When to Use Fallen Leaves Around Your Landscape

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Our abundance of trees provides us with an abundance of fallen leaves at this time. What you do with them matters, as not all leaves (or needles!) are the same and should not be used the same way.

Let us start with pine needles as this is our most abundant tree, and the one that ‘defines” the “Pineywoods”.

Do pine needles make the soil acidic? This myth persists despite repeated answers to the contrary. I guess it is a “chicken or egg question” of what came first? Is the soil underneath pine trees typically acidic because of the pine needles or did the pine tree prefer to grow in acidic soil and, thus, you have pine needles on top of low pH (acidic) soil?

The answer is that pine trees are really well adapted to our acidic soils and so it makes perfect sense that you’ll find pine needles on top of low pH ground.

But, one says, “aren’t the pine needles themselves really acidic?” It is true that pine needles have a pH of 3.2 to 3.8 (neutral is 7.0) when they drop from a tree. If you were to take the freshly fallen needles (before the needles decompose) and turn them into the soil right away, you may see a slight drop in the soil pH, but the change would not be damaging to the plants.

Certainly, they are acidic but as the microbes (decomposers) in the soil break them down, they will neutralize them. So, you can use pine needles (also called pine straw) and have no worries.

Pine straw is an excellent, abundant mulching material that will keep the moisture in, suppress weeds and eventually add nutrients back to the soil. We need to get away from thinking that pine needles should only used as mulch under azaleas, blueberries, or other acid loving plants. Those pine needles look good in a number of gardens and landscaped beds.

You can also add them to a compost pile. They will slowly break down over time. If you run them through a shredder, they will break down faster. A general rule of thumb is not to add more than 10 percent of pine needles to your compost pile.

Can oak leaves be used as a garden mulch? Yes. While oak leaves are slightly acidic as well, an oak leaf mulch should also have little effect on the soil pH. Shredded leaves are an excellent mulch for vegetable gardens, fruit trees, perennial flower beds and around trees and shrubs. Oak leaves can be shredded with a lawn mower or a leaf shredder.

The only drawback to using oak, sweetgum, or other large leaves is that they can mat down and ultimately limit the porosity.

Home vegetable gardeners know that adding organic matter (including leaves) will ultimately help their garden. Shredded leaves can be spread across a vegetable garden and tilled in. Over several weeks the shredded leaves will break down nicely, loosen the soil and will increase organic matter.

But do not add pecan, hickory, and walnut leaves to a vegetable garden. Never ever! They have a naturally occurring herbicide-type compound that inhibits other plants from growing. Of the three nut trees listed above, the black walnut has the most bio-chemical, juglone, by far.

This natural herbicide activity is a complex phenomenon called allelopathy. Doing some research, I found that allelopathy was first documented by Pliny the Elder, a Roman natural science author, in 77 A.D. In his writings he noted the toxic effects of black walnut on neighboring plants in the landscape.

Allelopathy involves a plant’s secretion of biochemical materials into the environment to inhibit germination or growth of surrounding vegetation. Allelopathy is a good thing to the tree that produces it as it enhances tree survival and reproduction.

To wrap up, you should absolutely use pine straw and all tree leaves except those from walnuts, pecans and hickories in your landscape. Their benefits are numerous and with our abundance of leaves, we can greatly improve our gardens and landscapes.

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