Pine needle mulch — it’s not just for blueberries anymore

At this time of year, when pine trees are shedding their old needles, my neighbors Mark and Olga are happy to have me rake up the needles that fall from their trees onto the street and their driveway, and trundle them off in my wheelbarrow to use as mulch. What the couple may not know is how much I enjoy working with this tree debris.

The needles weigh almost nothing, are supereasy to rake and to use in the garden, and they smell so, so good! Whether warmed by the sun or damp from the rain, fresh-fallen pine needles have (to me, at least) a delicious sweet-spicy aroma that I can’t get toomuch of.

There’s another bonus to pine needles. For years — decades certainly, if not longer — common wisdom held that you should only use pine needles and oak leaves as mulch for plants that need a slightly acidic pH, such as blueberries, strawberries and azaleas. While it’s true that pine needles and oak leaves do have low pHs — ranging between 3.2 and 3.8 — they will not have an acidifying effect on your soil.

In looking for details on this I found dozens of references to research done by Dr. Abigail Maynard at the Connecticut Agricultural Research Station, which apparently shows that adding these particular organic materials to the soil does not cause any significant drop in pH. However, I could not find the actual study.

The best information I

was able to find was on the site of the University of Wisconsin Extension Office. Here’s the essence of it:

“Many materials are acidic before they are broken down. Pine needles and oak leaves both have pHs ranging between 3.2 and 3.8 when they fall fromthe tree. If those materials are incorporated into the soil before decomposing, theymay have a small effect on the soil pH.

“If those raw materials are used on the soil surface as mulch, they will have very little effect on plant growth because the roots are not growing in this material. As the leaves and needles break down, they are neutralized by the microbes that are doing the decomposing work. Most compost has a pH of 6.8-7.0, which is very neutral.”

The article suggests that one reason gardeners have traditionally avoided using pine needles as mulch is because observation shows that hardly anything grows beneath pine trees. That’s not because the soil is super-acidic. Rather, it’s because pine trees cast dense shade, and their roots are “so numerous and shallow that they out-compete other plants forwater and nutrients.”

Gardeners have also probably been influenced by the advice to not use more than 10% pine needles in a compost pile. But this advice is given because the needles have a waxy coating and are very dry — they take a long time to break down.

The important thing is to mulch — both to protect tender roots from freezing and thawing during our changeable-weather winters, and to add organic matter to the soil. Whatever leaves you can find (other than poison ivy!), gather and use them in your garden and around shrubs.

One of the reasons I love using pine needles is because their fine texture makes them easy to tuck around plants. Also, they don’t clump but remain fairly light and fluffy— good insulation— and allow water to easily penetrate. Another plus is that they look attractive in the garden.

With oak leaves, I either let them decompose for a year or so in a pile or I shred them before using them as mulch. Otherwise, these tough-textured leaves form a heavy mat. This is great for weed suppression, but is not good for water penetration.

One final note: the fact that pine needles and oak leaves do not significantly lower soil pH is no reason not to continue using them as mulch around acid-loving plants.

PHOTO BY PAM BAXTER Pine needles make a fine and sweet-smelling mulch.