Mulching trees maximizes water uptake
We all have seen and felt the beauty and power of forests! No one is there to water or feed; it is more like strength in numbers! By contrast, our urban trees have turf to compete with. When trees and shrubs are planted in turf/grass, competition for nutrients, water, oxygen and space occurs below ground between turf roots and woody plant roots. Turf wins because its dense fibrous root system prevents woody plants from getting their feeder roots into the top few inches of soil.These shallow roots on a tree or shrub are called feeder roots because they are specialized for absorbing oxygen, water, and minerals from the soil. These feeder roots are concentrated in the upper soil layers where oxygen, water, and nutrients are most abundant, therefore making feeder roots the main source for the plant to receive these elements. The oxygen is needed for root respiration. Without the oxygen, roots will die off. The water and minerals are needed for processes like photosynthesis, which is the foundation of the tree’s life. As a result, the growth, health and sustainability of the trees and shrubs is compromised in turf areas.
We can give the trees something that forests naturally give the trees — decomposting leaves and wood around the base of a tree — and it is called mulch! Mulch provides nutrients as the mulch decomposes, decreases water evaporation from soil thus preventing roots from drying out and dying off. In summary mulch optimizes root production, water uptake, and establishment of newly planted trees and shrubs. The more similar the mulch is to a forest floor, the better it is. For more detail, check out this technical report on Fresh Woodchip Mulch (PDF) from E. Thomas Smiley, PhD, Urban Forestry, of Bartlett Tree Research Laboratories, the research wing of Bartlett Tree Experts.
Never do the “volcano effect” as it puts the mulch in contact with the tree bark, rotting the tough outer bark and making the tree vulnerable to rotting, insects and suffocating the feeder roots at the base of the tree. For more information on “volcano mulching,” refer to Understanding Volcano Mulching.
Refer to How to Plant a Tree for more on mulch when planting a tree.
Mulch:
- Decreases water evaporation from soil.
- Serves as a sponge that prevents runoff around plants growing in heavy clay soils or on sloped sites.
- Helps to control seed germination and growth of weeds.
- Insulates soil and buffers extreme summer and winter soil temperatures.
- Reduces soil compaction from mowing equipment.
- Prevents damage to stems and trunks by lawn mowers and weed cutters.
- Improves soil health (increases microbial activity, nutrient- and water-holding capacity, soil pore spaces, and air penetration) as it decomposes.
How to Mulch an Established Tree
Remove grass and roots in the area around the base of the tree to the width you will be laying the mulch. Ideally it would be best to take mulch to the drip line, but in an urban area this may be unrealistic.
Make sure the dirt is smooth and even as uneven causes puddling of water resulting in uneven watering of roots.
Cover with about 2 to 3 inches of mulch, making sure there is a gap of 3 to 6 inches between mulch and the trunk of the tree. Arranging the mulch in this donut hole design will prevent smothering the root collar area as well as rot and disease. All roots need oxygen and the root collar area is where most of the oxygen and nutrients are concentrated. The root collar area is where the root flare originates. Covering the root collar area with mulch will significantly affect the health of the tree. Imagine if you got only 30% oxygen when you breathe.
Lay the mulch sloping away from the root collar. Make the mulch thicker on the perimeter of the mulch donut hole to help protect the trunk from damage by mowers and weed eaters. If you dig a 2-4″ deep trench around the outside of the ring, the mulch will naturally fall into it. Avoid using the plastic edging because when roots hit the plastic edge they interpret it as a barrier, and begin growing in a circle. This can lead to girdling roots, which strangle the tree and prevent sustainable outward growth.