What are the care methods for citrus trees after freezing damage

After a citrus tree is damaged by freezing, the aboveground part suffers varying degrees of injury or dieback, while the underground part is generally less damaged. Post-freeze care measures mainly make use of its relatively intact root system to promote the sprouting of new branches, restore the canopy as quickly as possible, and gradually achieve the goal of normal growth and fruiting. The specific measures include the following aspects.

1. Treatment of the freeze-damaged canopy. According to the severity of the freeze injury, and following the principle of promptly cutting off the freeze-killed parts while retaining as much healthy tissue as possible, the canopy should be treated at the appropriate time and to the appropriate degree:

(1) For mild freeze injury with leaf curling, part of the curled leaves should be pruned off in time, and about 0.3% urea solution should be sprayed 2 to 3 times to prevent increased leaf drop.

(2) If the leaves are freeze-damaged and withered but remain attached without falling, while the twigs have not died, the withered leaves will continue to consume water and expand the freeze injury, so the dead leaves should be knocked off, picked off, or pruned off in time to prevent the twigs from dying.

(3) For branches and trunks that have partially suffered freeze injury, it is advisable to first promptly prune away the portions of freeze-damaged branches and trunks that can be identified with the naked eye, and then wait until new shoots sprout and the boundary between dead and living tissue is clear before thoroughly cutting away the already freeze-killed parts.

(4) For severely freeze-damaged trees whose entire canopy has died, they should first be sawed off, while the remaining trunk and the bases of the main branches should be left untreated for the time being. After the first batch of new shoots growing on them has matured, saw them back to a position slightly lower than the boundary between the freeze-damaged part and the healthy part. For trees that have been cut back in this way, preserving the roots and “keeping them alive” is the primary task. When large numbers of new shoots sprout, apart from removing the thin, weak, tender shoots in overly dense areas, as many as possible should be retained to accumulate photosynthate to nourish the root system, and from among them 2 to 3 vigorous-growing ones should be selected and retained, appropriately pinched to promote branching, and trained into the framework of a new canopy.

(5) Branches and trunks broken by snow damage should be saved and retained as much as possible. For large branches that are torn, some branches and leaves should be appropriately pruned away to reduce water consumption, then they should be aligned in their original position, wrapped with plastic film, bound with ropes, and supported with props to fix them in place, and the binding should be loosened only after the wound has healed firmly later. If they cannot be saved, they should be sawed off as early as possible. After sawing off a trunk, the wound surface is relatively large, so it should be smoothed with a sharp knife and coated with a protectant to prevent mold and rot and promote healing. The protectant can be yellow clay slurry plus fresh cow dung plus 2,4-D at 100 mg/kg; or it can be prepared from 250 g of petroleum jelly (or water-based asphalt) plus 5 g of carbendazim. According to recent reports, a “Sanling ointment” prepared from 1 g of “920”, 50 g of carbendazim, and 10 kg of petroleum jelly also has a very good effect in protecting wounds.

2. Strengthen soil, fertilizer, and water management. For freeze-damaged citrus orchards, the general requirements are that the soil should be cultivated less, fertilizer should be applied lightly rather than heavily, and it is best to frequently spray dilute foliar fertilizer. From April to July, urea solution should be the main application, twice a month, and from August onward, phosphorus and potassium fertilizers should be added to the spray solution. Generally, freeze-damaged trees are prone to deficiencies of trace nutrient elements such as zinc, manganese, and copper. The deficiency symptoms can be identified according to the newly sprouted branches and leaves, and then, by means of foliar topdressing, the trace elements lacking in the tree body or a multi-trace compound foliar fertilizer such as Yemianbao should be sprayed. In years with abundant rain and snow, especially after the beginning of spring when there is more rain, trenches should be opened in time to drain away accumulated water to prevent root rot. After the weather clears, shallow loosening of the soil should be carried out in time to prevent soil compaction and raise soil temperature. In the high-temperature and dry seasons of summer and autumn, attention should be paid to drought prevention and drought resistance. Local materials can be used to mulch the tree disk, which prevents drought, lowers temperature, and also prevents soil erosion.

3. Prevent branch and trunk cracking and promptly prevent and control diseases and pests. Because freeze-damaged citrus trees have damaged branches and leaves and exposed branches and trunks, they are highly susceptible to sunscald in the high-temperature season, causing the bark to split and triggering resin disease and anthracnose, further weakening tree vigor. Therefore, protecting the branches and trunks is very important. A whitewash can be prepared from 5 parts lime, 1 part lime-sulfur mixture stock solution, 0.1 part salt, 0.1 part tung oil, and 20 parts water, and painted onto the branches and trunks. The newly sprouted tender branches and leaves of freeze-damaged trees are both precious and vulnerable to damage by diseases and pests. Therefore, diseases and pests that harm new shoots and tender leaves, such as aphids, mites, scale insects, leafminers, swallowtail butterfly larvae, and anthracnose, should be carefully inspected and promptly controlled.