Too mulch, too soon? Follow these tips for healthier garden beds

Truckloads of mulch — another harbinger of spring? Mulch is a popular amendment used to suppress weeds and give a neat, uniform appearance to planting beds. When used appropriately, mulch can also conserve soil moisture and provide nutrition and a buffer from the damaging effects of lawn equipment and foot traffic. But if done incorrectly, this yearly ritual could actually be counterproductive.

Mulching is a practice that predates written garden records, and is intended to mimic the natural environment in forests, where fallen leaves and branches blanket the soil and provide important nutrients to trees and other plants as they decompose. And fallen leaves make an excellent (and free) organic mulch. But more often, a longer-lasting hardwood mulch is the homeowners’ material of choice. Less work for the gardener — but what about the plant?

In the U.S., the use of hardwood mulch gained traction in the 1960s, when the housing and furniture boom caused the lumber industry to surge. Mountains of decomposing bark needed to be disposed of, and grinding them up a little finer created a dark-hued product that retained moisture, controlled weeds, and decomposed quickly, as in nature.

As the years went on, municipalities also began to ban the disposal of green waste (tree and shrub trimmings, grass clippings) in their landfills. These unwanted materials became a handy mulch additive. Today, mulch can be composed of any of these, plus other materials such as pallets, stumps and land clearing debris, which are ground and often dyed for a uniform appearance. The resulting mulch takes longer to decompose, releasing fewer nutrients.

Even worse, when layers of fresh mulch are added year after year, they can build up to the point at which water can’t readily penetrate — actually hindering the plant from receiving the moisture it needs. According to Extension experts, over-mulching can cause roots to grow up through the mulch in a vain quest for oxygen and water, or to form girdling roots that encircle the trunk. A thick layer of mulch can also harbor pests and diseases, especially when piled close to a tree’s trunk (the ubiquitous “mulch volcano”).

Additionally, the practice has a harmful effect on pollinators. Seventy percent of our native bees are solitary, ground nesting bees, making their homes in shallow tunnels beneath the soil. These bees cannot dig through mulch; even an inch or two is as impermeable as pavement to them. Instead, they must search for bare earth in which to construct their nests. The pupae overwinter in these tunnels, emerging in spring to begin their essential work of pollination. If a layer of mulch is dumped atop their homes before they’ve awakened, they’re in trouble!

What’s a gardener to do? First, get to know your mulch. Where did it come from, and of what materials is it composed? Avoid dyed materials and inorganic mulch such as stones or gravel, which add no nutrients to the soil. Arborist chips, which are produced when tree care companies chip debris, can be a good choice; this type of mulch usually includes branches, twigs and leaves, which mean added nutrients. This locally produced cover also has a lower carbon footprint than bags of highly processed materials that have been shipped long distances.

Next, be sure to apply the mulch correctly. When preparing to mulch around trees, first notice how a tree grows in nature. You’ll notice that the tree has a natural flare where the trunk meets the soil. This “root flare” should not be covered up. Spread the mulch no thicker than 3-4 inches, and avoid mounding it around trunks and stems. (The “lollipop” effect of a tree trunk stuck straight into a mound of mulch is not a healthy look!)

If you already have several inches of mulch on your beds and tree rings, consider raking it instead of adding more. This will freshen its appearance, and save you some time and money as well. Even better, choose to rake your fallen leaves where they belong — under the tree or plant that grew them. They’ll break down into nutrient-rich soil that will help your plants thrive. Add some native groundcover for added interest and to outcompete any weeds. Or if mulch you must, consider “mullet gardening” — a tidy edging of hardwood mulch along the front edge of beds, where it’s most visible, with compost or fallen leaves as cover at the back. It looks neat and fresh, and will nourish your plants — and their pollinators, too.

For more on best practices for mulching, Penn State Extension provides many resources, including:

extension.psu.edu/learning-how-to-properly-mulch-can-save-your-tree

In the garden this month

Although we haven’t reached our frost-free date yet (around May 11), there are plenty of garden tasks to be getting on with!

• Test soil and amend as recommended
• Plan new plantings on paper, being sure to note mature sizes of plants
• Plant trees and shrubs
• Begin hardening off cool-season vegetable transplants
• Direct-sow spring crops like leafy greens, carrots, peas, radishes, turnips
• Install an easy-to-read rain gauge near your garden, to help keep track of watering needed later in the season

Lisa Schneider has been a Master Gardener volunteer since moving to PA in 2017. Have a home gardening question? Email centremg@psu.edu.