Sure, mulch can help control weeds, improve soil quality, reduce moisture loss, and cool soil temperature. Not only that, but—drum roll—it can make your yard look more manicured and groomed. However, that doesn’t mean it doesn’t come without its questions every spring. When’s a good time to drop mulch? How much mulch do I really need? Should I remove old, dried-up mulch from last year?
Ahead, pros answer your pressing mulch questions and give lots of gardening tips.
Meet the Expert
- Bryant Scharenbroch is Assistant Professor of Soil Science at the University of Wisconsin—Stevens Point.
- Venelin Dimitrov is Senior Product Manager at Burpee, a seed company since 1881.
When is the Best Time to Drop Mulch?
Mid-to-late spring is best after the soil has warmed up and plants have begun to germinate. “Don’t put down mulch too early, or it will slow down your soil’s warming process,” Bryant Scharenbroch, Assistant Professor of Soil Science at the University of Wisconsin—Stevens Point, warns.
How Much Mulch Do You Really Need?
“It’s best to maintain a mulch depth between three and six inches,” Scharenbroch says. “If you’re using it to thwart weeds, you may want to add a few more inches; if you’re working in a wetter site, reduce it to one to two inches, as a soggy soil already fights for air, and over-mulching could lead to lower oxygen levels for your plants.”
Is It Necessary to Remove Old Mulch Come Spring?
“Existing mulch contains layers of decomposed organic materials such as twigs, branches, and leaves, which benefit the soil underneath by providing critical nutrients," Scharenbroch shares. "It also acts as a buffer, slowing down evaporation so more water gets to your plants while blocking the sun’s rays to prevent your soil from overheating and drying out. Plus, before it eventually becomes part of your soil, old mulch will turn into compost, helping protect plants from diseases and pests, and retaining nutrients and moisture.”
Another good reason to leave old mulch alone: You could accidentally pull out plant roots during the removal process.
Is There Anything You Should Do to Old Mulch in the Spring?
Snow or rain over the winter can cause older and finer mulch particles to mat, preventing sun and water from reaching your plants. If you notice matting in the spring, take a rake or cultivator to break up and aerate the mat before adding any new mulch.
“You may need to add some new mulch, as it decays at different rates depending on its physical, chemical, and biological characteristics,” Scharenbroch advises.
Some varieties will need to be “top-dressed” less often. For example, larger woody materials such as shredded bark and wood chips will decompose at a slower rate than small green materials like leaves.
What's the Difference Between Natural and Industrial Mulch?
The most common natural mulch consists of shredded wood and leaves, straw, pine needles, stone, sand, and living perennial plants. “When planted in the right spot, perennial plants are the best ground cover, as they prevent erosion, suppress weeds, cool off the soil, retain water, improve the soil structure, provide shelter for worms, and create habitats for beneficial insects,” Venelin Dimitrov, Senior Product Manager at Burpee, says.
Industrial mulch includes newspaper, plastic mesh, nets, black plastic, and shredded domestic waste like old doors and wood floors. “Industrial mulch will be there forever, so make sure you use it wisely,” Dimitrov warns. “It helps in areas where there’s a problem with stormwater management and where stones or plants could not be used. Some industrial mulch is composed of repurposed waste, like shredded tires, and is often used as mulch around playgrounds.”
What About Stone Mulch?
Stone mulch is made up of a variety of rocks and gravel, including lava rock, granite, quartz rock, river rock, and pea gravel, and comes in various shapes and sizes.
“It’s a great option for someone looking to stabilize garden areas vulnerable to washouts,” Dimitrov says. “The stones don’t absorb rainwater like other mulches; therefore, there’s less of a chance their surroundings will be washed out. But be wary—rock mulch can absorb heat from the sun quickly, which could bake shallow plant roots faster than normal.”
Why Does Mulch Come in Different Colors?
Black, brown, and red are the most popular colors, but mulch could come in any hue when dye is used.
“Red mulch, for example, is processed with dye and can run a bit when it rains, but this won’t harm your plants,” Dimitrov says.
Are There Any Mulches to Avoid?
Everyone should know where their mulch comes from, Dimitrov notes.
“I recommend staying away from grass clippings from golf courses, since [these] can contain herbicides, along with shredded wood from landfills, as this type of mulch can contain treated wood,” Dimitrov says.
Look for the Mulch & Soil Council logo to ensure you’re using mulch that’s safe from chemicals or toxic substances.