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Using Grass Clippings for Mulch


You Bet Your Garden
Question of the Week © 2018 Mike McGrath

Note: Hundreds of Mike's informative articles are available (in alphabetical order!) right here at the Gardens Alive website. To find Mike's answers to your specific garden problem, Click here and find your topic (like Weeds, Worms, Rhododendrons...) in our complete alphabetical archive of Questions of the Week.

Using Grass Clippings for Mulch

That's the title for this week? After 19 years and 10 months on Public Radio I suddenly decide to change my mind about grass clippings?! My head is spinning! I see poor little lawns starving, fatal compost being created, green slimy masses of woe-begotten mulch…!

Q. "My stomach churned when I read your comments that grass clippings should always be shredded and returned to the turf with a 'mulching' lawn mower. I am new to an acre and a half of lawn and just purchased a very expensive bagger attachment for an equally expensive zero-turn mower with the intent of using the collected clippings as a one to three-inch layer of mulch in my vegetable garden..."

---Joel in Elkton Maryland

A. This is a really long question, so let's break things up a bit and answer piece by piece. First, clippings from an untreated lawn do make an excellent mulch. In a test we ran at The Rodale Institute back when I was the editor of ORGANIC GARDENING magazine, dried grass clippings prevented weed growth, retained soil moisture and were judged to be the most attractive mulch; nicer looking than straw, compost, shredded leaves or wood.

The drying part is important. Fresh clippings must be spread out to dry until they're a tannish-brown before being used as a mulch or they'll get all slimy. Now, back to our question!

Q. Joel continues: "Grass mulch has a seemingly beneficial mix of solid to air volume fraction, especially the high volume of air. Compared to wood mulch, this larger amount of void space provides less resistance to air and water vapor transport, thereby allowing the soil to 'breathe' while still blocking weeds. I also suspect that grass mulch allows beneficial insects to flourish while minimizing the presence of rollie pollies, pill bugs, or whatever you call them. The surface roughness of the outward facing grass mulch may also prevent water from splashing back up onto the plant..."

A. …Which would help lower disease pressure, but not nearly as much as a mulch of compost, as some of the organisms in compost actually eat disease spores. Oh—and rollie pollie/sow bug/pill bug thingies are not plant enemies; they just recycle debris at the soil line.

Q. Joel continues: "I have contemplated two potentially harmful impacts of using grass mulch, but they are specific to certain plants. One would be increased thermal resistance that would keep the roots of heat-loving veggies like melons and eggplant cooler than their optimal growing temperature. The other is providing an environment for verticillium, fusarium, and other harmful microorganisms to flourish..."

A. The first depends on the season. If it's cold and cloudy, keeping the roots cool could be a major problem. But if its heat wave after heat wave, cool soil would be a blessing. And the two diseases he mentions are soil-borne wilts; they have no connection to mulch of any kind.

Q. This guy must buy ink by the barrel! He continues: "I would like your input on the hypotheses I have presented. As you may have guessed, I am a scientist and would like to test my claims of increased air/vapor transport, decreased radiative transport, relative insect population affect, water rebound, increased thermal resistance, and the proliferation of unwanted microorganisms. But for now, please help me feel more comfortable about my recent purchase. I'd even be willing to compromise and alternate between mulching and collecting grass when I mow."

A. I think he's done. Don't look. OK—now; most of these hypotheses are sound, and dried grass clippings do make an excellent mulch. I would especially consider their use in areas where fall leaves are not abundant and the making of compost—the best mulch—is challenging. But the clippings MUST be from a no-nonsense herbicide-free lawn. And the clippings must be spread out to dry first.

Now: Am I really changing my religion?

Not at all; I saved the best for last. Back in 1995, researchers at The Rodale Institute (in Southeast Pennsylvania) analyzed grass clippings to determine their nutrient content. In a single season, one acre of clippings yielded 235 pounds of Nitrogen—the primary grass food; 77 pounds of phosphorus and 210 pounds of potassium. That's about half the yearly recommended feeding for the average lawn. "Collect the clips and starve the turf."

But he just bought that million-dollar mower!

And if he keeps the blade super-sharp and doesn't let the grass get too tall he can mulch those nutrient rich clippings back into the lawn without harm. And since it seems to be his garden mulch of choice, he can also bag one or two runs a year; that should be more than enough to supply a nice two-inch layer for the average garden.

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