Sycamore Trees and Allelopathy

Question from Sue:

I saw a short article in the newspaper about Sycamore Leaves being allelopathic. I’ve been adding them to my compost pile and I am not sure this is a good idea now.  If I understood the article correctly, sycamore leaves can inhibit seed germination.

Answer from Pat:

If I were you I would stop using sycamore leaves in your compost pile, not because of their allelopathy, but because sycamores are prone to several pests and diseases, including mildew and anthracnose, a fungal disease that turns leaves brown and can sicken the entire tree. I have always advised gardeners to keep ground under their sycamore trees clean by raking up the leaves and twigs that fall from sycamores and replacing them with clean mulch. I also advise gardeners not to put the leaves and twigs into the compost pile but to bag them and send them to the dump. This is one way to help control anthracnose, mildew and pests on sycamores in a home garden. (See pages 87 and 88 in “Pat Welsh’s Southern California Organic Gardening, Month by Month” for more information on this subject.) The rule of thumb in regard to diseased leaves of any plant, such as rose leaves with rust, for example, is don’t add them to the compost pile. A hot compost pile that is kept cooking for 3 to 6 weeks can and will kill most pests and diseases, but few gardeners have the skill, strength or time to maintain a hot compost pile and keep it cooking for the length of time it takes to complete the compost process.

That said, I wouldn’t stress unnecessarily over what you’ve already done. Just don’t do it again. Various beneficial organisms in an organic garden can eventually kill many diseases.

Now regarding Allelopathy: Sycamore leaves are indeed allelopathic. However, they are not as strongly so as, for example, walnut leaves and wood, especially black walnut. Years ago a very knowledgeable local Farm Advisor, Vince Lazaneo, told me that in most cases the chemicals in leaves that inhibit seed growth will eventually break down in the composting process. He gave the eucalyptus leaf as an example. Local gardeners in Southern California often worry that it might be wrong to use chipped leaves, wood and twigs of eucalyptus as mulch. However, as I have explained elsewhere in this blog, eucalyptus mulch does not seem to harm plants, but must never be combined with soil until fully rotted. Once decomposed, mulch from eucalyptus even when combined with garden soil does not seem to inhibit seed growth. In my own garden, seeds have often germinated on the edges of paths that were covered with eucalyptus mulch. I imagine the same would be true of sycamore.