Regional Gardening Reports :: National Gardening Association

In the Garden:
New England
May, 2007
Regional Report

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Aside from providing a weed barrier and conserving soil moisture, mulch cushions tomato fruits so they don't sit on the ground and rot.

Right Mulch, Right Plant

You can tell it's spring by the smell, not just the floral fragrance of early-blooming plants or the earthy scent of the soil warming, but the woodsy aroma of freshly spread bark mulch. That ubiquitous mulch seems to be a one-size-fits-all solution to covering the soil around every type of plant, but there are better choices for certain plants and situations. You can use different types in different places so your plants benefit from the mulch best suited to them.

Where Bark is Best
For ease of maintenance, chunky bark chips are a good choice for mulching around trees and shrubs and for covering large areas where you don't want to have to replenish mulch every year. The larger the pieces, the slower they will decompose. The bigger pieces also stay put better than shredded bark on slopes where heavy rains can wash away lighter materials. They are not a good choice for walkways where sure footing is needed because they make an uneven surface and don't nestle together.

Shredded Bark and Wood Chips
These mulches form a nice dense blanket, which is both good and bad. Good for plants that crave moisture, bad for plants that are sensitive to too much moisture. Because the particles are smaller than bark chips, these mulches decompose more quickly and borrow some nitrogen from the soil as they do so. But then they enrich the soil with organic matter. These materials make a nice, soft walking surface for pathways, and although evil weeds are bound to spring forth from the mulch, they are easy to pull out.

It matters not so much which kind of woody mulch you use around trees, just that you spread one or the other in a ring around the base of the tree reaching outward to as close to the dripline (the tips of the branches) as possible. In studies at Kansas State University, lawn trees that were mulched had an average leaf area 200 percent higher than nonmulched trees that had grass growing right up to the trunks.

It's also important not to spread the mulch too thick because a layer of wood or bark mulch deeper than 2 inches can harm a tree when the fungi that decompose the mulch create a dry, water-repellent layer.

Chocolate Leftovers
I like the chocolate color of cocoa hull mulch, and the candy bar smell. Some years I cover most of my perennial beds with it, buying dozens of bags at a deep discount the second Saturday of May from a huge truck that loads up in Hershey, Pennsylvania, and unloads in a parking lot not far from my house. I feel like I'm purchasing contraband, so that makes it even more fun to use.

One downside to cocoa hulls, aside from the cost, is the tendency of a thick layer to turn slimy if it remains too wet, especially as it thaws in the spring. And it's so lightweight that it tends to shift with a hard rainfall. A dangerous drawback is that some dogs are attracted by the odor and consume the hulls, which can cause sickness, and even death if a dog eats enough of it, because of the theobromine, a constituent of chocolate. Fortunately, my Lab, who will try to eat most anything -- even plastic bottles she steals from the recycling bin -- has never been interested in the mulch.

Straw, Hay, Leaves, Grass Clippings
In a vegetable garden, it's hard to beat these materials for smothering weeds and conserving soil moisture, and for adding organic matter and nutrients as they decompose. I first put down several overlapping (very important) layers of newspapers on the walkways and in beds where I'm using transplants, then moisten the newspapers, and finally spread several inches of straw or hay or grass clippings on top. In garden beds I cut holes through the layers and set my transplants. At season's end, I rake the walkway mulch into the beds and turn it all under.

Gravel and Stones
Certain plants are susceptible to rotting if their crowns stay wet, especially in winter. Mediterranean herbs like lavender, thyme, and sage; perennial favorites like dianthus and penstemon; and alpine plants are in this category. Gravel or stone mulch can remedy the situation, even if it's just a layer a couple of inches thick atop the soil. With different colors available, this mulch can be as decorative as it is useful. I love the look of it, just wish a shovelful wasn't so heavy.

What's Up With Plastics?
Landscape fabric is, of course, very handy for areas of permanent plantings. In annual beds, both black plastic and clear plastic can be used to warm the soil before planting, and you can also use black plastic to prevent weed growth the entire season, cutting holes in the plastic for your plants.

But these impermeable plastics don't allow air or water to pass through the way IRT (infra-red transmitting) mulch does. IRT plastics transmit infra-red radiation to warm the soil, and they contain pigment (usually green or brown) that prevents the transmission of most of the visible light that encourages weed growth. Tiny holes allow air and water to pass through. IRT mulch is most beneficial on early-season plantings.

A specific type of IRT mulch called "red tomato mulch" can improve yields by reflecting far red light waves onto tomato plants. The mulch also warms the soil. The benefit appears to be greatest where there's lots of sunshine and when it's laid down early in the season.

Plastic mulch is best suited to warm-weather crops like peppers, tomatoes, and melons. The downside of all plastics is they don't do anything to replenish the organic matter in the soil. Be sure to mix in compost or other organic matter each year before you spread a new layer of plastic.

Whatever type of mulch you use, now's the time. Weeds are just as full of energy as gardeners are in spring.

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