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.
I wold like to offer a raised bed suggestion that I tried some months ago that I am very happy with. I used clay roof tiles from Urban Ore by standing them on their ends and burying the ends deep enough that they stand on their own. I did put stakes made from scraps of water pipe or conduit for added support. I was very pleased with both the aesthetics and the cost: about $40 for the materials to build a 4′ X 10′ x 12″ bed. I have some good photos, which I will email to the editor email address.
Hope this helps.
Sam Foushee, Emeryville
What a great idea! Are we going to see the photos of it? I am totally new to growing veggies etc. so any ideas, photos are very welcome! Thanks, Maria (The Netherlands)
Sounds like a great idea you have there. I would love to see photos. Please send.
Thanks Sam for your great design idea. Reuse materials such as you’ve used brings a great character and quality to a garden as well as being the best for the environment. We’ll be happy to receive your photos and share them with others. Yay for building with reuse!
I would LOVE to see photos of the clay roof tile raised bed!
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I’m thinking about lining my raised beds to prevent rodent damage. It seems to be the general consensus that galvanized hardware cloth is the best thing to use, but I was wondering if anyone had thoughts regarding the zinc, used in the galvanization process, leeching into the soil and/or plants?
My husband recently made me corrugated metal raised beds using used sheet metal from an old meta barn but I am curious if the siding will contaminate my garden…any suggestions or insight on if this is true?
I passed your question on to my coworkers here, and here is their response:
If you have any further questions or need clarification, please ask!
I am considering using old burlap bags to line my raised garden bed. So you have any comments or experiences with them? I want to use a non toxic material and commercial fabric liners as well as coir rolls have either latex or chemicals to hold them together. Any other non toxic suggestions would be appreciated. Thank you
Hi, did you make any headway in this regard? Do you use burlap ? if so how are you finding it working with plants, etc, growth, health…?
Hi- what a great article and resource for materials to use in building raised beds! Do you have a suggestion for how to go about testing scrap lumber for chemicals that might leach? I have a bunch of old barn wood that I’d like to use, but I have no idea if it has ever been treated or coated with anything. Would soaking it in water and sending the water off for a test work? Thanks!
I am interested to hear how Beck Cowles suggests how to make a raised be from her suggested heights over 12″ high? The widest lumber I could find was a 2 x 12 (2″ thick x 12 ” high.) How do you build raised bed that is 24″ high ? I wish she had given more information on how to build those taller raised beds so that you can grow broccoli, brussel sprouts, cabbage, cauliflower, corn, any tomato that she lists as needing a 24″ high raised bed. Building a 24″ high raised bed garden means you will have to mix lots of purchased soil to fill the bed. I am asking this question as my son, who lives in Chicago, in an urban area has lead in their soil. So, they are going to make raised bed that are lined with some suggested materials I have read about in other articles.
My suggestion would be to line the bottom with hay bales, then put the soil on top of those. Yes, eventually the hay composts, but not in a season, and eventually you can mix your older soil and the hay and put fresh planting soil on top.
Wouldn’t the hay bales decompose like compost and make the bed really hot??? That sounds like a bad idea.
Using the hay bales as a bottom layer in a raised-bed would probably not create a lot of heat, since it would be slightly anaerobic decomposition, due to being covered-up by the thick soil layer over the top of the bales. Yes, over time the hay would decompose, but much more slowly and certainly at a lower temperature than aerobic decomposition.
I’ve never tried hay bales as a spacer between contaminated deep soils and an upper raised bed soil, but I believe that it can work, without heating-up the planting soil.
-James, Education & Engagement Associate
I line the bottom of a new bed with sticks and limbs from our yard. It’s a German technique called Hugelkulture. The wood is a filler and holds moisture. It does slowly decompose and add to the soil structure. It’s an amazing process.
I have the same question.
I’m planting an organic vegetable garden in recycled wooden crates. I have no way of knowing if the wood was treated or not. Is there something I can use to line the crates to make it safe? I’ve seen some articles that say the chemicals the leach into the soil aren’t harmful but not certain I trust those. Any advice would be very appreciated.
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I made a bunch of raised beds for vegetables, but they aren’t doing as well as I had expected. One thing I did was put a wooden bottom on the boxes to keep out the gophers, was this a mistake? A neighbor told me a wooden bottom wouldn’t allow for proper drainage. Any help would be greatly appreciated.
Check out juniper lumber for raised beds. It lasts longer than cedar or redwood and comes form grassland restoration programs — it’s a native that has become invasive. It’s got more of the natural aromatics than cedar or redwood so it lasts even longer in ground-contact settings. Totally untreated and nontoxic!
Mead Clark in Santa Rosa carries it, and I think they deliver too.
Here’s some info: http://snwwood.com/Northwest-Hardwoods/Restoration-Juniper
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I’m hoping someone will take the time to inform me on the following scenario.
A potentially toxic wooden planter that has been painted over. I’m considering a barrier made of pond liner (toxicity unknown but the waterproofing is ideal to protect from leeching from the planter) and another layer of square grow bags for versatility in planting and soil renewal.
I appreciate any advice! Thanks!
its amazing blog i liked it thanks for sharing info
Great info! The breakdown on ideal depths is super helpful. I never knew about EcoDirectory – I’m definitely going to be referencing this resource in the future.