How a heatwave can affect when your trees bear fruit
Q: My tangerine tree, which usually bears in January and February, fruited much later this year. Also, my pineapple guava, which usually bears in September, fruited early this summer. I haven’t changed my watering schedule. Did the early heatwave have anything to do with this?
A: Early extreme heatwaves are much more damaging than heatwaves that occur later in the summer. In the early spring, when most plants are coming out of dormancy, new leaves are tender and vulnerable. They are light green which means that they haven’t had a chance to produce enough chlorophyll to absorb intense sunlight. Think about how easily you sunburn when you go to the beach for the first time in the summer. Kind of the same idea.
If a heatwave comes later in the growing season, leaves are mature and darker green (hopefully!) and have developed a thicker cuticle layer (wax-like leaf surface). Although intense heat will still cause some damage, it’s not as bad as it would be in the early spring.
Intense heat can affect fruiting by heat stress, but there are other stressors as well. Water stress is one of the most obvious. When extreme heat is in the forecast, pay close attention to your soil moisture. Mulching generously with bark or other organic matter (not gravel!) will protect surface roots from intense heat and keep them evenly moist. Give everyone extra water, preferably in the early morning. This may not totally prevent wilting and stress, but it will go a long way.
For smaller trees and shrubs, shade cloth can help as well. In high-wind areas this may not be practical, to say the least.
Intense heat can affect fruit production in other ways. Pollen can be deactivated by high temperatures. Flowers are especially susceptible to heat damage and may simply drop without forming fruit. Pollinating insects are not as active during extreme heat and will retreat to cooler, moister areas during the day. During the last heatwave, I didn’t see any bees on our flowering sage. Instead, our slowly leaking hose bibb was covered with them. This was a reminder of how important it is to maintain a reliable water source for insects and other creatures during a heat wave.
Q: You mentioned in a previous column a remedy for citrus leaf miners. I forgot what it was, and now my trees are heavily infested with them. Could you refresh my memory?
A: Citrus leaf miners cause squiggly lines inside of citrus leaves. This can result in curling in the affected leaves. In the case of minor infestation, this is not a huge concern. In major infestations where a lot of leaves are distorted, especially in young plants, it can affect the overall health of the tree. Since these pests reside inside the leaf tissue (and not on the surface) a soil drench is the only way to get rid of them. Soil drench pesticides are systemic, which means that all of the plant will be treated. In order to protect the bees, only apply it after the flowers have dropped. Imidacloprid (Bayer Advanced Fruit, Citrus, and Vegetable Insect Control) is very effective for major infestations.
Looking for more gardening tips? Here’s how to contact the Master Gardener program in your area.
Los Angeles County
mglosangeleshelpline@ucdavis.edu; 626-586-1988; http://celosangeles.ucanr.edu/UC_Master_Gardener_Program/
Orange County
ucceocmghotline@ucanr.edu; 949-809-9760; http://mgorange.ucanr.edu/
Riverside County
anrmgriverside@ucanr.edu; 951-683-6491 ext. 231; https://ucanr.edu/sites/RiversideMG/
San Bernardino County
mgsanbern@ucanr.edu; 909-387-2182; http://mgsb.ucanr.edu/