Q: Could you provide some basic guidance on how not to kill a tree? I see conflicting information everywhere!
Unfortunately, there are many ways to kill a tree, so I will try to cover the most common ones here.
Incorrect pruning, particularly “topping,” is the surest way to ruin a tree. If it doesn’t kill the tree outright, it will result in extremely dense growth at the ends of the chopped-off branches. This makes the tree top-heavy and unstable, resulting in catastrophic breakage. Never, ever, top a tree!
Allowing the tree to grow two or more codominant leader branches with a narrow crotch angle produces an unstable overall structure as well. The tree can split down the middle as one of the large branches breaks away. Prune the tree when it’s young to achieve an open branch structure.
Ignoring broken branches or crossing/rubbing branches can leave a tree vulnerable to insect infestation. Avoid using sealant on the pruned site since this can hinder healing.
Girdling is another way to kill a tree, and often this is done inadvertently. Remove tree stakes and supports once the tree is stable. Trunk wraps should not be left in place for extended periods of time. If the wraps were there to protect against sunburn, consider instead painting the exposed trunk with dilute white latex paint (1 part paint to 1 part water). Don’t allow vines to wind around the trunk. Although it may look attractive, they can girdle the tree. Don’t use a string trimmer around the base of a tree. This is a very common cause of trunk damage and tree death since it both damages the trunk (opening the way for insects and disease to enter) in addition to girdling.
Avoid using weed cloth or plastic sheeting as weed barriers over the root zone of the tree. These can prevent water and air from reaching the roots. They can also encourage damaging fungal growth. Organic mulch is fine, but don’t allow it to touch the base of the trunk since it tends to hold moisture and foster fungal growth.
When a heat wave is in the forecast, prevent water stress by deep watering well before the temperature rises. Overwatering a drought-stressed tree can result in trunk splitting as the thirsty tree tries to take in as much water as possible all at once. As you can imagine, a split-open tree will not last very long.
Avoid planting in areas with poor drainage. When digging a planting hole, fill it with water and note how long it takes for the water to drain. If the hole is still full of water days afterwards, you may want to replace the dirt and find another site.
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; 951-955-0170; https://ucanr.edu/sites/RiversideMG/
San Bernardino County
mgsanbern@ucanr.edu; 909-387-2182; http://mgsb.ucanr.edu