An example of Armillaria, which can be fatal to your tree. (Photo by Laura Simpson)
Q. Do you have any advice on how to properly care for a mature tree?
Mature trees do not require as much attention as young, newly planted trees, but they do need some maintenance.
In Southern California and other dry climates, supplemental watering (especially during the summer months) is highly beneficial. Mature trees should be watered in the early morning to avoid water loss to evaporation. Water at the drip line, which is the edge of the canopy. This means that if your tree has a canopy spread that reaches 15 feet (for example) from the trunk, place a slow-running hose 15 feet from the trunk and let it run until the water has penetrated to about 9 inches below the soil surface. Move the hose occasionally so all the roots get watered. The amount of time it takes to allow the water to reach deep enough depends on your soil conditions, but you can use a soil core sampler to measure water penetration.
Never water near the base of the trunk and don’t let sprinkler overspray wet the trunk. Wet bark can encourage fungal growth, particularly armillaria, which can be fatal to your tree. Keep grass away from the trunk as well. Grass roots are extremely dense and will compete for water and nutrients that you want to go to your tree. Additionally, trimming the grass away from the trunk with a string trimmer can cause bark damage and girdling. Don’t let ivy or other vines grow up the trunk, since that can cause girdling as well.
Pruning a large mature tree does not have to be done frequently, but there are some situations that may require it. Dead or diseased branches should always be removed. Sometimes pruning is needed to correct a weight imbalance, particularly when there is dense growth at the far end of a branch (this is called “end weight reduction”). Pruning to open up the canopy should be done in areas prone to high winds since a lollipop-shaped tree can catch the wind and break in half. Ideally, this kind of pruning should be done early in the tree’s life so the main structure can start off balanced and open and only minor corrective pruning will be needed as the tree matures.
Never, ever top a tree! This results in an ugly, misshapen, and weak tree that will grow to be dangerously top-heavy. The stumps where the branches used to be will produce a tremendous number of succulent, dense sprouts in an effort to recover from the butchery. No amount of corrective pruning will restore the tree to a stable, healthy shape. If the tree breaks, it could cause significant property damage (or worse).
Removing large branches from a mature tree can be very dangerous, so I strongly recommend consulting with a certified, licensed, and insured arborist. These professionals can examine your tree and offer competent advice on keeping your tree healthy and safe, unlike some random dude with a chainsaw in a home center parking lot.
Los Angeles County
mglosangeleshelpline@ucdavis.edu; 626-586-1988; http://celosangeles.ucanr.edu/UC_Master_Gardener_Program/
Orange County
ucceocmghotline@ucanr.edu; http://mgorange.ucanr.edu/
Riverside County
anrmgriverside@ucanr.edu; https://ucanr.edu/sites/RiversideMG/
San Bernardino County
mgsanbern@ucanr.edu; 909-387-2182; http://mgsb.ucanr.edu
