A raised bed garden is a garden with the growing medium built up above the normal landscape. Raised bed gardens can be free standing, mounded upon the landscape, or enclosed. For the school garden, I recommend enclosing the garden, and with wood. Using this method, the garden will be more permanent, not erode over time, and be easier to maintain. Having a number of raised beds makes the division of labor easier, and facilitates “ownership” as youth are made responsible for a particular bed. This pattern also allows for some experimentation, planting one bed in a particular way and another in a different way or with different seeds/plants.

REASONS TO BUILD ONE

A raised bed garden:

1. allows you to start with optimal soil conditions, rather than augmenting existing soils over time so that they will support healthy growth;
2. lets you to build above poorly draining soils and high water tables that can drown roots;
3. is a controlled environment, so that you can be assured of starting your garden free of lead, construction waste (such as nails), and other contaminants;
4. can be built on any surface, even a roof or a parking lot;
5. facilitates terracing so that otherwise unproductive areas such as hillsides can flourish AND reduce soil erosion;
6. promotes root growth for higher yields;
7. is easier to work, and, with wooden sides, provides a “perch” for laborers;
8. allows intensive planting, so space is not wasted, and “weeds” are crowded out;
9. provides a barrier between itself and the surrounds, so “weed” encroachment is less;
10. keeps soils dryer, warmer, and lighter, so it can be planted earlier in the spring;
11. is already constructed for ready covering to extend the season after frosts.

MATERIALS
Native Americans used stone to build up and enclose their gardens, sometimes in the middle of streams or water canals connected to natural waterways. In this way, the water would irrigate the rood system and fish would “fertilize” the water. This was way before wetlands regulations!

Wood is easier to find and use, and it will generally be the economical choice, though lumber prices have climbed meteorically in the past few years. If you have access to cinder blocks, you can stagger the blocks with a 50% overlap and drive stakes through the openings into the ground to hold the bed sides in place. You can, if you like, plant the holes in the blocks with flowers that will attract pollinators and/or repel detrimental insects. (French marigolds and cosmos, among many others, work well for this.)

In choosing wood, treated lumber (see below) or naturally rot-resistant woods such as cedar are best. Untreated lumber can be used, but it will have to be replaced more often. But be careful about the type of treated lumber you use. Traditionally, pressure-treated lumber used CCA (chromated copper arsenate) or ACA (ammoniacal copper arsenate) as a preservative. Because of serious questions about the threat to health, the lumber industry moved to alkaline copper quaternary, which is high in copper but free of arsenic. If you use treated lumber, make sure that it is newly purchased to take advantage of these new precautions. It is best to avoid old railroad ties, as well, as they contain creosote.

DESIGN

If you are working with youth, have them design the garden. Even children as young as 9 or 10 can use graph paper, cut out rectangles, and design the lay-out. Rectangles in measurements that are multiples of 4’ (the standard industry size, so that you can reduce both sawing and waste) are easiest to build. Wooden walls should be at least 2” deep for strength, and at least 12 inches for depth. (In actuality, a 2”x4’ is not quite 2”, but is close enough.)The beds should be no more than 4’ wide, as this makes it easiest for gardeners to reach into the middle from either side. If the bed is not accessible from one side, than the depth should be no more than 2.5’. In this way, gardeners do not have to walk on or place heavy articles onto the garden surface, and this will help keep your soil lighter, and your plant roots aerated, a condition they will greatly appreciate.

Level the area first to create a flat base for the beds. To build on a hill, you will need to, of course, adjust the depth of the walls according to the slope so that the beds themselves are level. This can be done more easily with cinder blocks than wood.

You can make the bed as long as you like, though wooden beds longer than 8 feet will tend to bulge in the middle, and may come apart at the seams over time.
Most plants need at least a 6- to 12-inch rooting zone, but deeper would be better. With deep tillage, some of the rooting depth may come from soil at or below the existing grade. Beds built higher than 18 to 24 inches require retaining walls with foundations and supports.

Make pathways between raised beds wide enough for foot traffic. One foot is wide enough, unless your garden needs to be accessible to wheelchairs (see below). To conserve space, one option is to make most paths narrow, occasionally adding a wider path for access.

I have found that most people are comfortable sitting on the edges of a 2x4 frame, but, if you like, you can make seats on the corner by nailing on a triangular peace of 2x12 inch board, or whatever you used to build the bed. If you like, you can sink hollow pipes just inside the walls to anchor stakes for tomatoes or trellises for peas, pole beans, or any other vining crop (squash? cucumbers?).

MAINTAINING THE GARDEN

Soil in raised beds warms faster and dries out more quickly than soil at ground level. In spring and fall, these are good things. But in the summertime, it is not so good, and in the hottest weather, raised beds will need watering every few days. Using organic mulches, such as straw or hay, will ameliorate the problem. Soil temperatures are lower under organic mulches, less water is lost through evaporation, and, an added plus, weeds are suppressed. You will need to supplement natural rainfall during dry periods. Soaker hoses or drip irrigation may be placed directly on the bed. Overhead sprinklers or hand-held hoses can also be used, but because they wet foliage they are more likely to spread diseases.

I recommend avoiding power tillers in the beds. They are an unnecessary use of fossil fuels (or electricity usually generated from fossel fuels) and potentially destructive to the structural supports. If you are working with youth, power tillers don’t allow them to use their musclesThey also do not allow students to use their muscles, creating kinesthetic connections to the garden and building pride in accomplishment. In the fall, you can compost plant residue for reapplication the next year. You will want to add additional compost yearly. This can be done as a top-dressing in the fall, to be tilled into the soil in the spring. Tilling can be done by hand, with shovels. Over time, you will probably develop rich enough soil so that you will not need to till at all!

IF YOU NEED TO MAKE IT ACCESSIBLE

If your garden is to be a public space, you will probably want it to be accessible for people using wheelchairs. Construct raised bed walls about 2 feet high and limit the width of the bed to about 3 feet if there is to be wheelchair access on both sides of the bed.

ADA specifies, “The minimum clear width of an accessible route shall be 36 in (915 mm)…” For a person turning around the beds in the garden using a wheelchair, ADA specifies that “A 90 degree turn can be made from a 36 inch (915 mm) wide passage into another 36 inch (915 mm) passage if the depth of each leg [from turn to turn] is a minimum of 48 inches (1220 mm) on the inside dimensions of the turn.” The solution to this is a bit confusing. The width of the standard bed is 4 feet, or 48 inches, which meets the depth requirement of the leg. However, if you are building for gardening by people using wheelchairs, you will want to make the beds no wider than 3 feet or 36 inches. Therefore, to be truly compliant with ADA, you will want to move the walkway out another foot from the bed, or a total of 48”.

ADA further specifies that, “Ground and floor surfaces along accessible routes and in accessible rooms and spaces including floors, walks, ramps, stairs, and curb ramps, shall be stable, firm, slip-resistant…..” I recommend that you talk over all this over with people who use wheelchairs. Nothing brings down barriers to implementation like early inclusion!

WHAT GOES IN THE BEDS?

I recommend a mix of topsoil (50%), organic matter (about 35%), and coir or sand (15%). Peat moss is not recommended as it is not sustainably produced. Using coir, which is derived from coconut shells, opens the door to discussions about sustainability. Bogs “grow” at the rate of approximately a millimeter a year, and most of today’s existing bogs were formed at the end of the last ice age, around 9,000 years ago. 90% of coir, however, is produced in Indian and Sri Lanka, so needs to be transported (fossil fuels, again!) a tremendous distance to your garden. You can, instead, use a small amount of sand (about 10%), which will serve the purpose of increasing soil aeration and drainage. Note: As an educator, I find that such talking points are really helpful in getting the message across that sustainability is like ballroom dancing rather than playing solitaire. Ballroom dancing requires being highly conscious of your partner (Nature), and adjusting what you do to the rhythm of her/his dancing. It is a highly developed skill or art. Playing solitaire requires the dogged application of rules, and winning or losing has to do with luck, not any relationship to the cards. (I imagine there are card sharks who would disagree with me on this.)