Winter has finally arrived in the north state! Winter temperatures can pose a threat to the fruit and foliage of citrus, as well as other succulents, tender perennials, tropical and subtropical plants.
The most common type of frost in California is referred to as radiation frost. Radiation frost occurs on cold nights when the air is clear and still. Soil, buildings, plants, and other objects at the earth's surface act as a heat reservoir by absorbing heat during the day. Heat is lost, or radiated, from the earth's surface into the atmosphere and plants are damaged when enough heat is lost from this reservoir to lower the temperature at the surface to below critical temperatures. Greater damage occurs with colder temperatures or a longer duration of cold. Temperatures can drop more rapidly when the air is very dry and the dewpoint is low. If the dewpoint is very low a freeze can occur without the formation of dew and frost. This condition is known as a “black frost.”.
Avoid planting frost-sensitive trees in the lowest areas of the yard or garden, as cold air tends to flow downhill and can accumulate in such depressions. Trees planted near structures or walls, particularly those with southwest exposure that absorb and retain reflected heat during the day, may be damaged less during periods of frost because that heat is released during the night. Fertilize and prune during the spring and early summer so that new foliage will have enough time to mature before the onset of cold weather. Bare, moist soil stores more heat during the day and radiates more heat during the night, so rake away mulch, ground cover, weeds and leaves from around the tree during cold weather.
Monitor weather forecasts and take note of how low temperatures will go, and for how long. When the weatherman warns of frost, there are several precautions you can take to protect your plants.
- Identify cold spots in your landscape by monitoring with thermometers
- Identify plants at risk: citrus, succulents, tender perennials, tropical & subtropical plants
- Have supplies ready: either sheets or blankets with stakes or a framework to hold covers off foliage, or frost cloth (which can lie directly on plant foliage); lights; wraps for trunks; thermometers.
- Wrap trunks of tender trees, using towels, blankets, rags or pipe insulation.
- Move potted plants to warmer spots next to the house or under a patio cover.
- Water plants and trees. Dry plants are more susceptible to damage and moist soil retains heat better than dry soil.
- Cover plants just before sunset to capture and hold in as much heat as possible. Remove sheets/blankets daily if it's sunny, to allow soil to absorb heat during the day.
- Add heat by hanging a 100-watt lamp designed for outdoors in the interior of the tree, or use a holiday string of outdoor lights (not LED lights, since they don't give off heat).
If the larger branches of a tree or plant sustain frost damage, don't prune away damaged wood or remove the plant too soon. Wait several months in order to assess the extent of the damage and allow time for possible recovery during warm weather. As new foliage begins to grow, frost-killed twigs and branches will be apparent and can be pruned at that time. Postpone heavy pruning until the following year to allow trees to regrow to the point where damaged wood can be clearly determined.
UC Master Gardeners of Butte County are part of the University of California Cooperative Extension (UCCE) system. To learn more about us and our upcoming events, and for help with gardening in our area visit our website. If you have a gardening question or problem, email the Hotline at mgbutte@ucanr.edu or leave a phone message on our Hotline at 530-552-5812. To speak to a Master Gardener about a gardening issue, or to drop by the MG office during Hotline hours, see the most current information on our Ask Us section of our website.
Freezing temperatures damage plants by causing ice crystals to form in their cells. Frost-damaged vegetation withers and turns a dark brown or black. Ice crystals can also form in citrus fruit, causing juice vesicles inside the fruit to rupture and the fruit to dry out. Although frost-damaged fruit is edible after a freeze, it is susceptible to decay and often becomes unusable soon after the damage occurs.
Different types of citrus vary in cold hardiness. Mandarins and kumquats are the most cold hardy varieties of citrus trees. Mature mandarin trees, for example, can tolerate temperatures down to 20° for 4 hours before serious damage occurs to the wood. For Meyer lemons, the critical temperature is 22°, while oranges can sustain serious damage to the tree after 4 hours at 24°. The critical temperature for grapefruit trees is 26°. Keep in mind that these are general temperature figures. Other factors may interact with cold temperatures to influence the extent of frost damage. For example, greater damage results when cold temperatures occur over a longer time period. Mandarin fruit will tolerate a brief exposure to 24°, but several hours at 26° will begin to freeze the fruit.
Where a tree is planted can affect whether or not it is damaged by frost. Trees that are growing in open areas are more likely to suffer frost damage. Cold air tends to collect in low areas, so avoid these sites as well when planting citrus. On the other hand, trees can benefit from reflected and reradiated heat if they are planted near walls or other structures.
Certain cultural practices can also mitigate frost damage. Healthy, well-irrigated trees are better able to endure freezing temperatures. The full, leafy canopy of a healthy tree also helps shelter the fruit. In addition, moist soil retains heat better than dry soil and can actually warm the air near the soil surface. It is also a good idea to keep the ground around trees bare during the winter months. Bare, moist soil is better able to absorb and reradiate heat than soil insulated with mulch, weeds, or other ground covers. Fertilizing and pruning both encourage new growth, and delicate new growth is far more frost-sensitive than mature foliage. Thus, pruning and fertilizing of citrus trees should occur in the spring and early summer in order to give the new vegetation time to mature and harden off before cold weather sets in.
A light bulb (100-watt, designed for outdoor use), or Christmas lights hung in the middle of a tree can provide frost protection if the lights are the “old-fashioned” incandescent type that actually release significant heat. When a hard freeze is predicted, watering all night with sprinklers can also protect trees from frost damage. This only works if water is applied continuously. If the water supply is interrupted, more damage may result than would have otherwise occurred. Additionally, don't forget that this practice uses a lot of water and is considered inappropriate for our currently drought-stricken dry California climate.
If trees do show signs of frost damage, resist the urge to prune out “dead” wood in the early spring. Allow a few months for the trees to recover with the return of warm weather. Once new growth appears, the actual extent of the frost damage will be apparent and frost-killed branches can be removed without accidentally pruning out living material.
For more information see UC ANR Publication 8100.
UC Master Gardeners of Butte County are part of the University of California Cooperative Extension (UCCE) system. To learn more about us and our upcoming events, and for help with gardening in our area, visit our website. If you have a gardening question or problem, email the Hotline at mgbutte@ucanr.edu (preferred) or call (530) 538-7201.
Freezing temperatures damage plants by causing ice crystals to form in their cells. Frost-damaged vegetation withers and turns a dark brown or black. Ice crystals can also form in citrus fruit, causing juice vesicles inside the fruit to rupture and the fruit to dry out. Although frost-damaged fruit is edible after a freeze, it is susceptible to decay and often becomes unusable soon after the damage occurs.
Different types of citrus vary in cold hardiness. Mandarins and kumquats are the most cold hardy varieties of citrus trees. Mature mandarin trees, for example, can tolerate temperatures down to 20° for 4 hours before serious damage occurs to the wood. For Meyer lemons, the critical temperature is 22°, while oranges can sustain serious damage to the tree after 4 hours at 24°. The critical temperature for grapefruit trees is 26°. Keep in mind that these are general temperature figures. Other factors may interact with cold temperatures to influence the extent of frost damage. For example, greater damage results when cold temperatures occur over a longer time period. Mandarin fruit will tolerate a brief exposure to 24°, but several hours at 26° will begin to freeze the fruit.
Certain cultural practices can also mitigate frost damage. Healthy, well-irrigated trees are better able to endure freezing temperatures. The full, leafy canopy of a healthy tree also helps shelter the fruit. In addition, moist soil retains heat better than dry soil and can actually warm the air near the soil surface. It is also a good idea to keep the ground around trees bare during the winter months. Bare, moist soil is better able to absorb and reradiate heat than soil insulated with mulch, weeds, or other ground covers. Fertilizing and pruning both encourage new growth, and delicate new growth is far more frost-sensitive than mature foliage. Thus, pruning and fertilizing of citrus trees should occur in the spring and early summer in order to give the new vegetation time to mature and harden off before cold weather sets in.
Frost-sensitive young trees can be protected by wrapping them with insulating material from the ground up to the main branches. In frost-prone areas, you might also consider covering trees that are one to three years old entirely, using a light material to trap heat being radiated by the ground. A frame or stakes should be used to minimize contact between the cover and leaves because ice may form where the cover touches any leaves. Sheets, burlap, or tarps are a better choice of cover material than plastic because plastic transfers more heat and if leaves come in contact with it they are more likely to freeze. The cover should extend as close to the ground as possible and be removed the next morning after the temperature rises.
If trees do show signs of frost damage, resist the urge to prune out “dead” wood in the early spring. Allow a few months for the trees to recover with the return of warm weather. Once new growth appears, the actual extent of the frost damage will be apparent and frost-killed branches can be removed without accidentally pruning out living material.
For more information download the free UC ANR publication 8100 on Frost Protection for Citrus and Other Subtropicals.
UC Master Gardeners of Butte County are part of the University of California Cooperative Extension (UCCE) system. To learn more about us and our upcoming events, and for help with gardening in our area, visit our website. If you have a gardening question or problem, email the Hotline at mgbutte@ucanr.edu (preferred) or call (530) 538-7201.