Raised Bed Vegetable Gardens – How and Why

 With all the interest in building raised beds the last few years, perhaps we need to look at the whys and wherefores of raised bed vegetable gardens.

What is a raised bed?

A raised bed is any planting area that has been worked so the planting surface is 4″ or higher than the original soil level. Raised beds are usually 2′ to 5′ wide and any length desired. As you can see below, the deeply prepared soil of a raised bed gives the plants’ roots more room to grow in.

The narrow beds have a number of advantages. Garden jobs such as weeding, controlling insects, and harvest are all made easier with a reachable garden’ These jobs can be performed from the pathways between beds and the gardener never needs to step on the garden bed.

Staying off the bed is important. Plants are short lived and need healthy roots to produce fruit. Just walking next to a plant in soft earth can shear of a large amount of root. Compacted soil also has fewer voids for water and air penetration, and root systems use both of these to digest and transport minerals.

To get the most out of a limited space, these raised beds need to be spaced carefully. Narrow beds permit easier monitoring of your garden for pests, but you need to allow room to move among and care for the plants. Access and spacing are important, and once established, raised beds help keep the areas defined. You concentrate the enriched soil and compost in the beds, not of the pathways. The aisle areas can be mulched to suppress weeds, and a visually attractive, low maintenance, high-production garden is the result.

Bed preparation is the most important part of an intensive gardening effort. Once it is properly prepared, your garden soil will be loose, with good texture (or structure), and rich in air, water and nutrients. This deep preparation promotes deep root growth and greater root mass, and it also puts the roots into a deeper, more stable moisture zone. We’ll cover the subject of soil preparation in a later section.

The deep bed preparation and enrichment that is part of an intensive system also allows us to grow more in less space. We can plant as close as practical without reducing the amount of food each plant can absorb. This close spacing allows us create micro-climates where plants shade the top soil and reduce soil temperatures, and reduce evaporation even more.
 

How to Make a Raised Bed

Making a raised bed garden isn’t rocket science and it isn’t a superman contest. It is fairly simple, low cost but time consuming the first time you do it. The initial work to create a deep, prepared bed does pay off in succeeding years. You have healthier soil, healthier plants and a more productive garden. Let’s talk about the steps…

1) Loosening and cleaning up the soil.

The first order of business is to break up the (normally) compacted earth. Ideally, you will use a garden fork to break up the compacted soil and any ‘hard pan’ that may be present. The idea is not to turn the soil over or totally stir up the soil, but rather to break up the compacted area you want to give to your roots. Remove any weeds during this step.

Add any soil amendments (soil conditioners that are mixed into the soil are called amendments) to the broken up surface of the soil, and fork the ground up again. As before, the idea is to not turn cover the soil, but simple let the amendments to filter through the loosened dirt. Our goal is to create a gradual change from enriched soil down to the subsoil. A sudden change in soil composition will form a barrier to water movement in the ground. Biological agents in the soil will continue this process in the future.

The soil you have to work with dictates the amendments. Heavy clay or loose sandy soils require more amendment than loam, but all soils benefit from organic amendments. Organic matter in the soil acts as a sponge that holds water, air and dissolved nutrients, ready to be absorbed by the plant root systems.

NB: Steer manure is classified as a soil amendment, not as a fertilizer. To be a legal ‘fertilizer’, there needs to be a minimum guaranteed amount of nitrogen in the product, and steer manure fails the test. It is a good soil conditioner, though.

2) Double-digging

(Note: here is a point on which our Master Gardener instructor and I, plus most of the gardeners in the class, disagreed. Dr. Downer was not convinced it was worthwhile, but his viewpoint was perhaps for larger scale agriculture while ours was the home garden. My opinion is backed up by the Peace Corps, but take my advice with a grain of salt… )

Double digging is discussed in this video, Home vegetable Gardening, Part 1, starting at about minute 19… (http://www.uctv.tv/search-details.aspx?showID=6675&subject=ag)

Double digging is the basic step for true deep bed prep. The idea is to remove a shovel-depth of soil and set it aside, add amendments to the bottom of the resulting hole, and fork the amendments into the exposed subsoil as we talked about above. Go as deep as reasonable. Push and pull back on the handle of your fork as you go down to break up and loosen the soil. Move the fork or spade back a few inches and repeat until your trench is complete.

In a narrow bed, move from side to side, then back a few inches and do an entire bed in one pass. On wider beds, it may be easier to make one long, narrow trench, then make another pass next to it, and move soil from side to side as you double dig. In either case, the idea is to move the top layer of earth, go down another dig deeper, then add the top of the next section to what you just broke up.

Replace the soil you removed previously and mix in amendments as you go, then fork this upper layer up. You now have a loosened area 16″ to 24″ deep with passages through the soil for the exchange of air and water. This deeper cultivation will promote deeper moisture and root penetration. In really tough ground, you may not be able to go as deep as you like, but the next time you work up the soil, it will have been softened by the probing plant roots.

The final step is to rake out the resulting bed, moisten the soil, and let it rest for a day while the soil moisture stabilizes. You can now add your drip system and cover the soil with a weed prohibiting layer of organic mulch.
 

Shaping the beds

If you are lucky enough to have enclosed beds, you can skip this section as your beds are already raised and shaped.

If you don’t have an enclosed bed, you just need to shape the beds. I have grown for many years without sideboards and it works quite well. The earth is removed from the aisles and placed on the bed. A string or board is handy to keep a straight edge.

My goal is to form a mound with an aisle on either side, rake that flat, and firm up the loose edges. Re-raking the tops and edges a couple of times give you a nice, flat, truncated pyramid with flat top and 45 degree sloping sides. Since we encourage drip irrigation, a flat top works well; if you do this at home and must irrigate with a hose or sprinkler system, a lip along the edges of the beds will prevent run off and erosion.

I firm the sides up with a shovel or my boot, and the ground will pretty well hold its shape through a growing season. I also add a 4″ deep layer of wood chip mulching the aisles between beds and this helps retain the edges. Finish off as above with your drip lines and mulch.
 

Growing on Raised Beds

Why do we go to all the work of double digging, enriching and piling up our garden beds? For one thing, raised beds produce twice as much as the same space devoted to tradition row culture, and twice as much for the amount of water used. Add our protective, insulating layer of mulch to the bed surface and the water requirements drop to 25% for the same amount of food.

One study done here in California confirmed that, when properly controlled and managed, intensive raised bed gardening produce 4 times the amount of food per acre using 1/2 the water used in commercial growing.

Your garden may not reach this level of production for a few years. It takes time to create the life systems that drive soil fertility, but California’s soils are generally rich in nutrients. Adding organic materials to them is all that is required to balance the soil pH and release the locked up nutrients. The only nutrient we really need to add is nitrogen which is notoriously soluble and liable to wash out of the soil. Our watering practices, application of compost, growing nitrogen-fixing plants, and other permicultural practices will supply the nitrogen we need.

High production is the result of high fertility.  Maintain your soil to maintain your garden.

Alternatives to Enclosed Raised Beds

Mankind has been growing crops in rows since the invention of the plow. Man recognized early on that most plants did better in plowed ground and when grown on top of a row.

Some crops are grown at the bottom of the rows, or ‘ditches’ (potatoes are a good example), and some crops are sown directly on essentially level ground, wheat being a notable example.

Rows created as ridges may be narrow or wide, depending on the crop. I like to make wider ridges to allow for several rows of vegetable to share the space and water system. When planting on ridges, you get most of the benefits of the raised bed garden without all the work. The down-side is that the exposed edges require more work to maintain and keep weed-free.

Planting in furrows (the ditches between the rows) should be reserved for just a few plants. Potatoes are planted at the bottom of the furrow, and as they grow, the rows and enrichments are gradually pulled up over the growing plants, leaving the last few inches of the plant tops exposed. Potatoes form at the leaf-nodes along a stem, and not from the roots. This banking up of earth allows for more tubers to form.

Planting (or ’sowing’) seeds on level ground is the way wheat has been grown for thousands of years. It has the advantage of low effort, but leaves seed exposed to birds and the elements and is rather primitive. I use this method to sow wildflowers and cover crops (nitrogen fixing ‘green manure’ crops grown during the rainy season – if we have one…). The seeds are often raked into the surface of the soil to offer some protection from predation and get them into firm contact with the soil.