Gardening in raised beds, a common practice before colonial times, is enjoying a resurgence of popularity among home vegetable growers. In parts of the world with greater population densities or less tillable land, gardening in beds is still the norm.
What are "Raised Beds"?
The "raised" part means that the soil level in the bed is higher than the surrounding soil, and "bed" implies a size small enough to work without actually stepping onto the bed. A bed should be no wider than 4 feet, but length can be whatever suits the site or gardener's needs. Wider beds can be subdivided into sections accessible from planks or stepping stones.
The bed does not have to be enclosed or framed, but if unframed, the use of power tillers is feasible. Framing offers several other opportunities, however; and a properly maintained bed will not need power cultivation.
Higher Yields There are many reasons for the raised bed revival, but probably the most important is more production per square foot of garden.
In a traditional home garden, good management may yield about .6 pounds of vegetables per square foot. Records of production over three years in a raised bed at Dawes Arboretum near Newark, Ohio, indicate an average of 1.24 pounds per square foot, more than double the conventional yield.
Raised beds do not require the usual space between rows because no walking is done in the bed to cultivate or harvest. Hence, vegetables are planted in beds at higher densities - ideally spaced just far enough apart to avoid crowding but close enough to shade weeds.
Improved Soil Conditions Another reason for greater production in a given space is the improvement of soil conditions. Soil compaction can reduce crop yields up to 50 percent. Water, air and roots all have difficulty moving through soil compressed by tractors, tillers or human feet. Plows, tillers or spades have been the usual answer to this problem, but gardeners can avoid the problem completely by creating beds narrow enough to work from the sides.
Soil organic matter content can be increased greatly without getting bogged down. Raised beds also help in situations where compaction is not the only culprit. Homeowners may have low spots unsuited for conventional gardens because of ponding or excessive erosion from runoff. Raised beds rise above these, with frames as a foundation. Gravity becomes an ally, not only in avoiding soggy soils but in reducing a problem common - alkaline soils. Saturated soils get a dose of lime every spring via percolation.
In a raised bed, gravity reduces percolation to a trickle from capillary action. Soil acidity can be maintained in the 5.8 to 6.8 pH range that vegetables prefer.
Better Drainage Growing plants in raised beds is a logical choice for gardeners with heavy, poorly drained soils. Raised beds permit plant roots to develop in soil held above waterlogged or compacted zones. This provides a more optimum soil environment for root growth. As beds are built up, compost or other forms of organic matter may be incorporated, further improving soil structure, drainage and nutrient-holding capacity.
Ease of Working The gardener shares some benefits from raised beds as well. The first, and most important, is the increased ease of timely planting and harvesting. Most people avoid working traditional gardens in rainy weather to avoid compaction and muddy feet. Because raised beds are designed for walking around, not in, there is no reason for mud to delay operation. Spaces between beds may be left in sod, mulched or even paved with stone or brick.
Maintenance Because plants are growing above the level of walkways, less stooping is required for weeding, watering and other chores. Intensively planted raised beds provide dense foliage cover, shading out much weed growth.
Expanded Growing Season Better drainage speeds soil warming and allows earlier spring planting. In wet seasons, soil dries out faster, permitting planting to proceed between rains.
Ease of Pest Control Pest control becomes less difficult in raised beds. If burrowing rodents are abundant, the bottom of the bed can be lined with poultry wire or hardware cloth. Rabbits and groundhogs can be discouraged by placing their favorite foods in a framed bed with a low fence. The narrow dimensions of beds even make bird netting suspended on flexible conduit frames practical. Weed control with plastic mulch can be achieved economically, as the width of the bed can be spanned by one roll.
Water Conservation The narrow dimensions of beds are advantageous for water conservation. There are several watering systems that ensure the water gets only where it is needed. Canvas soaker hoses, perforated plastic sprinkle hoses and drip-type irrigation disperse water in a long, narrow pattern well-suited to beds. They also reduce disease by directing water to the soil instead of wetting leaf surfaces as with overhead irrigation.
For those who are producing for more than just family or friends, raised beds may not be the answer. Certain vegetables, such as squash, melons and sweet corn might do as well on ground level due to the extensive space they shade.
Using Difficult Sites Raised beds make gardening possible on sites where growing plants would otherwise be impossible. Rooftop gardens and raised beds on top of solid rock are examples. Terraced raised beds turn hillsides into productive growing areas while reducing soil erosion potential.
The Psychological Affect One of the banes of the traditional row-type garden is that everyone makes it too big. And by the middle of the growing season, the watering and the weeding have become major chores.
Gardening is a true pleasure with raised beds. First, you can produce the same amount of food with approximately half the garden area. And more importantly by using raised beds you have broken the gardening tasks (like weeding) into smaller work units. You feel that you are weeding an individual bed instead of the entire garden. Success is yours in small packets and you will have a tendency to continue the whole gardening season.
Types of Raised Beds Temporary raised beds The temporary raised bed is nothing more than marking out the boundaries of the desired bed and till the enclosed soil. Once prepared, the bed of loosened soil remains slightly raised above the surrounding areas. Adding compost or other organic matter to the bed raised it even farther. Keep off the tilled beds. Beds will flatten over time due to settling of the soil and gradual erosion of the soil into the walkways or down-hill directions due the fact that the bed is not contained within a frame.
Shallow bed This is a 3 to 6 inch bed of compost. No soil is used at all thus keeping the weight to a minimum which makes it suitable for rooftop gardens. If compost is not available, plants can be successfully grown in fresh organic matter of many kinds. Such bed 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.
Tire bed/garden This is the ultimate in raised bed gardening... very cheap to construct, can be used just about everywhere and it's portable. The "garden" is made from an old tire and a small sheet of plastic film (for example, a garbage bag). Construction is simple and elegant. Lay a tire flat on the ground. Note that the top rim is a mirror image of the bottom rim. With a knife 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 stands up along the walls of the tire. Now turn the top rim that has been cut off upside down. It fits like a lock on the bottom rim, holding the plastic firmly in place. Fill with growing medium, usually starting with lightweight, airy materials on the bottom and soil or compost on the surface. If the plastic is trimmed to near the bottom of the tire, the garden will essentially be a portable "shallow bed garden."
Permanent raised beds If your situation allows it, you will find that permanent raised beds are the most desirable way to go. Framed or walled raised beds may be used for vegetable gardens. Although there will be initial expense and labor in constructing, the finished product should last for years and years. Beside controlling erosion better than temporary beds, framed beds permit deep soil amendment.
Frame/Wall Construction Materials Many types of materials may be used to construct the frame/wall of raised beds. The most commonly used are: naturally rot-resistant lumber such as redwood or cedar I. treated landscape timbers II. used railroad ties III. bricks IV. concrete blocks V. stones VI. tires Avoid the use of creosote or pentachlorophenol-treated lumber for bed frames. These chemicals can leach out and injure plants. Be aware that the cement in block will raise soil pH over time.
Which material to use is entire up to you.
Personally I have tried all of them and have finally settled on using concrete
blocks. The concrete blocks are available almost everywhere, relatively
cheap, consistent in size (I use only the 8x8x16 inch blocks), last forever,
lend themselves to endless designs, easy to add an additional row to make
deeper beds, and reasonably portable.
There has been concerns raised about the safety
of using treated timbers in food gardens. Pressure-treated (PT) lumber
used chromated copper arsenate (CCA) or ammoniacal cooper arsenate (ACA)
as the preservative.
Studies done by Texas A&M Agricultural Extension Service showed no movement of these compounds into the surrounding soil. Pressure-treated lumber has no effect on plant growth or food safety.
If you are uncertain about the safety of treated lumber, place a heavy plastic liner between the treated lumber and soil used for growing plants to prevent direct contact of plant roots with the treated lumber. Be careful not to tear the plastic when tilling the bed. It's your call... personally I wouldn't take the chance. There have just been too many instances where the scientists have proclaimed something completely harmless only to reverse themselves a few years later. Why take the chance when it is completely a moot point by using other materials? Creosote, which is used to treat railroad ties, can cause injury or death to plants that come into direct contact with it. After a few years the effect diminished. Old, discarded ties usually do not injure plants.
However, injury may occur if ties are still oozing black, sticky creosote or smell intensely. With these potential problems, plus the great weight of the ties, I personally stopped using them years ago. Construction Tips There are only a few guidelines to remember in raised bed construction: Keep the beds narrow and match their length to the site and the watering system. Typically, the beds are laid out in a rectangular pattern. A convenient width for the bed is 4 feet. At this width, the center of the bed is easily accessible from either side. Using the concrete blocks this is three blocks wide. If the bed is accessible only from one side, limit the with to about 3 feet. Most gardeners find it uncomfortable to reach farther than 3 feet to tend the bed.
VII. The length of a raised bed is not critical. It is limited only by the dimension of the growing area. It is wise, however, to break up long distances into shorter beds. To prevent soil compaction, foot traffic and garden equipment such as wheelbarrows should not be permitted to go through the raised beds. For example, instead of building one long 50-foot bed, construct two 24-foot long beds with a 2-foot walkway between them. This will save you many steps, frustration of having to walk all the way around the long beds, and will end the temptation of jumping across the bed.
The depth of your raised beds is to a great extent up to your discretion. Most plants need at least a 6 to 12-inch rooting zone, but deeper would be better. I build only with concrete blocks and have two different designs... one is 8 inches deep (a single block) and the other is 16-inches deep (two layers of blocks). Using this approach I have beds for shallow rooted crops and then I also have beds for crops such as carrots which can make use of 16 inches of prime growing soil.
Note that the blocks have been offset on the upper layer, thus giving greater stability as I do not use mortar, they are "dry stacked". The length is flexible and can be any length you wish (personally I limit the length to approximately 24 feet per bed). I also plant small plants into some of the "holes" of the blocks... plants for pest control like marigolds. Other "holes" have short pieces of 1-1/2 inch diameter PVC pipe set in concrete to act as support posts for season-extending cold frames and for summer trellises for vine crops such as cucumbers or A north-south orientation is best for low-growing crops, allowing direct sunlight to both sides of the bed. Beds that will contain taller crops such as pole beans, trellised peas or caged tomatoes might do better on an east-west axis. Thus, lower-growing crops could be planted on the south side of the bed and still get full sun.
VIII. Even if the soil is heavy clay, at least one-fifth of the volume of the bed's root zone should consist of existing soil. There are a lot of good minerals in clay and by loosening it up with compost or peat and coarse sand, it will make a good growing medium. Add a little garden fertilizer and test the soil after the first crop year.
As a professional nurseryman for over two decades, I have experimented with many different soil mixes. Here is the "formula" for the soilless mix that I use in my own raised beds.
5 parts by volume of coarse sand, IX. 3 parts by volume of sawdust (I use aspen sawdust from a local saw mill... avoid high resin-type sawdust such as pine), X. 2 parts by volume of peat moss, XI. 1 part by volume of aged manure, XII. 1 part by volume of compost I use a cement mixer to blend the above materials. If the materials are too dry, add a couple gallons of water to the mix.
As a work saver, I strongly recommend that you put down a layer of weed barrier material (available at most garden centers) beneath both the raised bed and the paths. I mulch my paths with about 3 inches of aspen chips. This way the paths are never muddy and I can get into the garden to work at any time...and it makes for a really neat looking garden.
XIII. Organic material constantly decomposes and disappears, so replenish your raised beds with compost regularly. Cover the beds with 2 inches of leaves, compost or a cover crop like crimson clover each autumn or early spring where the climate is harsh. Turn under cover crop or leaves in spring, and plant again the next year.
Raised bed possibilities are endless. Beds elevated 2 feet or more offer the promise of gardening without bending and can have benches built on the sides for even more convenience. Because a bed warms up quicker than the ground, it can easily double as a cold frame by covering it with a lightweight clear plastic cover. Imagine being able to start plants early in beds with covers and never having to transplant them! Supports for poles, cages and trellises can be mounted to the frame for longer life and ease of installation and removal.
Many of the same principles used in raised beds are being adopted on a larger scale in field crops. Ridge tillage, solid seeding and controlled traffic are all new techniques designed to deal with drainage, weed or compaction problems and to increase productivity. Traditional gardens with orderly rows on wide intervals have mimicked their larger farm counterparts for years. Maybe it is time for them to change their role model to the new farm, or the ancient garden.