Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
1/22/01
Contacts:
Mark Gleason, Extension Plant Pathology, (515) 294-0579,
mgleason@iastate.edu
Elaine Edwards, Continuing Education and Communication
Services, (515) 294-5168, eedwards@iastate.edu
Yard and Garden Column for the Week Beginning Jan. 26
Using Organic Mulches in the Landscape
By Mark Gleason
Extension Plant Pathologist
Iowa State University Extension
As December's lovely snowfall slowly congeals to crusty sludge, one thing is clear: Iowa's drought is not over yet. Rainfall has been below normal over much of Iowa for most of the past 18 months. And meteorologists and other seers are predicting more of the same.
Some Iowa communities imposed watering restrictions last year, and more will do so in 2001 if weather patterns don't change. Now, during the frozen-dog days of winter, is a good time to develop a plan for fighting drought stress on trees and shrubs during the 2001 growing season.
Landscape mulches deserve to be an important part of the plan. Mulching is a practical way to help woody plants survive and thrive in urban landscapes, and its popularity is increasing rapidly. Although recent research has taken some of the guesswork out of using mulches safely and effectively, misuse of mulches remains common.
The benefits of organic mulches are many. For one thing, organic mulches around trees and shrubs mimic natural environments. In forests, for example, a moist layer of decaying leaves, twigs and branches typically covers the forest floor. Most of our woody landscape plants evolved in forests. Mulching landscape trees and shrubs can restore some aspects of the forest's root zone environment, even in a sun-baked Iowa yard distant from any forest.
Since wood chips and other organic mulches hold water like a sponge, underlying soils tend to be moister than unmulched soils. So more water is available to tree or shrub roots, especially in dry weather. A steady water supply encourages good root growth, and good root growth means better tolerance of drought periods.
The value of mulching is backed up by practical research. Multi-year studies at the Morton Arboretum in Illinois and at Cornell University in New York clearly showed increased soil moisture, increased root growth and better establishment of young trees under wood chip mulch than under bare ground.
Mulched zones around woody plants may be at least as valuable for what they keep out as for what they hold in. In the Midwest, trees and turfgrass often do not coexist very well. Because of their greater density and close proximity to the soil surface, turfgrass roots have a competitive advantage over tree roots.
Turfgrass competition for soil water becomes especially acute during drought periods. Many homeowners realize from personal experience how challenging it is to maintain high-quality turfgrass under the canopies of mature trees. Often the best solution is to buffer trees and shrubs with a mulched zone extending from near the trunk out to the drip line (the area under the tree directly below the canopy). That means sacrificing some grass to safeguard your trees' health.
Another advantage gained from separating trees and turf is prevention of mower blight (mower collisions that damage trees' bark and cambium), which frequently reduces the vigor and life span of landscape trees. And the wider your mulch ring, the less lawn you'll have to mow every week.
How much mulch is enough? In Iowa, a depth of 3 to 6 inches after a few days of settling will not only encourage steady root growth, but also suppress weeds. Add more mulch every few years to keep pace with decay and maintain the thickness of the mulch layer.
Which organic materials make the best mulch? The answer will depend on your own situation and preferences. Wood chips make superb mulch, and they are often available free or at low cost from towns or companies that generate chips from utility line-clearance work. Cypress bark, pine nuggets, or other decorative woody mulches are more costly, but you are paying for a prettier mulch, not a more effective one.
Organic mulch is not a free lunch for your trees and shrubs. There are a few potential risks or drawbacks associated with mulch. Fortunately, they can be avoided easily with a few precautions. Let's look at a few of these problems and their solutions.
Can wood chips from a sick tree spread disease to other plants? It's very unlikely if you take a few simple precautions. First, keep the mulch about one foot away from trunks of trees or shrubs. Likewise, don't mix chips with soil when transplanting; instead, keep the chips on top of the soil surface. These measures prevent direct contact between any disease organisms and your plants. And if you pile fresh chips for a month or more before use, any disease organisms will almost certainly die off.
"Sour mulch" smells like ammonia. It can result when chips have been stored in a poorly drained location or in piles higher than 4 to 6 feet. The problem is a lack of oxygen during chip decay. The remedies are clear: avoid using mulch that smells like ammonia, and store chips in ways that avoid the risk of sour mulch.
Some people think that mulch attracts rodents. Jim Pease, Extension Wildlife Specialist at Iowa State University, says that the only concern is to avoid mulches that contain seeds (some types of hay, for example). No seeds, no rodents - and no rodent injury to tree trunks. As an added precaution, keep the mulch one foot away from the trunk.
Does mulch rob nitrogen from the soil? This won't happen to any extent unless that mulch is fresh wood or leaves. With this kind of mulch, the best advice is to pile the mulch for at least a month before use, to let some decay happen. Partially decayed or coarse-textured mulches break down too slowly to tie up much soil nitrogen.
Most people don't realize it, but handling mulch can trigger an allergic response in some people. About 1 in 20 people are allergic to the spores produced by specialized bacteria, called thermophilic actinomycetes, which colonize piles of decaying wood chips. If these rare individuals inhale the spores while shoveling chips, they can experience temporary lung congestion. The solution? Wear a pollen mask (about $2 at any drug store) while shoveling mulch.
The common thread in all these mulch "problems" is that they can be avoided easily once you are armed with a little mulch know-how. If your trees and shrubs could talk, they'd all be enthusiastically in favor of mulching. It's the cheapest, most effective and most environmentally sensible way to keep your woody landscape healthy in Iowa's drought-prone climate.
ml: isugarden
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