Oak leaves can be used for garden mulch
“The oak tree:
in cherry blossoms.”
— translated from Matsuo Bash, Japanese poet (1644-1694)
Many poems have been written about the strength and durability of oak trees, but perhaps none as succinctly as Bash’s haiku. Oak trees focus their energy on producing their own flowers, called catkins or tassels, as well as growing a canopy of leaves that will eventually fall to the ground and enrich the surrounding soil with nutrients.
The third part of my series of columns about using autumn leaves in the garden (see also Nov. 8 and Nov. 15) addresses a question I received from Bob, who wrote: “You didn’t mention oak leaves in your article. I’ve always believed they are only good for acidic plants.”
It’s true that the leathery leaves of oak trees (Quercus spp.) contain substances called tannins that serve as a defense against predators by making the leaves, bark and acorns acidic and bitter-tasting. Although ingesting a lot of tannins would be toxic to humans (and grazing animals), small amounts of tannins are regularly consumed in tea, wine, dark chocolate, cranberries, rhubarb, walnuts and other foods.
When you sip a glass of red wine, it’s the tannins from the oak barrel in which the wine was aged that produce the rich flavor. When you bite into an unripe pear or plum, the astringent taste and tough texture of the fruit come from the tannins that haven’t yet broken down.
Depending on the species of tree, freshly fallen oak leaves have a pH around 4.5 to 5.5 (pH below 7.0 is acidic and above 7.0 is alkaline). The acidity level decreases with rainfall and as the leaves decompose. However, oak leaves are slow to break down, so the key is to shred the leaves and add a source of nitrogen, such as bloodmeal, to expedite the decomposition process.
Mixing oak leaves with other types of leaves will also lower the acidity if you want to use them for mulch this winter. Keep in mind, though, that most of our garden plants thrive in slightly acidic soil with a pH between 6.0 and 6.5. It doesn’t take long for the mulch to reach this level and even to become slightly alkaline.
In fact, scientists have learned that mulch doesn’t impact soil pH beyond the top two inches; it’s the underlying bedrock that has the most effect. For instance, soils formed from granite bedrock are acidic, whereas soils formed by basalt rock are more alkaline. Therefore, it’s a good idea to test the pH of your soil before adding any amendments.
I apply a few inches of leaf mulch that includes some oak leaves to my garden beds, making sure not to pile the mulch up against tree trunks and plant stems to avoid rot or disease. Come spring, I’ll gently work fresh compost into the mulch and that becomes my garden soil. Any remaining large clumps of leaves will be added to the compost or leaf mold pile where they will continue to decompose.
On the other hand, I don’t recommend using diseased leaves for garden mulch or leaf mold. My sycamore trees have anthracnose and my photinia hedge has leaf spot, both of which are types of fungal diseases. Since fungus spores can overwinter in leaf litter, I rake these leaves up and discard them. Making leaf mold is a cold composting process, which does not kill fungus spores.
Some gardeners say it’s OK to compost diseased leaves as long as the temperature of the compost pile stays higher than 160 degrees for several days. While commercial composting systems are effective at killing pathogens in organic matter, the home gardener’s compost pile may not reach/maintain the necessary level of heat to eliminate all pathogens.
Here’s the takeaway message in haiku format:
An oak leaf’s tannins
should not dissuade gardeners
from making leaf mulch.
Rhonda Nowak is a Rogue Valley gardener, teacher and writer. Email her at Rnowak39@gmail.com. For more about gardening, check out her podcasts at https://mailtribune.com/podcasts/the-literary-gardener and her website at www.literarygardener.com.