California Avocado Commission — According to a new report released by the California Department of Food and Agriculture (CDFA) and the Climate Science Alliance, Southern California agricultural regions will face more climate extremes — notably higher daily maximum temperatures and more frequent and intense heat events — in the coming decades. As Dr. Ben Faber notes in a recent blog post, although avocado groves have been successful in California, the trees are “poorly adapted to high temperatures and low humidity.” Therefore, avocado growers throughout the state should be aware of the impacts of heat on their trees.
Avocado trees cool themselves by moving water through the roots and out their leaves through leaf pores known as stomates. As the water evaporates from the leaves, it helps maintain leaf temperature as well as prevent the trunk and branches from overheating. During a heat wave, an avocado tree tends to close its stomates to prevent excess water loss, which results in the leaf temperature increasing. In extreme circumstances, the leaves and stems can overheat and literally cook.
California avocado varieties have different sensitivities to heat. While ‘Pinkerton’, ‘Lamb Hass’ and ‘Reed’ varieties are less susceptible to heat, ‘Hass’ and ‘Fuerte’ are most sensitive; and ‘Sir-Prize’ and ‘Sharwil’ tend to be somewhere in the middle.
Protection Measures
The timing of heat waves in California tends to coincide with peak season — when trees are producing their summer flush. When the temperatures is 100˚F or above, the trees tend to drop their fruit and flowers, thus excessive heat can impact the following year’s bloom.
Well-watered trees will better tolerate the stresses imposed by a heat wave. Thus, growers should irrigate their trees in advance of a heat wave to ensure they are fully hydrated. Apply an additional 50% of the budgeted amount of water. If the heat wave lasts for several days, be certain to provide daily pulses of irrigation.
Because avocado roots grow near the surface of the soil, mulch also helps trees manage heat by keeping the soil (and roots) cool.
Cover crops also can aid in keeping groves cool. The cover crop can prevent water runoff, improve water retention in the soil, lower the air temperature in the grove and keep the soil cool. In addition, cover crops will help improve soil structure, suppress weeds, increase soil organic matter and provide a home to beneficial pollinators and insects.
Harvest During a Heat Wave
Best practice is to make every attempt to wait and harvest fruit when the temperature is below 90˚F. No fruit should be harvested when the temperature is above 95˚F.
When fruit is harvested during warmer temperatures, place the field bins in the shade to avoid exposing the harvested fruit to sunburn. If fruit is left exposed to direct sunlight, the sunburn will only show up after it has been packed. Bins should be covered with bin covers or light-colored tarps. And bins should be sent to the packinghouse promptly to avoid decreased fruit quality caused by water loss.
Heat Damage
If you see significant leaf drop in your groves due to excessive heat, the following actions are recommended:
- As soon as possible, whitewash branches exposed to the sun with special attention paid to branches on the west and south sides of the tree.
- Trees that lose a significant portion of leaves cannot efficiently move water, therefore restrict irrigation amounts to ensure you avoid creating wet, soggy conditions that can lead to root rot. It’s best to irrigate less frequently and with smaller amounts of water.
- Do not prune your trees — leave hanging leaves in place to protect the tree from sunburn. Once new tree growth has occurred (in the next 3 – 6 months), pruning can take place on living wood.
- Adjust fertilization as you would with a frost-damaged tree: reducing the amount of fertilizer until the tree is re-established. If you see signs of a particular nutrient deficiency, adjust fertilization accordingly.
For more information about managing heat in avocado groves, growers can view the following articles on the California avocado growers’ website:
