Beware of mulch!

11/27/2001
Beware of mulch!

"What do you mean--'Beware of mulch'??," you're cursing at me as you spread yet another "protective layer" over your already mulch-encrusted garden in a final spasm of pre-winter activity. Your back is aching and your fingers are cold. I say, throw in the fork and come inside. Make yourself a steaming cup of tea, grab a cookie, and read this article.

I mean precisely that: Beware of mulch, as most of us Americans know it--more or less finely shredded hardwood bark. In my opinion and experience, in the perennial garden, hardwood mulch often does more harm than good--especially in areas of the country that suffer cold, wet winters.

I had already taken this heretical position before moving to France. In my slide presentations on all kinds of gardening subjects, I told hundreds of shocked garden club listeners to lay off the mulch. Now, I've moved to a continent that--having long ago been stripped of its cheap timber-- is practically mulchless, and yet--what do you know?--full of flourishing gardens. Needless to say, I feel vindicated and thus reinforced to once more bear the standard.

Once upon a time--perhaps in the early 1970's--American gardens weren't yet suffocated under an ever-deepening crust of shredded wood. Some brilliant person--I'll bet it was a marketer in the wood industry--hit upon the idea of selling for cold, hard cash what had heretofore been a waste product of the industry: tree bark. Why not shred the bark and market it to American gardeners as a labor-saving miracle product? Apply a couple of inches of the stuff a couple of times a year and...no more weeding, save on watering, protect your plants from winter freezing!

Needless to say, the idea took off like wildfire. Mulch became a major industry. But, in the garden, how does wood mulch measure up?

First, the admitted pluses. More than any other benefit, shredded bark mulch reduces the emergence of weeds from the soil below. However, sturdy perennial weeds like dock can still penetrate. Second, bark mulch does help conserve water, but watch out for some finely milled and very raw (that is, not at all broken down) products that tend to form a crust that actually sheds water. It also helps prevent plants from heaving out of the ground during freeze-and-thaw cycles during the winter, essentially by keeping the ground frozen.

Now, the minuses. One minor disadvantage is that practically nothing will self-sow in your garden if it is mulched with shredded bark. The seeds simply can't get in contact with the soil to germinate. This means that your biennials will never reappear, and that your garden will lack the lucky surprises of perennials and even shrubs and trees that sow themselves here and there. Gardeners with bark-mulched beds can usually be identified as those complaining that their foxgloves "die" every year, without "coming back."

Perhaps my biggest issue with bark mulch is with its value as a protector of plants through the winter. In all but the very coldest parts of the U.S., it's not the freezing temperatures but the perpetually cold and very wet soil that kills sensitive perennials over the winter. Just make a mental list of your most "cold-sensitive" perennials. I'll bet they're mostly plants that require good drainage to thrive.

These plants--such as lavenders, hummingbird mints, and hosts of others--tend to rot out at their crowns when winter soils remain waterlogged. During the winter, without leaves, these wet-sensitive plants have no means of translocating the extra water around their roots via transpiration (the movement of water from soil into roots and out through the stomates, or pores, in the leaf surfaces).

A mat of shredded bark around these perennials maintains the soil even wetter over the winter, and keeps the zone around the crown of the plant soggy as well--instead of airy and insulated, as it should be. That's why all the old gardening books say to mulch with crunchy oak leaves or airy evergreen bows...but no one wants to go through the trouble anymore, when you can just have a few cubic yards of the bark stuff delivered to your driveway.

You'll find that the majority of your winter-rot-sensitive perennials come through the cold much stronger if planted in very gritty soil, and left unmulched by bark. Use the above old-fashioned recommendations instead. Or just leave unmulched; you'll be surprised at the success of that approach if your soil is well drained. Alternatively, mulch these plants with a thin layer of coarse sand or gravel. This treatment allows water to quickly drain away from those sensitive crowns, greatly enhancing winter survival.

The second problem with bark mulch is that--unlike compost or shredded leaves--it can take a long time to decompose. And since it consists of almost pure carbon, it ties up valuable soil nutrients--notably nitrogen--during it's long breakdown process. The consequence is that these nutrients will not be available to your plants. The result? Markedly poor growth unless you bathe your perennials in soluble fertilizers, which organic gardeners are loathe to do.

Try a test for yourself: Prepare a new bed, adding soil amendments uniformly, and plant it. Now, keep half of it mulched with bark and keep the other half open or mulched with compost. The difference in plant growth will be apparent at the end of even the first growing season.

The slow breakdown of bark mulch leads to what is perhaps its most serious problem. After a few seasons, repeated bark mulching starts to smother both perennials and shrubs. Between landscapers offering to "freshen up" the mulch in the spring and protect your beds through winter with yet more mulch, pretty soon your plants can barely keep their chins above...mulch. I've even met some gardeners who seemed more intent on the "fresh" look of their mulch than on the appearance of their plants.

The reason the ever-deepening mulch layer is a problem is that all plants are sensitive to their planting depth. Some perennials can be said to be exquisitely sensitive. Half an inch too deep and they dwindle; half an inch too high, and the crowns dry up and die. Even apparently more stalwart trees and shrubs will dwindle away if planted too deeply. You can recognize this mysterious dwindling syndrome among woody plants by leaves that become smaller than normal and dwindle to almost nothing near the twig tips. Pretty soon terminal growth starts dying off, and at that point, death of the entire plant is imminent. (Watch out for this during the first year of newly planted stock. Careless technique often results in stock planted too deeply--even without the mulch layer. Lift threatened plants to a correct level.)

Plants, of course, can't distinguish between soil and mulch in "figuring" their effective planting depth. So, every time you add another couple of inches of mulch, you've effectively sunk your plant deeper in the ground.

By now you're probably tearing your hair, wondering what to do and feeling duped by all the gardening experts into killing your plants with a lethal layer of bark. Here are some simple techniques to help you and your garden avoid the hazards of too much mulch.


*If you love that freshly mulched look, remove old mulch before applying new.

*Add a generous application of a nitrogen-rich fertilizer along with the new mulch layer. For organic growers, this could be blood meal.

*Use a one-inch layer of compost or shredded leaves, topped by a bare one inch of shredded bark mulch.

*Go cold turkey, and mulch only with aged manure, compost, leaf mould, or shredded leaves. Yes, you will have to weed a bit more often, but your plants will thrive hugely.

*Use coarse sand or pea gravel as mulch in rock gardens and around all plants requiring good drainage.

*Use the old-time remedy of fluffy oak leaves or evergreen branches to protect cold-sensitive plants.

*Refrain from cutting back perennials in the fall. Their dead stalks and leaves provide the best protection from winter's cold. (See Au jardin: Fall perennial garden cleanup: To do--or not?)

*Whatever you do, never mulch over the crowns of perennials; mulch around them instead.

That luscious eryngium-oenothera combination in the photo is a perfect example of plants that just say no to bark mulch. Bark mulch has never touched the beds at the Clos de Coudray perennial nursery and display gardens in Normandy, where the photo was taken.

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