Garden Q&A

Question:

We have fir and ornamental flowering trees at our Chester Springs house. What is the proper way to mulch around the tree trunks? Our yard service adds new mulch on top of the old each year, and some of the mounds are a foot tall.

- Jane Schwartz
Answer: A foot tall? No, no, no.

Why do we mulch around trees? The most important thing is to conserve moisture. Next is protection of the trunk from lawn mowers and string trimmers (yes, string trimmers constantly hitting the bark of a tree trunk can cause injury). Then come the goals of weed suppression and tidiness. Note the word suppression. Mulching is not a weed-elimination concept, no matter how much you or semi-trained services may want it to be. Any mulch that is thick enough to eliminate weeds is so thick as to cause other problems.

The size of the mulched circle is not critical - 4 to 6 feet in diameter is good. With a very young tree, it should extend at least to the drip line (the perimeter of the tree's branch spread), since that is roughly the extent of the roots, and moisture conservation for young trees is all the more important.

It is the shape of the mulch application that matters the most. It should be no more than 4 inches thick. Right at the trunk, the mulch must not cover the flare - the very bottom of the trunk that gets wider. At the point where the flare and soil (soil, not old mulch) meet, the thickness of the mulch layer must be less, preferably about 1 inch. Thick mulch against the trunk invites pathogens and burrowing, gnawing critters like voles to start gnawing where you can't see them. At the perimeter of the mulch circle, a raised ring of mulch - sort of a doughnut - will help keep rain (and sprinkler irrigation) at the root zone.

The worst shape for mulch is a cone or volcano that is thinnest at the perimeter and highest at the trunk - that just makes the water go elsewhere and makes the pathogen problem worse.

Mulch piled upon mulch annually - particularly the shredded bark or licorice root so loved by contractors - will compress and form a water barrier, and/or become an oxygen-depleted mass that really encourages growth of unwanted organisms. The mulch must be loose enough that water passes through it quickly to reach the soil of the root zone. Do a test; with a spray attachment, turn the hose on your copious mulch for three minutes or more. Wait a minute, then dig in and see if any of that water has soaked into the soil. Odds are the soil is dry.

I have found that spreading a pre-emergent weed control (corn gluten is an organic product) on top of the loose mulch will help substantially in keeping new weeds from sprouting. (Ahem, the mulch zone needs to be weeded, by hand, before the mulch is applied.)

Teach all this to your lawn service - maybe other customers' trees will be liberated as well.

Q: How are the new disease-resistant American dogwoods holding up? I refuse to plant any until I hear the new ones have survived the last few years.

- Ann Hozack
A: The most serious affliction of the native Eastern flowering dogwood (Cornus florida) is dogwood anthracnose, which took a huge toll in the 1990s. Powdery mildew can be a problem, and though it is unsightly, it's rarely fatal.

The cultivar 'Appalachian Spring,' which has an upright growth habit and prolific blooming, was found growing wild on Maryland's Catoctin Mountain and is considered anthracnose resistant.

Beyond the straight species, there is a resistant hybrid of C. nuttallii, the Western flowering dogwood, and C. florida. The cultivar is 'Eddie's White Wonder,' but it is rarely found in the Eastern states, even though it is hardy in Zone 6. (The Philadelphia area roughly straddles the Zones 6-7 boundary.)

The other resistant hybrids are combinations of the Asian C. kousa and C. florida, which makes them half-American. They bloom later than C. florida, and foliage is emerging at the same time, as opposed to the native trees' habit of blooming before the leaves develop. Several of these hybrids were released by Rutgers a number of years ago and are widely available. Look for 'Ruth Ellen,' 'Constellation,' 'Aurora,' 'Galaxy' and 'Stardust.'

An excellent pamphlet published by the University of Tennessee, "Dogwoods for American Gardens," can be found at www.utextension.utk.edu/publications/pbfiles/PB1670.pdf.