March and April are the months we see the most mulch work done around the county. Mulch can have great benefits retaining moisture, and preventing weeds from taking over, but there are limitations to mulching. It is ultimately a solution for a problem we’ve created, and this post goes through whether you need mulch, and, if so, how to best use it.
Do I need to apply mulch to my trees?
In a natural setting, trees provide their own moisture retention and weed control through the leaves they drop. Even evergreen trees lose their needles, and provide a bed of leaves underneath their canopy, closing the circle for the nutrients lost creating those leaves, and providing that extra layer of protection for the trees. Here are some steps to take to decide if you need mulch:
- If the tree is in a natural area, and has a healthy leaf layer underneath it, mulch is not needed
- If the tree has significant space without lawn underneath its canopy, and leaves are recycled into the soil, mulch is probably not needed.
- If the landscape asks for a very manicured look, with leaves removed, and the tree has limited growing space, it may be beneficial to apply mulch, to simulate the natural setting of nutrient regeneration.
What kind of mulch should I use?
Different trees have different preferences for soil nutrients, acidity, and other soil characteristics. This makes proper mulch selection dependent on the soil needs and tree species.
Trees evolved with leaves under their canopies, and while different leaves decompose at different rates, this is a great mulch to use for tree health. Using a mulching mower on the leaves already there or buying leaf mulch of similar species can be a great way to improve the soil health. If you are using a non-specific source of leaf mulch, be aware that this may not always be compatible with the tree you are trying to help out. Oak leaves, for example, can be very acidic, and if applied repeatedly, may alter soil pH. This is one of the cheapest and best solutions for tree health, but regular application is needed, and without maintenance, the leaves may spread beyond the landscaped area. This can be prevented by combining leaf mulch with other types of heavier mulch, or expanding landscape areas.
Pine straw mulch
An option in between the natural look of leaf mulch, and the more manicured look of wood mulch is pine straw. While this is essentially similar to leaf mulch in quality, it provides a different look to the landscape. Applied in the right amounts, it lasts for a long time, with little need to reapply.
Shredded wood mulch
Shredded wood mulch typically comes in hardwood and pine bark mulch. Both have their value, and stay around for a long time. However, wood mulch can often leach nitrogen and other nutrients from the soil, so take care with applying this. If the tree is already struggling because of nutrient deficiencies, this may not be appropriate. Combining wood mulch with compost or leaf mulch may alleviate some of this, but unless you require the look of wood mulch, applying leaf mulch might be more appropriate. It is more readily available, so in the absence of the other solutions, this may work.
While technically all mulch is “alive” with bacterial activity, planting non-invasive groundcovers underneath trees can be the best solution to your landscape, while also enhancing its aesthetics and function. Ferns and low running plants are common in trees’ natural settings, as well, and provide the same values of mulch, without the need for constant reapplication. Be careful installing plants around trees, however, as you may damage root systems.
While not necessary a traditional mulch, compost can be used with the other mulches, or by itself, to reintroduce lost nutrients from lost leaf cover, or other sources of stress.
How to apply mulch
One great way to remember how to apply mulch is the 3-3-3 rule, popularized by Casey Trees. Apply a ring 3 feet in radius, 3 inches away from the trunk, no more than 3 inches high.
Unfortunately, some practices have gotten ingrained in the landscape industry that will harm mulch. Aside from applying mulch when not necessary, and excessively removing leaves, which are often driving by aesthetic preference, there is also a strong tendency to hide the roots of trees. This creates a practice known as volcano mulching, and can cause trunk infections, unstable trees, and ultimately the premature death of trees. Following proper mulch application guidelines can help prevent this problem. One example is shown on the right, where urban foresters recently found a newly planted tree, which had had mulch applied for 4 inches above grade, already showing signs of fungal growth.
Rage against the Mulch volcano:
On leaf mulching:
Information on other types of mulch and groundcover and their benefits and drawbacks:
Find where to get mulch for pickup and delivery at the Arlington County Trades center: