How to avoid pitfall of too much mulch


January 05, 1991|By Amalie Adler Ascher

Mulching can be a tricky business. Like extra amounts of TC pesticides or fertilizer, overdoing a mulch can be too mulch, er, much of a good thing. Some observations on mulching from various sources are worth passing along, especially since the time is at hand.

As a summertime measure, mulch laid on flower or shrub beds chokes out weeds, removing them from competition with plants that then can gain full access to the supply of nutrients and water. Mulch holds down the evaporation of moisture from the soil, while keeping it cool and keeping plant roots from overheating. Organic mulches (as opposed to plastic sheeting) break down and combine with the soil, improving its structure and water-absorbing capacity. Spread early in the warm season, mulch keeps soil temperatures from rising.

Cold weather mulching operates in reverse.

PD Its purpose is to tuck in perennials that might otherwise suffer

damage if, through the freezing and thawing of the ground, their roots are pushed above the surface. Bulbs and rosebushes benefit from mulching, too. For plants in general, in fact, especially those that are young and sensitive or marginally hardy, a mulch shelter for their roots in a severe winter could offer them an edge on survival.

Spreading mulch too early in the fall slows the ground's freezing, and as a consequence could prolong the hardening off of plants and the wait for them to enter dormancy. That could do them harm. Late-season mulching, therefore, is timed to take place after the ground freezes.

Always mulch after a rain or a thorough watering to ensure that plant roots don't enter the winter in dry soil. Before spreading mulch, remove weeds and cultivate the soil to loosen it; apply any fertilizer or pre- emergent herbicides if you plan to use them; and blend old mulch still remaining into the soil.

As to the problem of using too much: The build-up of mulch to too-high levels is "becoming a national horticultural disaster," according to a recent report in The Leaning Post, the monthly newsletter of the Men's Garden Club of Montgomery County. The source of the material was an article by Dr. Frank R. Gouin that appeared in the Delmarva Farmer, an agribusiness newspaper for the mid-Atlantic region. Dr. Gouin is a professor of horticulture at the University of Maryland. The most common problems arising from overmulching, he says, are the suffocation of shallow-rooted shrubs and small trees such as dogwoods, azaleas, rhododendrons, hollies, yews and the like. Mulch lying up against woody stems for any length of time can bring on stem cankers, which lead to decay. Roots growing in mulch can die if the mulch is allowed to dry out.

The mulch around shallow-rooted trees and shrubs and the trunks of trees should be no more than 2 inches thick. (Around perennials, it may be an inch or two higher -- the measure taken after the mulch settles.) Old mulch can be reactivated to the proper level by fluffing it with a rake or a hoe.

The newsletter says Dr. Gouin is partial to fresh or aged pine bark or pine nuggets, or composted shredded hardwood bark mulches. Also pine needles and partly decomposed leaves from a compost pile. Avoid fresh wood chips, hardwood bark, sawdust or shavings; and pine bark containing a high percentage of fresh white sap wood. These materials will compete with plants for nitrogen. Dr. Gouin also discourages the use of peat moss as a mulch because it crusts, sheds water, and is very acid.

When to clear a winter mulch away from the base of perennials depends on the weather.

Denver horticultural consultant Jim Borland, writing an a fall issue of American Nurseryman, says that while it is probably a good idea to remove or push back mulch from herbaceous perennials in spring to enable new growth to emerge, in areas likely to be hit by late spring frosts, it might be just as well to leave the mulch in place.

Baltimore Sun Articles
Please note the green-lined linked article text has been applied commercially without any involvement from our newsroom editors, reporters or any other editorial staff.