Anna Mason a summer intern with Veggies for All in Unity spreads mulch around garlic plants Wednesday. Mason is a sustainable agriculture major at Unity College. Credit: Gabor Degre | BDN

This story was originally published in June 2020.

Mulching is a great way to suppress weeds, retain moisture and, depending on the substrate you choose, add some nutrients back into the soil of your garden plot. It doesn’t seem that difficult to just lay a bunch of wood chips or newspaper over your plot, but there is actually more to it than that.

In fact, many gardeners who are using mulch for the first time make one fatal mistake that can render their mulch much less effective than it would be otherwise — they apply the wrong thickness.

“If you pack [mulch] thick enough it makes an impermeable barrier for weeds,” said Eric Gallandt, professor of weed ecology at the University of Maine. “One of the biggest problems for mulching gardens is this skimp on it.”

Thickness really matters. When it comes to weed control, any mulch can be effective as long as you put down a thick enough layer.

“When you spread [mulch] out and it’s only a couple inches thick it may look good but it compresses really quickly,” Gallandt said. “Anywhere you have gaps in your mulch you’re going to have weeds coming through. You have to make sure you have your mulch layer a sufficient thickness if you want it to prevent weeds for the whole season.”

Matthew Wallhead, ornamental horticulture specialist and assistant professor at the University of Maine Cooperative Extension, said that a good rule of thumb is to have at least two inches of mulch, but generally no more than six inches.

“Sometimes for certain situations, if you’re really trying to control weeds, you might go to a deeper mulch,” Wallhead said. “If it’s less than 2 inches you’re probably not going to get adequate weed control.”

However, different mulches require different thicknesses. Sonja Birthisel, postdoctoral research associate at the University of Maine, said that natural and organic mulches such as hay, straw or leaf litter will need an extra thick layer of mulch. Not only will microbes in the soil eat away at the material over the course of the season, but the once-fluffy material also will quickly compress and leave gaps of exposed soil.

“You need a pretty beefy 3 or 4 inches of mulch, particularly if you’re going to be using hay or straw mulch so that you’re fully blocking that light so weeds can’t come up,” Birthisel said.

The same goes for upcycled, DIY paper mulches.

“That newspaper is going to break down quicker than wood or wood chips,” Birthisel said. “You’re going to want several inches of newspaper.”

The University of Maine Cooperative Extension offers a helpful table that guides gardeners through accumulating sufficient mulch to cover the area and depth of their gardens. Wallhead added that garden center employees can help you make sure you are purchasing enough mulch.

“Tell them what you’re doing, [ask] if they have any suggestions and make sure you’re applying a deep enough mulch layer,” Wallhead said.

On the flipside, though, Kate Garland, horticultural specialist at the University of Maine Cooperative Extension, warned against letting too much mulch accumulate over the years, especially in ornamental gardens.

“A common issue is thick, thick layers of mulch that accumulate on ornamental gardens,” Garland said. “Landscapers are going in every year and adding mulch over and over again without raking off the old mulch.”

If you are worried about mulch piling up, Garland recommended cutting wood chips or bark mulch with compost.

“Your mulch will break down a little bit faster and not accumulate as much,” she said.

Watch: Organic farmer Tom Roberts shares some tips for spring gardens