Best wood for raised beds in vegetable gardens
Q: I’d like to build some new raised beds for vegetable and herb gardens. I’m wary about what materials would be safe around edibles. I like to recycle when possible, so can I use reclaimed wood?
A: Without knowing the history of the wood — if it was treated with preservatives, if it was part of a pallet that transported chemicals, etc. — it’s hard to say how safe it would be. In order to kill hitchhiking insect or fungal pests, pallet wood, for instance, could have been either heat-treated or fumigated.
Data is limited on which compounds leach out of wood that can be absorbed by harvestable plants, including in what plant tissues those compounds may accumulate. The degree or rates of leaching depends on the treatment chemicals themselves plus the environmental conditions of the site, like soil type and acidity. Being surrounded by soil, root crops like potato and carrot might be inherently more at risk than tomatoes or pole beans. That said, as one study illustrated, even when soil becomes contaminated with leached copper, it doesn’t necessarily result in contamination of the consumed plant part (say, carrot roots vs. carrot foliage), or result in levels deemed hazardous to human health.
Lumber that is pressure treated with a compound that contains copper, known as ACQ, should be OK. Reclaimed, treated lumber over 20 years old could be a problem because lumber treated with a combination of chromium, copper and arsenic (CCA) was not discontinued until 2003. A safer choice would be to use either untreated rot-resistant wood or wood which can be painted or stained to slow weathering and decomposition. Otherwise, wood with an unknown history can be separated from the bed’s soil with use of a plastic liner. Alternatives to wood or plastic include metal, bricks or stone. Concrete “cinder blocks” are also questionable in terms of leaching, though, so sealing them with paint might be the best solution. (Plus, it adds a nice canvas for some creative artwork!)
Our page The Safety of Materials Used for Building Raised Beds addresses these concerns and provides links to additional discussions on material safety.
Q: I’ve noticed there seem to be different categories of herbicides. How do I know which to look for to get weeds under control?
A: Pesticides, which include herbicides, are broadly categorized both by how they work and how they’re manufactured. Each has its advantages and disadvantages and situations for which they’re an appropriate choice. Many weed issues can be
managed without herbicides, and we have a new web page titled Manage Weeds Without Chemicals where you can learn more. An important first step for either approach is to identify the pest (weeds, in this case) so you can then investigate your options.
One division among pesticides is organic versus synthetic; different products using the same active ingredients could span both categories since products can be formulated in multiple ways. (Keep in mind that organic is not necessarily safer for you or the environment.)
Another division is contact versus systemic. Contact pesticides,
as the term implies, need to physically contact the pest in order to have an effect. With regard to weeds, contact sprays only impact the above-ground growth — only the parts the treatment contacts. In order to kill roots on perennial weeds to keep them from resprouting, a systemic product is needed instead, because these are absorbed by the plant (usually via foliage). In comparison, for insecticides, a systemic inside the plant’s tissues needs to be eaten to impact the pest.
How many species a pesticide impacts in its group (insects, weeds, etc.) determines whether it’s broad-spectrumor selective .As those terms suggest, the former is fairly non-discriminating in what it effects; the latter is more targeted, though essentially no pesticide only impacts a single pest species. Depending on how they’re used, broad-spectrum pesticides could potentially harm beneficial insects or desirable plants just as easily as the pest. With regards to herbicides, selective products may target broadleaf weeds but not affect grassy weeds.
Herbicide products that prevent seeds from germinating are called pre-emergent. After weeds germinate, you’d need to use a post-emergent. The two aren’t interchangeable because they function in different ways (which is why you can usually apply a pre-emergent around established plants), though a few products use chemicals from both to achieve long-term management.
Reading a pesticide label in full is always recommended, because it includes details like pests/ weeds targeted, plants for which the treatment is intended (such as cool-season lawns), and advisories about sensitive plants or wildlife.