This story was originally published in June 2020.
Mulching is a great way to suppress weeds, retain moisture, and add nutrients back into your garden soil, depending on the substrate you choose. While simply laying a layer of wood chips or newspaper on your garden might not seem like much, it actually takes more than that.
In fact, many gardeners who are new to mulch make a fatal mistake that makes mulch much less effective than it could be: applying the wrong thickness.
“If you pack your bags [mulch] “If you put down a good layer of soil, it creates a barrier that weeds can't penetrate,” says Eric Galland, a professor of weed ecology at the University of Maine. “One of the biggest problems with mulched gardens is under-mulching.”
Thickness really does matter: When it comes to suppressing weeds, any mulch will work, as long as you apply a thick enough layer.
“When you spread [mulch] “If you put down a layer of mulch that's only a few inches thick, it may look good, but it will quickly get compressed,” Galland says. “If there are gaps in the mulch, weeds will grow through them. If you want to keep weeds out all season, you need to make sure your layer of mulch is thick enough.”
Matthew Wallhead, an ornamental horticulture specialist and assistant professor with the University of Maine Cooperative Extension, said a good rule of thumb is to have mulch at least 2 inches thick, but usually no more than 6 inches.
“In certain situations, if you really want to suppress weeds, you may need to apply a thicker layer of mulch,” Wallhead says. “Less than two inches probably won't provide enough weed suppression.”
But different types of mulch need different thicknesses. Natural, organic mulches like hay, straw, and leaves require a thicker layer, says Sonia Bartisel, a postdoctoral researcher at the University of Maine. Not only will soil microbes eat away at the mulch over the seasons, but the once-fluffy mulch will quickly become compacted, leaving gaps with exposed soil.
“You want a fairly thick mulch, three to four inches, especially if you're using hay or straw mulch, to really block out the light and keep weeds from growing,” Barticell says.
Same goes for upcycled DIY paper mulch.
“Newspaper breaks down faster than wood or wood chips,” Bartisel says. “You'll want several inches of newspaper.”
The University of Maine Cooperative Extension offers a handy chart to guide gardeners on how to gather enough mulch to cover the area and depth of their garden, Wallhead said, adding that garden center employees can help you make sure you're buying enough mulch.
“Tell me what you’re doing. [ask] Listen to any advice they have and make sure you have a thick enough layer of mulch,” Wallhead says.
But Kate Garland, a horticulture specialist with the University of Maine Cooperative Extension, cautions against letting too much mulch build up over the years, especially in ornamental gardens.
“A common problem is a thick layer of mulch that builds up in ornamental gardens,” Garland says. “Landscaping contractors go in there every year and add more mulch over and over again without scraping off the old mulch.”
If you're worried about mulch piling up, Garland recommends cutting up some wood chip or bark mulch along with the compost.
“The mulch will break down a little quicker and won't build up as much,” she said.
