Thinking of building a raised bed this summer? There are several reasons why you might choose to build an above-ground structure in which to grow vegetables, herbs, and other plants:
-To avoid toxics in the soil such as lead or arsenic
-To avoid the drainage problems that heavily impacted or clay soils can present
-To reduce back strain. Raised beds can be built at a good height for a wheelchair.
-To add an aesthetically pleasing feature in the garden
-To easily add nutrients to the soil
-To improve yields by reducing the amount the soil is walked on and compacted
-To clearly separate the growing area from the pathways for easier weed control
-To garden in paved spaces
In this article, we will give you information for choosing the best building materials, recommendations for raised bed linings and depth, and a list of our favorite resources for plans.
When choosing materials for your raised bed, consider durability, toxicity, environmental impact, affordability, aesthetics, maintenance requirements, and how permanent or portable you want your raised bed to be.
Reused materials are the most affordable and have low environmental impact because they don’t require forestry, mining, manufacture, or long-distance transport. Find free or low-cost materials on Craigslist or Freecycle, at your local salvage yard, or in your own backyard! The Ecology Center’s Bay Area EcoDirectory contains resource listings, including local salvage yards.
Building with low-tech methods usually saves money and minimizes environment impact. A low-tech method that permaculturalists favor involves building a mound of soil and straw directly on top of the existing ground, without a structure to contain it. This method builds nutrients and adds tilth to impacted land, but if your aim is to avoid toxics, it might not be sufficient.
|Soil/straw mound||Very low||None||Low||Free to low||Look into permaculture methods|
(broken concrete pieces)
|Salvaged material has low impact||See note||High||Free to low||Concrete less than one year old can leach lime, affecting soil pH|
|Low if reused, medium if new||See note||High||Low when reused||Lining the bed can prevent lime from entering soil|
|Wood logs||Low if scrap||None||Medium to high||Free to low|
|Bamboo||Especially low when grown domestically||None||Medium||Moderate|
|Metal siding||Low if reused||Low||Medium to high||Low to high||Line with fabric to delay rusting|
|Naturally treated reused wood||Low||Low. See note.||Medium to high||Moderate||Use nontoxic sealant on untreated wood (Flaxseed oil or wax)|
|Plywood, pressboard and painted woods||High if new, lower if reuse||Very high||Medium||Moderate||These materials are never recommended due to toxic components|
|Pressure treated wood||High||Can be very high||High||High||See section below on pressure treated wood|
|Redwood or cedar (new)||Lower if FSC certified or is very high||Low||High||High|
|Stone (new)||Can be high. See note.||Low||High||High||Impact depends on where the stone was quarried, weight and transport.|
|Plastic lumber||Medium (does not decompose or recycle)||Moderate||High||High||HDPE and LDPE are best plastic choices|
|Railroad ties||Hazardous||Very high||High||Moderate||Creosote, not recommended for use.|
Lining a Raised Bed
Raised beds can be lined to make them more durable and to avoid the leaching of toxic substances into the soil. A lining can make an existing raised bed safer, but if your raised bed is made of creosote railroad ties or arsenic treated wood, it’s best to remove the wood from the yard to prevent continued migration of the toxics. For lining, use landscape fabric found at garden supply stores or cloth fabric from clothing. Avoid non-porous plastic, as it can retain too much moisture and discourage beneficial critters. Lining the bed with hardware cloth will help keep out gophers and moles. Stapling gopher/rat mesh to the bottom of the bed will thwart those pests.
Treated and Untreated Wood
Some pressure treated wood is toxic, while some is not. How can you tell which is which?
The most toxic treated wood has been pressure treated with chromated copper arsenate (CCA), which can be identified by staple-like indentations and a greenish tint, which can fade. Not all CCA wood has these identifiers. Wood that is pressure treated with Alkaline Copper Quartenary (ACQ) contains a very high level of copper and consequently, also appears green. ACQ treated wood is considered safe by conventional sources at this time. If the wood was purchased or the structure was built prior to 2003, the lumber was most likely treated with CCA.# To be sure, you can obtain an arsenic test kit, available at many hardware stores and online.
If your existing raised beds were built with CCA treated lumber, you may wish to remove the structure to avoid the continued migration of arsenic through the yard. Dispose of it at a local waste facility; it’s categorized as construction debris. If this isn’t feasible, you might choose to grow ornamental plants in the beds rather than food. Even if the soil is replaced, the arsenic will continue to migrate into new soil.
If you use untreated wood, you may wish to use natural wood treatments like flaxseed oil or wax. Linseed oil can contain toxic additives, so it’s best to avoid it. A wide variety of borate-based “washes” exist to make wood resistant to decay and insects. They are safe around food plants.
Determining Raised Bed Depth
You will want to make your raised bed deep enough for healthy root growth. Plants will be stunted and may not produce well if their roots can’t reach down far enough into soil. If the raised bed is built shallowly, the roots will venture into the ground below unless it is too compacted. A floor or barrier is needed if you don’t want roots growing into contaminated soil. The following recommended Soil Depths are taken directly from Golden Gate Gardening:
-6 to 10 inches: basil, beet, carrot short, chervil, chives, cilantro, lettuce, onion, green, parsley, peppermint, radish, spinach, thyme, dwarf cherry tomato, watercress, oregano, sage, marjoram.
-10 to 15 inches: carrot, celery, Chinese cabbage, garlic, leek, lettuce, mustard, oregano, potato, strawberry, chard, dwarf patio tomato.
-15 to 18 inches: all beans, collards, cucumber, kale, pea, pepper, squash, short vine tomato.
-18 to 24 inches: broccoli, brussel sprouts, cabbage, cauliflower, corn, any tomato.