When we stand back and consider how best to care for a tree, we often consider its height, branches, leaf health and moisture levels.
But we can’t neglect a crucial aspect—the mulch at our feet.
Mulch ranks right up there with planting technique, species selection, right tree/right place, monitoring for pests and target pruning as the essential, foundational themes of best management practices tree care. As such, it pays to consider all aspects of this fundamental subject in order to provide good tree care for clients.
Separate ‘T’ and ‘O’
Let’s first discuss where mulch belongs in the landscape. Our job is to create areas in the landscape that “look like what Mother Nature would have created on her own,” with natural mulch falling in place under trees – the natural mulch being the leaves, fruits, twigs and bark.
One of the basic principles of sustainable landscape design is to separate turf from trees and other ornamentals structurally, as a landscape designer puts pen on paper and draws in landscape beds with trees and shrubs. When done properly, there is no turf next to trees. Put differently, there are no trees swimming in an ocean of turf. They are separated, with mulch in place around the trees and shrubs, and the turf plants growing in a mulch-free sod.
Why is separation such an important tenet? There are several reasons.
First, trees and turf have different needs. Generally speaking, less water and fertilizer is needed for trees than for turf. Separation keeps the fertilizer and water away from the tree roots, to some degree; sure, there is overlap of the tree and turf roots, especially in mature landscapes as tree roots expand into the turf stands, but it’s best to try upfront.
Second, separating turf from trees eliminates turf maintenance near trees, thereby reducing mower blight to the trunk.
Third, as the tree debris falls to the soil surface or the applied mulch decomposes, it recycles nutrients back into the soil.
Fourth, there’s the benefit of increased aesthetics. Ask any landscape designer: It not only looks tidy, but separation also creates “mass/void,” a powerful and effective landscape technique.
Many, many types of mulch, both organic and inorganic, are available. Both have their place. Yet, many, if not most, arborists and horticulturists have a strong bias for organic materials. The main consideration is what’s best for the tree, but there are other factors:
Cost: Some materials are pricier than others, depending on source and the need for processing.
Delivery distance: The “buy local” movement has created awareness of this consideration, especially in an effort to reduce the consumption of fossil fuels required to move the product.
Ability to decompose: The potential for the mulch product to decompose into humus is a viable benefit for tree growth.
Need to replenish: The need to buy and apply more product over the seasons can be a drag on the bottom line.
Weed suppression: The capacity for mulch to suppress or prevent weeds from growing is a plus.
Water retention: Especially during the summer months and droughts, the potential for a product to retain moisture in the root zone is very beneficial.
Color: Largely a customer preference issue; it should be investigated upfront.
Heat sink over tree roots: The tendency for the material to intensify or soften summer and winter heat is an often overlooked consideration.
Pros and cons of organic
Like all other inputs for tree care, there are advantages and disadvantages when it comes to organic (derived from plant sources) and inorganic (usually rock or rubber) mulch.
Most commonly, the “pro” column for organic materials includes its ability to degrade and replenish the soil, moderate soil temperatures in summer and winter, conserve soil moisture, stability for walking paths, reduce radiated heat around plants and keep mowers/string trimmers away from tree trunks. On the down side, organic mulch can blow away, creating the need to add more mulch; hold too much moisture around roots; be hard to obtain; and be hideous in color, depending on the processor.
The positives of inorganic mulch include that it tends to stay put and is less likely to blow away to grassy areas, lasts longer in the landscape, can be neutral and pleasing in color and can be plentiful, depending on local sources. Unfortunately, it usually adds heat to landscape plantings, can be hard to walk on, is so-so for weed suppression and moisture retention, doesn’t degrade and improve the soil and can also be hideous in color.
Woody mulch vs. rocky mulch
The next question becomes: “Where should each be used?” It boils down to this:
Rocky: Install next to buildings in plant-free zones, where reflected heat may kill the plants, on certain slopes where light wood chip mulch may tumble onto hardscape surfaces and in windswept areas.
Woody: Everywhere else.
Depth and placement
The issue of mulch depth is really one of volume as well as thickness. For most trees, a 2- to 3-inch layer of mulch over the tree roots is a desirable guideline. Using this depth will produce the benefits discussed above.
Lesser amounts will benefit trees to a reduced degree, and greater volumes will actually cause a decrease in aeration and water penetration to the roots, much like the effects of a thatch roof – good for houses, bad for trees.
When it comes to placement, I always lean on my favorite mulch quote by Bruce Freidrich of The F.A. Bartlett Tree Expert Co.: “Mulch is a tree root treatment, not a trunk treatment.”
This important distinction serves to keep mowers and other turf maintenance equipment away from the trunk and tender bark, while preventing the negative effects of bark in close contact with the trunk, such as increased potential for decay and armillaria root rot. Excessive volumes of mulch placed on the lower trunk or root plate, also known as “mulch volcanoes,” may produce a small benefit in that, if young grounds maintenance workers are lazy or simply unaware, the mower they’re operating will tend to be stopped by the abundant mulch before they strike the trunk; but, in the long run, this benefit is far outweighed by the prevention of the other maladies.
In addition to the considerations of source, heat, weed suppression, availability, cost and moisture retention potential, the perception of the material by clients shouldn’t be overlooked. For example, if the name of a subdivision or office complex suggests an inorganic mulch, i.e. Quarry Creek Estates, then it just makes more sense to use inorganic mulch, at least in some places in the neighborhood or property. A campus such as Oak Woods Office Park would be a companion to this selection factor on the organic side.
With or without fabric
A final consideration for mulch utilization is the decision whether to install a plastic or geotextile fabric beneath the mulch itself. These products add very little benefit, if any, to a planting bed or around trees and shrubs. In fact, in many situations, as the mulch degrades and wind blows weed seed in, weeds often germinate right in the fabric or on top of the plastic. Actually, this also commonly occurs in planting beds with inorganic mulch.
If a tree or shrub is planted on a slope, a fabric may provide some capacity to hold the mulch in place, but, overall, it is considered an unnecessary cost. The worthiness of the cost of the product and hassle of installation, as well as the inconvenience of removal when the landscape is being renovated, is dubious.