Allelopathy -- today I learned some plants actually do better under allelopathic trees

Like most people, I grew up hearing that you didn't want to plant a garden too near a black walnut tree or certain oaks because there was "something" in the trees that killed the plants beneath it. When I got older and did some research, I learned about allelopathy and knew that the "something" was one or more chemicals produced by certain parts of the tree. I also added a few more species to my "no-no" list -- like eastern red cedar, bay trees, and certain maples; and found that the part that effects other plant's growth can vary from species to species -- cedar trees, for example, produce their toxins in the leaves, so other parts are fine around other plants. (So, you CAN use cedar boards and logs for hugelkulture and raised beds.) What I didn't know until this morning is that some plants actually do better under certain allelopathic trees!

I just read a couple of studies (one from the U. of FLorida and another study) where they grew several known allelopaths -- including, believe it or not, broccoli -- in conjunction with crops like peas, wheat, rice, rye and corn. They found that (just a couple examples) under walnut trees, corn production went down, but under Leucaena, wheat and tumeric went down but maize and rice actually increased.

http://edis.ifas.ufl.edu/hs186

The second study sprouted and grew peas and corn watered with solutions made from cedar foliage. Apparently peas are not affected by cedars, but corn -- though it will sprout, does not achieve as great a biomass.

http://escholarshare.drake.edu/bitstream/handle/2092/956/Poster%2020.pdf?sequence=1

There is a lot more on this out there and I am going to read it avidly. I have acres of red cedar and lots of walnuts, oaks and other problematic trees. This may have huge impact on my future garden!

You are exactly the kind of forum member I look for when choosing which posts to read. Thank you for sharing your research; this forum allows me to learn a little more about one thing while I investigate another item on my list of 'to be researched' subjects.

Thanks Susan. The one thing I know for sure in life is that there is a LOT more out there than we can ever possibly know. That's why I love this forum so much -- everyone here knows something I don't, so the learning never stops. For me, that is a really great thing!

My .02:

Linda Chalker-Scott has a small chapter on allelopathy in her great book The Informed Gardener Blooms Again. You can find it here.

One issue that she doesn't directly address is this:

While the chips of these aromatic species won't necessarily inhibit growth or otherwise damage plants, they will inhibit fungal consumption of lignin, and other microbial processes that break the chips down into soil. So if one of your primary considerations with mulch (and if often is) is soil building, you're better off with a species that didn't evolve aromatic oils to inhibit fungal growth.

HI Leonard,

You really make my point that this forum is a great place to learn new things. The idea of fungal growth being inhibited by the chips from aromatic species is something I had not considered. I agree that for soil building that could be a problem. I am going to check out that book -- sounds good.

There is another problem associated with wood chips in general (not just those containing aromatic oils and resins, but all wood chips)... they do take up nitrogen from the soil while decomposing so, under ordinary circumstances, could make it necessary to add nitrogen for the first couple of years they are in the soil. You can easily get around that by putting extra manure on your crops under the mulch, or by simply starting out with well-seasoned wood chips.

I have personally not found cedar sawdust to be a problem because my chief aim in using the cedar sawdust and chips I get here (free and by the truckload from the mill a mile away) is as a thick mulch. Our summers are so scorchingly hot that we really need deep mulch to keep the roots cool, and I like the sawdust over straw or similar mulches because it is not seedy. (We have just restored our glades to eliminate invasive species, so I don't want to introduce more.) I also like the cedar sawdust for the very fact that it does NOT break down quickly -- neccessitating continual replacement -- and because the site of our main garden is a former livestock pen (goats) and heavily nitrogenous already. If anything, the sawdust is helping to balance out that heavy nitrogen load from all the years of animal use.

I guess it really depends upon your ultimate goals, and like pretty much everyting else, one situation is going to be different from another. In some cases it might not be a good idea to use aromatic species or any wood at all as mulch, while in others, it may be exactly what you need.

There is another problem associated with wood chips in general (not just those containing aromatic oils and resins, but all wood chips)... they do take up nitrogen from the soil while decomposing so, under ordinary circumstances, could make it necessary to add nitrogen for the first couple of years they are in the soil.

The book also has a great chapter entitled "The Myth of Nitrogen-Nabbing Woodchips". Enjoy!

Leonard Barrett wrote:

There is another problem associated with wood chips in general (not just those containing aromatic oils and resins, but all wood chips)... they do take up nitrogen from the soil while decomposing so, under ordinary circumstances, could make it necessary to add nitrogen for the first couple of years they are in the soil.

The book also has a great chapter entitled "The Myth of Nitrogen-Nabbing Woodchips". Enjoy!

You are a font of knowledge Leonard! Another treasured myth busted! I guess I am going to have to update my store of lore.