Healthy trees grow lots of branches and leaves — their canopy — as thickly as they can, to gather all the sunlight they need, Butler said. “Trees feed themselves through their leaves, so when you’re standing under a tree, you shouldn’t see a lot of blue sky. If you see a lot of blue sky, that’s an indication of a problem,” he said.
Another sign of trouble: bare branches sticking out of the canopy,
More likely than not, Rolshausen said, thinning canopy is the result of a soil-born disease such as phytophthora, aka root rot, that’s caused by excessive water.
“Homeowners have a tendency to over-irrigate a tree that’s not doing well, but soil-borne diseases actually thrive in wet soils, so that’s making things even worse,” he said. “Trees don’t like standing water on their root systems because they can’t breathe.”
Sprinklers that constantly hit tree trunks are another contributing factor, Butler said. People tend to plant lawns around trees, then water everything the same way. That’s detrimental to the trees, which need sporadic deep watering instead of the frequent shallow watering most people give their lawns.
“If you see green moss or algae on a tree in Southern California, it’s usually a sign that the tree is being hit by irrigation water,” Butler said. “Trees are not equipped to deal with sprinklers hitting their trunks all the time and it will probably make the tree sick.”
Sprinkler sprays can actually spread the disease to other trees, Rolshausen said; drip irrigation is better for trees. Water slowly and deeply along the drip line of the tree, then make sure the soil dries completely before you water again.