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Sunday, November 6, 2016

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.

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