Frost protection for citrus
This article was first published Nov. 27, 2015.
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.
Leaves, green wood and fruit are more frost sensitive than hard wood. Ripe fruit is more cold tolerant than green fruit because sugars in the juice act like antifreeze and lower the temperature at which it will freeze. Fruit is slightly less cold tolerant than leaves, and trees with fruit are less tolerant than trees without fruit, so in addition to preventing damaged fruit, picking fruit before a predicted freeze will not only save the fruit, but will help protect the entire tree as well.
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