Applying organic wood mulch is the single best type of tree care.
If you do only one thing for your tree this year, apply wood chip mulch.
What is mulch and why is it good for tree care?
Any material placed on the soil to protect and cover it is classified as mulch. This can include stone, straw, shredded rubber, bark, pine needles, sawdust, and wood chips.
What is organic mulch?
Organic mulch can include shredded bark, wood chips, grass clippings, pine needles, dry straw, leaves, and other organically derived materials. The opposite, inorganic mulch, consists of things such as crushed rock, stones, polyethylene, etc. Organic mulch is preferred because the risk of soil pollution is eliminated and the material breaks down over time into substances that the tree can use.
Benefits of organic mulch
Organic mulch benefits tree health in many ways. The most immediate benefit of mulch is soil moisture conservation. A layer of mulch can reduce evaporation by as much as 73%. Water is also absorbed and held by the mulch where it is slowly released into the soil below.
Soil erosion and water loss are greatly reduced due to the slowing down of the speed of rain and irrigation water by the mulch. This allows the water to better infiltrate the soil and the mulch also conditions the soil to increase the infiltration rate leaving less water available to runoff.
Mulch reduces the competition of weeds for nutrients and water. A layer of mulch 3″-4″ thick will very effectively prevent weeds from becoming established. The lack of weeds can also decrease soil moisture loss as weeds increase evapotranspiration. This is especially helpful in any location with an arid climate, as we go through the annual hot & dry period for weeks at a time.
The fertility of the soil is increased by the presence of mulch. As soil organisms break down the mulch through their activities nutrients are slowly released into the soil as nutrients that trees can take up through their roots. This a better alternative to using chemical fertilizers as there is a much lower risk of these nutrients leaching into groundwater and causing pollution due to the rapid releasing of elements into the surrounding soil before the tree can uptake them.
Mulch improves soil structure by increasing the porosity of the soil. Porosity is the presence of small pockets of open spaces in the soil. These open spaces are filled with water and oxygen. Both of which are critical for healthy root systems. A soil with low porosity is far more difficult for trees to thrive in.
Soil temperatures are moderated by the layer of mulch making the soil cooler in the summer and warmer in the winter. Soils with a moderate temperature and good aeration are ideal for root growth. Healthy roots equal healthy trees. Mulch can also prevent roots from freezing during extremely cold weather.
Some soil-borne diseases can be reduced as well. Water drops can splash disease spores from infected soil to healthy soil. As discussed earlier, mulch reduces the splashing effects of water thereby reducing this risk.
This is important as certain tree diseases can be fatal and lead to tree removal.
How to apply mulch to trees
Mulch should be applied around the tree in a layer about 3″-4″ thick. Begin near the trunk as spread it outward as far as you can. It should be a minimum radius of 3 times the trunk diameter to be effective.
For example, if the diameter of the trunk is 3″ then the wood mulch should be spread in a circle with a radius of at least 9″. Make sure to avoid putting the mulch into direct contact with the trunk. The tree trunk cannot handle having the mulch in direct contact as this creates a dark, moist environment perfect for insects and diseases to get established and damage the trunk.
Mulch should be refreshed annually or even more frequently in hotter climates such as in Phoenix, AZ.
Tips for buying wood mulch
- Composted mulch is better for tree nutrition than raw wood chips or bark as it is already partially broken down
- Wood chip mulch lasts longer than composted mulch, is less expensive and can be dyed in many colors
- Avoid mulch with preservatives or fungicides added as this can be toxic to the tree and prevents nutrient breakdown
- Colored mulches are ok as long as there are no strong preservatives in the coloring
1. RUSSELL, J.C.. 1939. The Effect of Surface Cover on Soil Moisture Losses by Evaporation.Proc. Soil Sci. Soc. Amer. 4:65-70