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Wood Mulch! What is it Good For? (Actually, Some Things….)


Q. We lost some trees to storm damage and I want to keep the leftover wood chips for mulching walkways in the back yard and between the raised beds in our garden. These areas are far away from the house, but your repeated warnings about wood mulches have made me cautious. What do you think?

---Muriel in Montgomery County, PA

A. I think you have perfectly described the most acceptable uses for wood mulch; and the material you have at hand is vastly superior for those uses than any kind of bark or wood mulch you could buy.

Now, before I explain those statements, I have to say that your email is also a refreshing change from the vast majority we get on this topic, which all seem to start out with "I know you're against the use of mulch…"

For the record, your honors, I am NOT against mulch! I love mulch! Mulch is a good friend of mine! A lot of the blame for the "I know you're against the use of mulch…" misconception is due to my repeated warnings about products made by the "chipping industry". When landfills stopped accepting green waste decades ago, developers, arborists and contractors started sending their trucks full of tree trimmings and wooden construction debris to chipping plants instead of the dump.Then landscapers so successfully sold the finished product to their customers that most people today think that the word 'mulch' means wood or bark.

Which it does NOT! Mulch is anything that's used to cover the surface of the soil to prevent weed growth and retain soil moisture. Thirty years ago, wood mulch was fairly rare; you either had to flag down a road crew trimming trees and beg them to dump their load or have an arborist leave the chips behind after you had a tree cut down.

And I feel strongly that such 'old school' material is vastly superior to store-bought 'mulch'; because at least it's real wood from local trees. There have been many reports of construction debris—including pressure treated wood containing arsenic and creosote—being chipped up to make generic wood mulches, especially the dyed ones. Those nasty colorings can do an excellent job of hiding the true nature of suspicious wood. (Not to mention that some of the colors they use make your landscape look like the inside of a Burger King.)

Our listener knows where her wood came from and she's keeping it on site—that's re-use, recycling and a very small carbon footprint. You should keep any chips from your own trees—or even a load from a local tree trimmer—if you have the room and appropriate places to use it in your landscape.

And her uses are appropriate, especially the keeping weeds down in pathways. I've used wood chips from my own trees and the local power company's tree-trimming crews to keep the weeds down in the lanes between my own raised beds. You could also pile the chips up a foot or two deep as a 'killing mulch' to try and smother something like a patch of running bamboo after you scalped it low to the ground.

But don't use wood mulch within thirty feet of a house or car because any wood mulch can breed the artillery fungus that stains those expensive objects with zillions of little tar ball-like spores. (See this previous Q of the Week for the stained details.)

And don't use wood or bark as a mulch underneath disease-prone plants like tomatoes, roses and lilacs for the same basic reason; wood is the perfect incubator for fungal spores, including ones that spread disease, like black spot. Compost makes a great mulch for any plant, but it's absolutely essential for these and other Drama Queens of the Garden.

In fact, I'll go further and add that compost—your own properly made black gold or store bought—should always be your first choice for mulching raised beds and other forms of vegetable growing gardens. And wood mulch should be the last choice. As documented in this previous Q of the Week, a massive study conducted by three different Universities found that two inches of compost prevented weeds just as well as two inches of shredded bark, but the compost mulched plants thrived, while the wood mulched plants suffered.

Compost rules! Wood mulch drools!

And no mulch should ever touch a plant; you always want the stem or trunk to be open to the air. Piling mulch onto a plant as opposed to around it invites rot and disease to take hold and insects and vermin to nibble away undetected.

And yes, that means that every 'volcano-mulched' tree is wrong, wrong, wrong! I want everyone reading this to get up! Get up out of your chairs! Grab hoes and rakes and go out and pull that mulch away from every poor tree you can save! And if anybody asks what you're doing….

"Tell them you're mad as hell and you won't take it anymore!"

And finally, never use ANY mulch right next to the foundation of a home; keeping moisture in the soil next to your house creates a super-highway for subterranean termites to get right up close to your framing. Always leave a foot of bare, dry soil around the perimeter. (You'll find more info on this very important topic in our termite Qs of the Week.)

Q. Let's squeeze in a last question from Barbara in Saunderstown, Rhode Island, who writes: "I've heard your warnings about using wood chips many times, but my husband has been cutting and splitting logs from an old cherry tree, and we have lots of sawdust and chipped-up bark. Isn't there something we can do with it?"

A. You can absolutely use the bark chips for weed control in pathways and other non-veggie, far-away-from-stainable-surface areas, and you can mix the sawdust right in with them. But don't use fresh wood of any kind—especially sawdust, with its small particle size—as a mulch anywhere near WANTED plants. Fresh wood---again, especially in the form of sawdust—will suck all the Nitrogen out of the soil and starve your poor plants to death.

And whatever you do, don't ever till sawdust or other forms of wood INTO your soil; that'll turn the area into a killing field for plants for years to come.