Mulch falling from the trees

My new perennial bed near the back of the house is covered with needles from a big white pine (Pinus strobus) nearby. 

Pine needles on the new bed in October

When the area was lawn, I didn’t notice how many needles fell from the tree, perhaps because our focus was on raking them off the grass.

    Now I realize that those pine needles are providing natural mulch without my intervention. Is that a good thing? A couple of persistent urban legends warn against pine needle mulch.

    In the South, you can buy compressed bales of pine needles, commonly called pine straw, in various shapes and sizes at the garden center. 

Bales of pine straw at Space Coast Landscape Supply in Florida

Proponents point out that this mulch is pretty, it’s slow to decompose because of the needles’ waxy coating, it doesn’t blow around or pack down to create a water barrier, and it’s less dense than bark mulch. A 40-pound bale of pine straw covers as much ground as 30 cubic feet of other much heavier mulches. By contrast, big bags of bark mulch that I purchased recently contain 3 cubic feet each. I can barely lift them, and a bag covers a dishearteningly small area.

Bark mulch, heavy as all get out

    Tossing pine needles around has got to be easier than schlepping those bags of bark mulch. Having the tree and the wind spread pine needles is even less work. (My recent mulch favorite, arborist wood chips, also has the advantage of being less dense than bark).

Arborist wood chips make great mulch too

    It’s often suggested that pine needle mulch will acidify soil. The argument is that since the needles have a lower pH than average soil, adding them to soil must make it more acidic. Many gardeners hesitate to mulch with oak leaves for the same reason. But science doesn’t substantiate this.

    Soil has tremendous buffering capacity. As Robert Pavlis points out in this blog post, rain becomes acidic as it falls, picking up carbon dioxide from the air and converting it to carbonic acid. This is true even without the pollution that causes acid rain. If rain hasn’t changed soil pH over millennia, you’re not going to do it with a few pine needles or oak leaves.

Pine needles can't overpower soil's buffering capacity

    Second, pine needles contain a group of organic compounds called terpenes that people worry may suppress growth in the garden. Terpenes act as insecticides, preventing insects from eating the needles. They may also help suppress growth of other plants under the tree. But once the needles fall from the tree and hit the ground, the volatile terpenes quickly float into air or water—that’s why the needles stop giving off that pleasing pine scent. Plants can and do grow under our white pine.

Shade lovers growing at the base of the white pine

    Recent research suggests terpenes help trigger rainfall by combining with free radicals and oxygen in the atmosphere, forming aerosols of polarized molecules that collect water and grow into clouds. Trees aren’t as passive as we used to think!

Conifers don't just wait for rain

    I’ll soon be able to assess whether pine needles are improving soil, suppressing weeds, and helping to retain moisture in my new bed. I’m optimistic.

Conventional wisdom that's wrong

With fall garden work in my future—spreading compost, chopping leaves for mulch, and generally preparing for the cold months ahead—I’m glad to learn that some conventional gardening wisdom turns out to be false. 

Time to prepare the garden for winter

My heroine Linda Chalker-Scott, professor of urban horticulture at Washington State University, has the research to prove it. Here are three myths we can forget about:

Nitrogen-sucking mulch
It used to be an article of faith that high-carbon mulches such as the ones I use—shredded leaves and wood chips—would use up nitrogen in the soil in their decomposition process, thereby leaving plants short of this important nutrient. 

Does wood chip mulch cause nitrogen deficiency?

Chalker-Scott found that it doesn’t happen. So we can go on spreading organic mulch without adding high-nitrogen chemical fertilizer to make up for the imagined loss. 

     There may be a narrow zone of nutrient deficiency just at the mulch-soil interface. That’s actually a good thing, because it’s where young weeds would try to get started. The lack of nitrogen will help keep them from succeeding. It doesn’t hamper the growth of plants with established root systems, though, because they feed further down under the soil surface.

Acidifying soil
Another shibboleth is the proposition that adding oak leaves or pine needles to compost or using them as mulch will acidify soil. Soil is not as simple as we used to think. Adding acidic components to soil is not like putting vinegar on your salad. The soil has a buffering system that prevents swings in its pH. People who mulch with oak leaves year after year see no change in their soil’s pH, and pine needles, widely used in the Southeast, make some of the best mulch there is. 

Pine needles make great mulch

If you really want acid soil, you have to add chemicals such as ammonium sulfate, which only works temporarily. Here in Massachusetts our soil is naturally acid, so we don’t need to add chemicals to grow acid-loving plants like blueberries and rhododendrons.

Winter wraps
Some people in my town wrap their broadleaf evergreen shrubs in burlap during the winter. This is intended to prevent drying caused by sun and cold winter winds. There’s some justification for this; leaves can indeed be killed when they lose moisture to evaporation and the frozen ground offers no replacement. 

     You may also see ads for anti-dessicants sprays that purport to prevent evaporation from leaves. Chalker-Scott found that using these materials is more likely to prevent your plant from taking in carbon dioxide, photosynthesizing, and avoiding overheating. 

    To my mind, wrapping rhododendrons and boxwood shrubs in burlap cancels the benefits of growing them in the first place. One of my main reasons for planting evergreen shrubs is the chance to enjoy their green leaves in the winter landscape.

Greenery is welcome in winter

     It turns out you can protect your shrubs best by making sure they’ve had enough water before winter starts and by mulching generously to hold that moisture in the soil. That way they can lose some water without ill effects.