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In My Garden Blog

April 10, 2008
New England
By Kathy Bond Borie,
Richmond, Vermont

Keep mulch away from the trunk and don't cover the root flare, where the roots begin spreading out from the trunk.

Ode to Mulch

Spring is the time of year when a gardener's fancy turns to thoughts of ... mulch. The semi-annual ritual of mulching may not be as high on the list as romance, but it certainly is as much a rite of spring, giving gardeners SOMETHING to do in the garden when we're itching to become reacquainted ... with our plants, that is.

Mulch can do your plants a world of good -- or kill them. Simple as that. Poorly piled mulch is a pet peeve of mine, but more on that in a minute. When it comes to how thick mulch should be and what type of mulch is preferable, there's no "one-size-fits-all" approach that's appropriate for all plants. So as you're raking off the old and spreading the new, consider the needs of different types of plants as well as our personal preferences.

If you've been reading gardening books for a number of years, you may have run across the classic ode to mulching by the Grand Dame of "I'll-do-it-my-way" gardening, Ruth Stout. In her 1971 work, The Ruth Stout No-Work Garden Book, which she wrote at the spry age of 85, she attributed her gardening successes to a healthy dose of ignoring authorities and to an 8-inch layer of mulch. Boasting of gardening from her couch, she eschewed plowing, spading, tilling, cultivating, and any manner of weeding. For her, mulching was the only effort she needed to expend in the garden because it made those other practices unnecessary.

I'm a great fan of newspaper topped with straw in the vegetable garden, and the thicker the better. But the idea of mounds of hay surrounding my ornamentals jars my aesthetic sensibility, and besides, things get a little more complicated in perennial beds, around trees and shrubs, and especially around fruiting plants.

Perennials
The common practice of mulching perennials in the fall helps mainly by providing winter protection for the crowns of the plants, which give rise to new growth in spring. But many plants -- especially those susceptible to rotting in cold, wet soil (such as agastache and lavender) -- can be killed by mulch if it suffocates the crowns. The looser the mulch, the better, so it doesn't compact and limit air circulation or hold too much moisture. Good choices are bark chips (they compress less than shredded bark or wood chips), hay, straw, or evergreen boughs. Avoid maple leaves, which compact readily, and shred any other leaves so they don't mat. You can use a gravel mulch, which won't hold water; even better, grow rot-sensitive plants in gravelly soil year-round.

Come spring, remove the winter mulch because it can harbor disease organisms that have overwintered in the soil. Removing the mulch allows the soil to warm up faster, encouraging plants to break dormancy, so don't be too eager to expose the crowns if there's a chance of more snow or frigid temperatures to come. The alternate freezing and thawing of soil in late winter/early spring can easily kill plants that survived the worst of winter. After raking up the old, spread fresh mulch 2 to 3 inches thick.

Trees and Shrubs
These plants have the same aversion to mulch touching their trunks or stems as perennials do to compacted mulch on their crowns. Proper mulching is especially important in fall because you don't want to give rodents a hiding place where they can tunnel right up to the tree trunk and chew away all winter. Even in summer, you want to leave a couple of inches of mulch-free zone around the base of the plants.

In spring, you can just add some new mulch to the old unless disease was a problem last year. Then it's best to remove the old before spreading the new.

This brings me to my pet peeve: volcano mulching. You know, those trees with mulch piled up against the trunk like volcano cones? I have to resist the urge to screech on my brakes, take out my rake, and level out these mulch piles whenever I see them. Which is often. These cones direct the water away from the tree so the roots don't get the water they need, and neither do they get the needed air because a big pile of mulch is inhibiting the flow. Air can penetrate through a moderate thickness of mulch, but when the root flare (where the roots spread out at the base of the tree) is deeply buried, the roots can suffocate. This is why excavation projects that raise the soil level around a tree will eventually kill it.

Research at Ohio State University demonstrated that wood or bark mulch deeper than 2 inches can harm a tree because the fungi that decompose the mulch can create a dry, water-repellent layer. Also, the smaller the pieces of mulch, the faster they will decompose and temporarily use up the nitrogen in the soil, leaving less for your plants. So a 2-inch layer of bark chips spread in a flat ring around -- but not touching -- your trees is the best approach. Besides, then the mulch police will leave you alone.

Fruits
Here we do an about-face, at least when facing blueberries. Blueberry growers in the mid-Atlantic region have been greatly increasing their yields by spreading 6 to 12 inches of hardwood sawdust around plants. The theory is that the roots grow vigorously into the lower layers of mulch, and a better root system can support a bigger crop of berries. The top layers of mulch protect the roots from cold damage, although it's not clear whether that would be enough to protect plants in the deep freeze of northern New England.

Sawdust is the preferred mulch because blueberries need its acidity, but you also can use other organic materials along with a high-acid fertilizer. I'm going to try this approach this summer on my small blueberry patch.

Grapes are another fruit that benefits from an acidic mulch, and strawberries like a slightly acid soil (pH about 6-6.5). Strawberries need a loose mulch, such as hay or straw, during the winter to protect the crowns from the cold without smothering them. Apply enough to give a 3- to 4-inch blanket after it settles. Remove the old mulch in spring and add new material.

Raspberries and other brambles will sucker more if they are mulched in between the rows, so you can avoid the problem by mulching only around the plants. Spread 8 to 12 inches of straw or 3 to 4 inches of wood chips. Add some extra nitrogen to make up for that used in decomposition of the mulch.

Now, the Caveats
Mulching to conserve soil moisture is most valuable on well-draining soil, more problematic on poor-draining soil if it keeps the soil too wet. Of course, the best solution would be to improve the soil because plants won't grow well there anyway.

Also, slugs adore mulch. Encircling plants with a necklace of sharp sand, diatomaceous earth, or wood ashes can help keep them away.

Although  volcano mulching is indeed a tragedy, the point the author
makes about mulch volcanos diverting water from a tree's roots is
somewhat misguided. 

A mature tree's roots can grow up to three times the width of its
canopy (or drip line, the very edges of the branches). Unless a
mulch volcano is wider than the tree's canopy (that's a lot of
mulch), it would not divert significant amounts of water from the
roots. In the case of a recently planted tree, however, the roots
are likely to be contained to a much smaller area (the size of the
pot or burlap ball that the tree arrived in); watering just beyond
this area will encourage roots to grow farther as they seek out the
water. 

Hello Annie,
Thanks for writing. Yes, tree roots can spread quite far from the
trunk, and the outer root zone is where watering is most important
in established trees. Newly planted trees with a more concentrated
root ball are the ones more at risk from poor mulching practices
that divert water away from the roots. In our area, arborists often
place watering bags around newly planted trees that surround the
trunks and provide slow watering to the root zone.       

Hi Kathy,
I've read a lot about the perils of volcano mulching, but not much
about remediation.  We've bought a house with a large 30+ year old
Norway maple that has been volcano mulched (in fact a 1-2 ft
retaining wall kept everything against the trunk) for many years.
After reading about the perils of volcano mulching, we've removed
the wall and most of the soil.  Will this damage the large mass of
now exposed roots and eventually the tree?  So far, besides the
exposed circular root mass, the other parts of the tree looks
healthly.

Hello Patrick,

If a large mass of roots are suddenly exposed, that could put stress
on the tree. It might be a good idea to cover the roots with a few
inches of mulch but just don't bring it all the way up against the
trunk. The surface of the soil around the trunk should be about
level with or just below the root flare -- where the roots begin to
spread out from the trunk. With such a dramatic grade change,
though, your best insurance would be to consult an arborist. Good
luck. 

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