Tuesday, May 10, 2011

Are Oak Leaves Safe for Garden Mulch?

Everyone knows that walnut trees produce a potent growth inhibitor as a way of protecting their territory.  Juniper trees are rumored to do so as well.  But what about oak trees?  Live oak trees, which are our most plentiful hardwood, make small, very fibrous leaves that take forever to decompose.  For that reason, as I explained here, I've been tending to use them as mulch rather than adding more than a small percentage to my compost pile.  And because I'd heard they might contain a plant growth inhibitor, I only used them on the walkways, never near the vegetables I was growing.

But now as weeds invade my beds and rows of tomatoes, squash, greens, onions, herbs, beans, garlic, peppers, cucumbers, potatoes, and other lovely deliciousness, I wonder if I can use oak leaves as mulch near the plants themselves.  Oak leaves are certainly something we have plenty of.

It turns out the question of oak leaves containing growth inhibitors is fairly debatable.  Definitely they contain lots of tannins.  That's why if oak leaves fall in a container of water, the water will turn brown like tea.  As far as I can tell, tannins do play some role in plant growth regulation, in the plants that produce them.  It might be reasonable to expect them to affect plants growing in the same soil in which tannins from other plants are being released.  But whether that means tannins from oak leaves would inhibit vegetable seed germination and growth, I don't know.  One benefit would be that many animals find the taste of tannins unpleasant, so perhaps oak leaves would repel unwanted pests.

More definitively, oak leaves release phenols in their first month or so of decomposition, which do inhibit seed germination and growth.  So I'd conclude that only leaves that have rotted for a month or more should be added to the beds itself, but that fresh leaves are fantastic for walkways.  In fact, this is the perfect way to work with nature instead of against it.

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