Early season hot spells are common in the central San Joaquin Valley. When daytime temperatures are above 90 degrees for several days in row in early May, the heat can cause serious damage to new seedlings and transplants, and also disrupt flowering and fruit set.
When temperatures are really high, above 100 degrees, the excessive heat can kill young plants, even those that are receiving what should be adequate irrigation, which keeps the soil consistently moist.
Soil that has been well-amended with compost or humus that contain beneficial micro-organisms will hold water better and longer than native soil. The micro-organisms also attach to root systems and improve the plants’ ability to draw up moisture.
Check soil moisture levels with your finger to avoid overwatering which also kills plants; allow soggy soil to dry slightly on the surface before watering new plantings again.
Fruit and nut trees will shed immature fruit and nuts if stressed by drought. Deep irrigate citrus and other fruit and nut trees during early season hot spells by letting water slowly soak the soil underneath the canopy for several hours. The soil should be moist to a depth of at least 8 inches all around the canopy. A bubbler attachment on a hose works well.
Provide shade for new transplants during hot spells. Use moveable market umbrellas and shade cloth structures to cast shade throughout the hot days.
Nighttime temperatures in May should drop low enough to allow new transplants to recover before the next hot day, but if plants are severely scorched or wilting seems permanent, pull out the poor things and wait until next year to try planting again.
Home gardeners will see the first signs of heat damage on their tomatoes. When temperatures are above 90 degrees, tomato flowers fall off.
Hormone sprays are not effective in improving fruit set during hot spells, but the old-fashioned method of shaking or tapping the tomato plants at midday when the flowers are open, does seem to help fruit set.
Choose tomato varieties that are more heat-tolerant for better success in our hot climate.
Beans also drop their flowers in hot weather. Bush beans finish their fruit set when the heat arrives and can be pulled out.
Pole beans are more vigorous than bush beans; flower production will slow or stop during the hot months but pole beans sometimes resume flowering as both nighttime and daytime temperatures cool in late summer.
Beans that are heat-stressed are very susceptible to pest insect infestations and to diseases carried by mites.
Apricots that are nearly ripe or ripe in May will develop pit burn when temperatures are above 104 degrees for several days. The inner portion near the apricot pit turns brown and gooey. Hybridizers have developed newer apricot varieties that are less susceptible to pit burn.
Early season hot spells are deadly to cool weather crops like lettuces and broccoli which bolt and set flowers (a plant survival mechanism) and to spring blooming annuals including pansies and snapdragons. Pull dying annuals out and add them to the compost pile.
Send Elinor Teague plant questions at firstname.lastname@example.org or email@example.com (“plants” in the subject line).