you cannot find that kind of fertilizer but are making the bed from organic materials, the micronutrients that will soon be released by the decaying organic material may be sufficient. One can often buy micronutrient formulations separately and inexpensively. These would be used in small amounts, following directions for a regular garden. Added micronutrients are a must with any system that is not based on organic matter, e.g. sand or gravel.

A quick way of providing these micronutrients, if they are not contained in the fertilizer, is to apply some manure or to water the garden with a manure tea made by soaking a bag of manure in a barrel of water for a few weeks. However, the manure or manure tea may not have an ideal ratio of the each micronutrient, and might even be totally lacking in one or more. If that happens the plant will develop symptoms of micronutrient deficiencies. Check on the web or in a library to learn what symptoms the deficiencies of various micronutrient look like. Often they involve discolorations of various kinds, e.g. yellow or white leaves, reddish color in veins, etc. You may be able to identify what particular nutrient is lacking and overcome the problem by adding only that one nutrient.

What else can be used to make the bed? Two Examples. Jeff McManus wrote from Bangladesh after reading about rooftop techniques in ECHO Development Notes to tell about how local gardeners grow vegetables in beds on top of the ground made from water hyacinth. Water hyacinth (Eichhornia crassipes) is one of the most prolific plants on earth. This floating weed chokes waterways around the world. He wrote that people in Bangladesh clear their ponds and rivers of the floating plants and pile them on the banks. They plant vegetables in the water hyacinth piles and these mounds become "floating gardens" in the monsoon season. The McManus family grows lettuce, papayas, tomatoes, and very productive roses in boxes filled with water hyacinth harvested from nearby nutrient-rich waters. They chop the plants into small pieces, let them compost for two weeks with daily turning, and plant directly in the compost. McManus mixes the compost with a little manure and some wood shavings, but does not add extra fertilizer, since the water hyacinth is an efficient collector of nutrients. The spongy plants hold a lot of water, so very little watering is needed for awhile. The box gardens work best with fresh material; reused compost seems to promote diseases in the plants.

ECHO collaborated in the 1990's with a US NGO, Center for Citizen Initiatives, and Russian colleagues to show what could be done in rooftop gardening in St. Petersburg, Russia. The "shallow bed" gardens we used there were bags of peat that had been used for a few years in a large greenhouse complex. They replace the bags periodically to ensure that they get the highest possible greenhouse yields, so the bags and peat only cost US$0.50. We cut the tops off, added dolomite and fertilizer with micronutrients and had very successful gardens. We also found a useful ingredient to incorporate into rooftop beds that is manufactured in Russia for insulation in buildings but is also used by nurserymen. This man-made product reminded me of lightweight volcanic rock, except that the largest particles were no more than half the size of a pea. Their name for it is "keramzit." I have seen a similar product in the USA, perhaps manufactured for growing orchids.

I am very interested in hearing your experiences with planting mixes or anything else you try that is relevant to rooftop or above-ground gardening. ECHO plans to have a special rooftop gardening location on our website where we will post updates and share highlights from your experiences in gardening "on the last frontier."

Planting in the new bed.

Planting seeds or transplants into shallow bed gardens made of compost is done as in any other garden. Planting directly into beds of organic material that has not yet decomposed requires some special techniques. Larger seeds like peas or beans can usually be planted directly if the medium is made of a material that packs closely enough together to remain moist most of the day and make close contact with the seed to keep it wet. Seeds must be deep enough into the medium to remain moist but shallow enough to be able to grow to the surface after germination. The top inch or so of many materials, e.g. nearly fresh grass clippings, tend to dry out. You may need to water a few times each day until they germinate. We have also had the opposite problem with older, matted grass clippings, which stayed too wet.

Smaller seeds, like carrots, require compost or soil or something of very similar texture to get started. You can cover the entire bed with compost or soil or just form a 1-2 inch deep trench in the packed down grass clippings, fill it with compost or soil, and plant in this trench. Even this small amount of compost will provide an environment for the seed and initial roots that is just like they would experience in any garden. A useful technique for germinating carrots in any garden is to place a board on top of the row. This ensures that the top centimeter of the soil remains moist. Look under the board daily until you see the first seeds germinating, then remove it.

Transplanting likewise can demand special care if the medium is not similar in texture to soil. If the bed is made of undecayed plant material that does not pack well, we either cover the top with 1-2 inches of soil/compost or make a small hole, insert the transplant, and fill in around it with several handfuls of compost or soil.

Keep a close watch on the appearance of the vegetables. At the first sign of nutrient deficiency, add a bit more fertilizer. With high-nitrogen materials like grass clippings, this may only need to be done once or twice, or not at all. With low nitrogen materials like wood chips or straw it will be necessary to add fertilizer frequently. A small amount of solid fertilizer can be sprinkled around the plants, taking care not to get it in direct contact with leaves or stems. Our wood chip gardens produce best if they are watered every other day with a solution of soluble fertilizer or manure tea. Most soluble fertilizers are made to pour directly on the leaves (some nutrients can be absorbed through the leaves of some plants). This is especially helpful if a deficiency has already appeared. If possible have a spray bottle on hand filled with a soluble fertilizer. Use it as "medicine" to spray plants when any deficiency appears. Do NOT spray manure tea onto the plants because they may contain disease microorganisms!

Refurbishing the Shallow Bed: Subsequent Seasons.

You may be surprised at two things: (1) how quickly the depth of the bed drops as the material turns to compost and (2) how quickly beautiful compost is formed. If there is no soil in the beds, just organic material, the material eventually turns deep black and may eventually looks something like peat. The bed must be refurbished after harvest whenever it has shrunk to less than the desired depth or has become so dense that it holds too much water and too little air. Alternatively, the bed can be recycled: dismantled and the compost which has formed in it used as the top layer in constructing new beds.

If the bed is still deep enough for another growing season, the only refurbishing needed is to apply fertilizer. Much of the bed, depending on its original composition, may by now have been converted to compost. The bed should not need as much fertilizer as it did when it was first constructed and planted and possibly may not need any addition of fertilizer, depending upon whether the now-decayed organic materials have turned into a good compost that provides all the nutrients needed for healthy plants. This is not a delicate system, like hydroponics, with exacting fertilizer requirements. I trust that any frustration at not finding rigorous details on the amount of fertilizer will be more than compensated by having a bed that allows some flexibility. More fertilizer will be needed if you have heavy rains that leach away nutrients. Learn what each of the vegetables or flowers that you grow look like when they are well grown, then watch for clues about what they may need from time to time. You may be able to find pictures on the Internet of what various nutrient deficiency symptoms look like for the particular plants you are growing

If the original organic material has completely turned to compost, then within one or two growing seasons the bed should be remade. The new bed will be easier to make than it was to make the original bed, because we are now starting with a considerable amount of compost. Rather than layering new organic material, e.g. grass clippings, on top of the bed, it is best to remove the composted material, layer the new undecayed material onto the place where you want the bed, then place the remains of the old bed back on top. We add some fertilizer (less than with a totally new bed) and water.

There are two reasons to refurbish in this way. First, the older material can become so dense that, if left at the bottom of the bed, aeration might be poor. This is not a problem when it is placed on top of the less compact fresh organic material. Second, it is much easier to plant into the composted material than it would be into the fresh material.

How Much Does a Shallow Bed Weigh?

We place great emphasis on developing very lightweight beds for rooftop applications. That is why we normally do not use soil and try to keep the depth to no more than three inches. Individual soil particles typically weigh approximately 2.75 times as much as an equal volume of water. There are spaces between the tiny soil particles, however, which can account for up to 50% of the volume of a good garden soil. It is the worst case (heaviest soil) that concerns us in considering any possible danger to the roof, so we will consider the weight after a drenching rain and assume that every space is filled with water. Such saturated soil weighs 1.9 times as much as an equal volume of water. Individual particles of organic matter typically weigh slightly more than water (1.1 to 1.4 times) and the spaces between them are much more than 50% of volume. So in a worse case, i.e. a totally flooded bed of fully decayed, compact organic matter, the weight would be at most 1.2 times that of water. In most cases, the weight will be almost the same as an equal volume of water.

The weight can still be considerable. This table compares the weight of 3" (7.6 cm) and 8" (20.3 cm) deep beds that are 4 feet wide and 8 feet long (1.22 m x 2.44 m), one with soil and one with well decomposed organic matter, both fully saturated with water.

Maximum weights of four rooftop gardens, each 4x8 feet (1.2x2.4 meters)

Depth	Weight
	well-decomposed organic matter	good garden soil
3"	598 lbs (272 kg)	947 lbs (430 kg)
8"	1,595 lbs (725 kg)	2,552 lbs (1,147 kg)

At ECHO we usually have no sides to the gardens in order to keep material cost to a minimum. If cement block sides were used, the weight and cost would be considerably greater. Based on what we have seen, people are often more cautious than necessary. As you can see in the pictures, some gardeners in Haiti are using concrete blocks for sides and depths equal to the 8 inches of the blocks. No problems have been reported although I saw perhaps only half a dozen gardens. If there is any doubt about safety, remember to put the heaviest items (like a barrel of water) directly over walls.

A Special Adaptation of Shallow Beds: the "Tire Garden"

Construction is simple and elegant and may cost nothing. What we want to end up with is a portable growing container that can be placed anywhere and moved if necessary. The bottom that will hold the potting mix in the container will be a piece of ordinary plastic, e.g. like painters use to keep paint from splattering on the floor. The plastic could even be a thick garbage bag. Lay a tire flat on the ground. Note that the top rim is a mirror image of the bottom rim. With a knife or machete or large-tooth saw, cut off the top rim. Place a piece of plastic inside the tire on the bottom rim, large enough so that an inch or two of plastic extends up along the walls of the tire. Now turn the top rim that you just cut off upside down and place it inside the tire where it will fit like a glove against the bottom rim, holding the plastic firmly in place. If the plastic is trimmed to near the bottom of the tire, the garden will essentially be a portable "shallow bed garden." If the plastic is left so that a pool of water is formed, it will be more like the "shallow bed in a shallow pool garden" that we discuss later.

Any suitable soil, compost or potting mix can be used to fill the tire. You will need to judge if fertilizer is needed and when it should be applied, based on what you use for a planting medium and how plants are growing. At ECHO we sometimes place an empty flower pot or a PVC pipe in the center so that we can see how much (if any) water is standing in the bottom and so judge when to water. More often though, we make sure the plastic does not retain too much standing water and we treat it as a straightforward shallow-bed container garden. We usually incorporate something with a lot of air space into the planting medium to reduce the weight of the tire garden. The same amount of planting mix goes farther because of the air spaces. At ECHO we sometimes use cola cans with holes cut into the sides so roots can penetrate the can. In the tropics one could use coconut husks or possibly pine cones or very light weight volcanic rocks.

The tire gardens are the "jeep" of above-ground gardening methods: portable gardens that can literally go almost anywhere. I met with some potential rooftop gardeners in El Salvador. After showing how to construct the garden, it was fun watching as their imaginations led them to move a tire garden to unlikely places for a garden: on a flat rock, on a steep hillside supported on the downhill side with rocks, on the roots under a tree, on a pile of rock or a junked car.

If there is danger of theft or damage by chickens and goats, the tire can be placed on top of something, even along the edge of the tin roof of a shanty. People often put pieces of iron or other heavy items on the roof of a shanty to keep it from blowing away. They do this because there is not enough framing to adequately secure the corrugated roofing. A few tire gardens might even help the roof stay in place.

The tire garden can be moved if necessary as seasons change. A growing vegetable may need to be moved where there is more sun or less sun or where there is less wind. If the garden is on a rooftop, it can be placed on sticks or stones so that air can circulate underneath, keeping the roof surface dry. If gardeners themselves have to move, they can take their gardens and their improved soil to their new home. When ECHO staff member Dan Holcombe (in picture) returned from his vacation to the church in Mexico City where he had a rooftop tire garden, he found it flourishing-on a different building. The church moved it in order to add a second story to their original building. Workers simply parked a pickup truck by the first building; the tires were handed down to men standing in the truck and then relocated to the roof of a second building.

Advantages of "tire gardens"

There is less potential to damage the roof. Though the shallow bed garden can be constructed easily on any sturdy flat rooftop, there are situations where this causes concern. Some cement rooftops may have small cracks. There is fear that because of the constant moisture and fertilizer beneath the garden, roots will begin to grow into the cracks. Nature shows us how this process can even break rocks as the roots expand.

For this reason, when presented with a range of possible garden construction methods for rooftops, many citizens in developing countries will chose the tire garden. The tire can be placed on three or four sticks, rocks or other items so that there is considerable air space beneath the tire. This allows the air to circulate and eliminates constant dampness. Also there is no contact of the roots with the rooftop itself.

Summary: Shallow Bed Gardens

In summary, Shallow Bed gardens or hybrid gardens that include a shallow bed, are the most foolproof of the methods we have tried. Especially when made of compost, it differs little from gardening in the soil. The main differences are its need for daily watering and that it's limited depth permits only shallow roots (to which plants show a surprising ability to adapt). The ability to grow vegetables in fresh organic material while it is being turned to compost is a very attractive feature of the method.

One might be tempted to think that the need to water frequently would not be a problem if the garden belonged to an unemployed person who presumably had a lot of time on his or her hands. However, the limited feedback I have received suggests that human nature and demands on time even of an unemployed person are such that the likelihood of a gardening project succeeding is much greater if watering needs are less frequent than may be required for shallow bed gardening. That is especially true when the garden is located on a rooftop that may be difficult to climb on to.

This leads us to the benefits of the next gardening method that we call "wick gardens."