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Make up Your Mind, Weather: Protecting Your Trees From Monsoons and Santa Anas

The weather in San Diego isn't nearly as unpredictable as it can be in other parts of the country. But it can still frustrate homeowners here because it tends to swing between opposites.

In summer, the weather shifts from cooler and foggier to suddenly very humid as the monsoon season starts up — and then it swerves in the other direction with the Santa Ana winds. Those bring super-low humidity levels and high heat, and all of that weather can take a toll on trees.

Luckily, San Diego is home to several tree species, both native and introduced, that thrive throughout this weird weather with proper help from you.

Competing Seasons

Summers in San Diego County generally see three distinct weather patterns. The first is a general summer pattern — hotter temperatures, fewer clouds, generally dry, as is typical of Southern California.

The second is the monsoon season, which doesn't affect San Diego like it does, say, Tucson. However, the county is on the outskirts of the North American monsoon, and the coastal and inland-valley communities get hit with uncomfortable humidity and low winds. The monsoon season lasts from mid-June to the end of September.

The third pattern is the Santa Ana, those winds that sweep Southern California year-round. They bring unseasonably hot and dry weather; in summer, the temperatures are scorching, the humidity is very low — single digits are common — and plants dry up quickly. It is, in effect, fire weather.

Because Santa Anas can appear at any point in the year, it's common to have a couple of weeks of humid monsoonal moisture followed by a sharp turnaround into Santa Ana weather, and then right back into humidity. For gardeners, these patterns make good tree care critical.

Damaging Trees

San Diego trees usually survive the season well; they just need help. During humid, windless weather, tree canopies that are very dense become incubators for fungal spores, with moist, warm air hanging out in the middle of the canopy. If there are any molds or other spores on the tree, the spores can multiply and damage the leaves and branches. The humidity also makes it harder for moisture from watering to evaporate.

When the Santa Anas hit, the weather seems to reverse. No clouds, severe dryness, and hot, hot weather. Trees are at risk of dehydrating as if they were in a drought. People now know to plant drought-tolerant trees, which helps to an extent, but the scorching weather is still difficult to handle.

Solving Problems

Pruning and trimming trees helps them survive the moisture, stagnant air, dryness, and heat that occur in summer. Pruning the canopy so that there are more ways for air to blow through the branches allows what little wind there is to push that stagnant, moist air out. The increased space also helps everything get a little drier, which is bad news for the molds.

Pruning also helps remove dead branches and old leaves. When Santa Ana season hits, that's less fuel in case a fire starts up.
Watering needs to be done in the morning so that most of the water seeps into the soil before evaporating as the temperature rises on hot, dry days. It can be done in the late afternoon, too, but don't water in the evening, especially when the weather is humid. Much of the water that hits the trees and surrounding grass (and that doesn't sink into the soil) won't evaporate properly overnight, and the moisture will promote more fungal growth among the roots and on the trunk.

Pruning and trimming should be done in late winter. If you missed that window, or you think there is a problem with the trunk or roots, contact One Tripp Tree Service for an evaluation.