Thursday, January 10, 2013

Oak, pine, and walnut are good mulch

There are several gardening superstitions out there, regarding what leaves are not good for mulch.  Nearly all leaves are good for mulch, a notable exception being salt cedar, which are covered with salt.  Nothing grows under salt cedar.
Pine needles and oak leaves are thought to be acid, but they aren’t; their toughness can cause acidity, however.   Ph is determined by the movement of calcium in the soil and its balance with aluminum; calcium, which is alkaline, moves with water, while aluminum, which is acid, stays put.  In winter, calcium leaches down, and the surface becomes more acid.  In summer, calcium moves upward with evaporation, and the surface becomes more alkaline.  If the area is excessively watered or shaded, the surface stays acid; pine and oak trees and leaves shade the soil all summer.  When piled thick as mulch, however, the neutral Ph of the leaves overwhelms the Ph difference caused by shading the soil with them, and good soil happens as they break down.
Pine needles are quite useful.  They can be used as base mulch; roots will readily grow through them to soil.  They work well as top mulch, allowing water and light to reach seedlings while shading the compost.  They make good path cover as well.
Oak and tough evergreen leaves are particularly good for keeping weeds suppressed year-round in a shrub border or perennial bed.  Tough evergreen leaves like laurel or magnolia are also good to spread under tomatoes from mid to late summer, to keep the fruit off the soil; many evergreens drop their two-year-old leaves at that time of year.  Madrone is one of these, but their leaves are not as thick as most evergreens and readily break down.
Walnut is another leaf with a bad reputation; they are said to have a natural herbicide in them, juglone, that stops the growth of plants.  This is true, but the plants that they stop are seeds; started plants grow just fine in walnut leaves, as do oak, walnut, and other nut seeds. 
This makes them exceedingly useful; I use walnut leaves as a pre-emergent herbicide to stop weeds from sprouting in my perennial beds, and to keep blackberries and other bird-dropped seeds from sprouting beneath my huge sweet-gum tree.  They break down before winter is over; other mulch is needed to keep soil covered and moist.
If you have a bed full of walnut leaf soil that you want to start seeds in, spread an inch or so of compost in the bed, and sprinkle your seeds into it.  For larger seeds, put down and inch of compost, spread the seeds, and spread more on top.  This keeps the seeds from touching the walnut soil long enough for them to sprout and grow.

Published at under The Natural Gardener #7.

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