Myth: Mulching Your Garden Is Good for the Environment

Mulch Ado About Weed Control

Anyone who listens to the news is going to be uncomfortable using chemical weed killer to control weeds in a home garden. Although many are touted as safe, herbicides do seem to have a way of getting linked to unexpected health hazards.

The obvious, safer solution is to use mulch instead. Mulches not only control weeds, they help the soil retain moisture. Some mulches actually feed the soil as they decompose.

Unfortunately, as is so often the case, it turns out that there’s a bit more to it. Some mulching options produce unwanted side effects, and it’s useful to know what they are.

Be “Eco-smart” When it Comes to Mulch

We all try to pick and choose which plants will be allowed to reside in our gardens, but weeds have ideas of their own.

Some weeds succeed because they’re invasive species with no local natural enemies. Some can poison the soil to prevent competition. Others are just splendidly adapted for the local environment. They’re tough customers, and they see that open space in your garden as a standing invitation to move on in.

Few gardeners enjoy weeding. And many of us are quite dubious about the safety of commercial herbicides, no matter what it may say on the label.

So as an alternative we use mulch, as recommended by Horticulturist Joseph Krol, of the Morton Arboretum. And while mulch is a useful weapon against weeds, not all mulches are the same.

The Cypress Crisis

Cypress mulch has been a perennial favorite. But the truth is that cypress mulch isn’t what it’s made out to be. Its popularity is based upon the idea that cypress mulch contains natural chemicals that fend off insects and resist rot. And it did—back in the days when the mulch was made from full-grown, fully developed, century-old trees.

But that’s no longer the case. The demand for cypress timber and mulch has led to an unsustainable over-harvesting of cypress trees, so much so that most cypress mulch now comes from immature trees that have not yet developed the properties that made its mulch so popular in the first place.

And it gets worse. Cypress groves thrive where lowlands meet the ocean, and they protect inland areas from hurricanes by cutting storm surge by as much as 90 percent. They’re also the pillars of their local ecosystems, protecting the region from invasive species and helping to maintain proper conditions for the local flora and fauna. A study by the Louisiana Coastal Wetland Science Working Group (SWG) estimates that the lumber in Louisiana’s cypress groves would have a one-time value of $3.3 billion if the groves were harvested.

That sounds like a lot of money until you read the rest of the report. If left in place as hurricane barriers and guardians of the local environment, the same groves are worth $6.7 billion per year, year in and year out.

Few groves will ever grow back once harvested. Mature cypress trees can live to be more than 1,000 years old, but their seedlings need the freshwater floods that used to replenish the land with fresh layers of silt on a regular basis.

Thanks to more than a century of dam-building, shipping canals and flood control, very little new silt makes it into the cypress groves anymore. Instead of being continually rebuilt, the land is slowly subsiding, and brackish water from the Gulf of Mexico keeps creeping farther inland. The cypress seedlings can’t survive.

It’s not impossible that our water management practices will change at some time in the future, and that conditions in the cypress groves will change with them. But until that happens, the cypress groves will not regenerate.

According to the Louisiana Forestry Association, loggers are erasing up to 20,000 acres of cypress every year. At that pace Louisiana’s best defense against hurricanes will be gone in less than two decades. Without the cypress groves to hold them back, even minor hurricanes could strike like Katrina. Buying cypress mulch just brings the disaster closer.

Mulch Options Evaluated

So what are your other options? We’ll cover the good, the okay, and the bad…and we may as well get the bad out of the way first—just like you’re going to do once you’re done reading, right? Right.

Mulches to Avoid

  • Rubber mulches aren’t a great choice. They’re made from chipped automobile tires, which, according to Linda Chalker-Scott, PhD, of Washington State University, contain zinc and other contaminants that leech into the soil and can kill neighboring plants. In addition, rubber mulch is not as effective at controlling weeds as wood chips. It’s also worth pointing that a hot summer day will make your rubber mulch stink to high heaven.
  • Cocoa Bean Shells look and smell nicer, but they contain theobromine, the same chemical that makes chocolate toxic to dogs. How dangerous it is depends upon the dog’s habits, and how many cocoa shells get eaten. Even the most voracious pups probably won’t eat enough to kill themselves outright. But they may very well experience diarrhea, vomiting, tremors, and an elevated heart rate. Not surprisingly, the ASPCA Animal Poison Control Center recommends avoiding the use of cocoa bean shell mulch in landscaping around dogs. If you’re a dog owner you may find yourself raking all that mulch back up again.

Beneficial But Problematic Organic Mulches

  • Last year’s leaves, shredded so that they won’t matt up, make excellent mulch. But that doesn’t help you if you haven’t got a lot of leaves.
  • Sawdust works, as long as it’s made from clean, untreated lumber. But who generates sawdust in sufficient quantity? You don’t want take chances with sawdust you didn’t create yourself, because if it was made from things like particle board or plywood it will leech all kinds of nasty chemicals.
  • Pine needles make fine mulch, but you need a stand of pine trees.
  • Pure, completely decomposed compost works well, because it makes the soil too rich for weed seedlings. But if the compost is applied before its ready, the weeds will move right in. Done correctly, using compost as mulch is a rich option for both controlling weeds and applying nutrients to the soil. For more on compost, see this primer from the USDA Natural Resources Conservation Service.

The Solution: Natural Wood Mulch

Fortunately there are other excellent options. High quality wood mulch, like compost, keeps weeds down and releases nutrients as it decomposes. You’ll want to be careful about the source, of course. Low quality wood mulches are often made from recycled wood products like old pallets and demolished buildings, and they may very well contain unwelcome ingredients like arsenic, creosote or lead-based paint. In which case, you’re right back to the same problem posed by chemical herbicides. 

So read the label. Good mulch is made from natural wood products rather than recycled trash wood, and the label will boast about it. And you’ll also see a statement saying that the mulch has been certified by the Mulch and Soil Council. Often made from pine, certified mulches are the byproducts of lumber milling and processing operations, and they’re not being allowed to go to waste.

The EPA says there’s nothing to fear from the red or black colorants used in good wood mulches. The red colorant is an ingredient in everything from cosmetics to paint, and in those applications we call it “iron oxide.” When it starts to appear on our cars we usually just call it “rust.” There’s normally going to be plenty of iron in your soil anyway, and adding a bit more won’t hurt anything.

The black colorant is ordinary carbon, and wood mulch is already roughly 50 percent carbon. Again, there’s no reason to panic. If you’re using Mulch and Soil Council certified mulch, a byproduct of the lumber industry, the area harvested to make the mulch is normally going to be allowed to grow back as a forest again. So as your wood mulch decomposes and releases carbon, new vegetation will be sequestering carbon back at the source.

When using wood mulch, don’t let the top layer remain chronically wet. And add a new layer every year. When kept wet, old rotting wood mulches can eventually breed “shotgun” or “artillery” fungus that may permanently stain homes and cars up to 30 feet away. The fungus shoots out spores that look like little tar balls, and they’re just about impossible to remove.

But used wisely, non-cypress wood mulches do have their place in a gardener’s toolbox. You’re not contributing to the demise of the cypress groves or trying to drown your neighbors down in Louisiana. You’re fertilizing your plants a bit and you’re helping them conserve water. You’re making your landscape more attractive, you’re not contaminating the soil—and it sure beats the beats the heck out of weeding.

The Morton Arboretum provides a guide to full range of organic mulches, including those shown here, plus how to apply them, and more.

What You Can Do

  • Don’t buy cypress mulch and don’t be shy about explaining the cypress mulch issue to your fellow gardeners.
  • Be aware that some mulch will leak poisons into your soil, including rubber, cocoa shells and recycled wood mulch.
  • Read the label. Make sure that your mulch was made from natural forest products and is certified by the Mulch and Soil Council.
  • You can’t beat chopped up leaves, if you’ve got a large supply. They’re a natural mulch and a natural fertilizer at the same time. And they’re free.
  • Make your own compost. Pure, fully-decomposed compost will keep weeds down for a season, while steadily enriching the soil below.
  • Don’t pile any mulch up against a building. Termites will use it as a highway to termite heaven.
  • Don’t pile any mulch against the stems or trunks of your trees and plants. It makes it easier for insects and diseases to attack the plant.

Written by Paul Frisbie