Are Oak Leaves Healthy For A Garden

Carol Savonen's former tomato bed was readied for winter by pulling out the old crops, adding two wheelbarrows worth of compost and a few handfuls of ground limestone and topped off with a 6-inch layer of fallen oak leaves.

Q. I have a large number of oak leaves that have accumulated in my yard after this last round of storms. I would like to compost them but have been told they are toxic to microorganisms and they should not be added to my compost pile. Is this true?

A. First I want to commend you for thinking of composting your oak leaves and not burning them. Burning leaves contributes to our poor air quality and also removes a valuable nutrient resource from your yard.

What you have heard about oak leaves is somewhat true in that they have high levels of tannins that will slow the decomposition process, but they can still be composted. All leaves and many fruits have tannins in them, freshly fallen oak leaves just have larger amounts. When composting a large amount of oak leaves, you may need to mix them with other types of leaves, straw or newspaper to keep your pile decomposing at a good rate.

Oak leaves among the frosty leaf litter along a trail at Rocky Ridge County Park in Pennsylvania.

To help speed up decomposition and improve the leaching of tannins out of the leaves it is best to shred the oak leaves before adding them to the compost pile. This can be done by forming windrows with the leaves and running a lawn mower over them, or stuffing the leaves into a garbage can and using a string trimmer to shred them. Once shredded you can use the leaves to create layers of brown and green materials in your pile.

For the best results:

Your compost pile should be at least three feet by three feet. Any smaller than this and the pile may not get hot enough to effectively break down the leaves.
During wet weather you may need to cover the pile so it does not get too wet, as this will slow down the decomposing process.
Turning the pile every couple of weeks to insure enough oxygen is getting to the middle of the pile will also help keep things decomposing.

Another common question we get about using oak leaves in compost relates to concerns that it will make the pile or finished product too acidic. It's true freshly-fallen oak leaves are more acidic than many other types of leaves, but once they have completely decomposed the compost tends to be only slightly acid or neutral. In native settings soil tested under oaks with decomposed oak leaves is usually not anymore acidic than surrounding soils without accumulated oak leaves.

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Composting your oak leaves makes a valuable addition to any garden soil, it uses materials you might otherwise discard, and is a simple process. Once composted, the leaves are no longer a fire danger — as they would be if you just left them as mulch.

For more information on composting go to the Nevada County Master Gardeners at http://ncmg.ucanr.org/Composting_Resources/ and the Placer County Master Gardeners at http://pcmg.ucanr.org/Composting_Information/.

The Shasta Master Gardeners Program can be reached by phone at 242-2219 or email mastergardener@shastacollege.edu. The gardener office is staffed by volunteers trained by the University of California to answer gardeners' questions using information based on scientific research.