I have a huge planter to fill but don’t want to buy that much soil so I want to partially fill it with wood. I’m going to plant herbs in it but I wanted to know if the wood I have would make eating the herbs inadvisable. I have roots and branches from a snake bark elm and some large pieces of lilac. None of the wood is treated but I know some wood is poisonous and wasn’t sure about these two.
Before you go ahead with using wood to fill in the planter, another trick
you might try is to put an upended smaller pot inside the large pot, if
the planter is too deep. What you are looking for is a potting medium
with good drainage.
I am not familiar with snakebark elm (there is a snakebark maple, and a
lacebark elm–might it be one of these?) so I can’t give a conclusive
answer about its wood or roots. The phenomenon of plants which are toxic
to other plants is called allelopathy. The most famously allelopathic
tree is the black walnut. Apparently, lilac wood (Syringa vulgaris) has
the ability to raise the phenolics content in the soil, according to a
2004 scientific article (now archived) I found, from the 2nd European Allelopathy Symposium.
To be on the safe side, I would avoid using the lilac and elm wood as
filler in your planter, since there are better options.
You may find the information below useful:
Local gardener Mary Preus’s book, The Northwest Herb Lover’s Handbook
(Sasquatch Books, 2000) offers a recipe for potting soil for herbs grown in containers:
- 8 quarts compost, earthworm castings and/or composted chicken or steer
manure - 4 quarts sphagnum peat moss
- 4 quarts perlite
- 4 quarts builder’s sand
- 1 cup all-purpose fertilizer mix (she has another recipe for this*)
- 3 tablespoons ground dolomitic limestone
- *all-purpose fertilizer recipe:
- 2 pounds fish meal or crab meal
- 1/2 pound greensand
- 1/2 pound steamed bonemeal
- 1 pound rock phosphate
- 1 pound kelp meal
Virginia CooperativeExtension also has information on soil mixes for growing edible crops in containers:
“A fairly lightweight mix is needed for container gardening. Soil
straight from the garden usually cannot be used in a container because it
is too heavy, unless your garden has sandy loam or sandy soil. Clay soil
consists of extremely small (microscopic) particles. In a container, the
bad qualities of clay are exaggerated. It holds too much moisture when
wet, resulting in too little air for the roots. Also, it pulls away from
the sides of the pot when dry.
“Container medium must be porous in order to support plants, because roots
require both air and water. Packaged potting soil available at local
garden centers is relatively lightweight and may make a good container
medium.
“For a large container garden, the expense of prepackaged or soil- less
mixes may be quite high. Try mixing your own with one part peat moss, one
part garden loam, and one part clean coarse (builder’s) sand, and a
slow-release fertilizer (14-14-14) added according to container size.
Lime may also be needed to bring the pH to around 6.5. In any case, a
soil test is helpful in determining nutrient and pH needs, just as in a
large garden.”