What are the main measures for freeze protection of citrus fruit trees?

In order to prevent sudden low temperatures and cold waves and minimize the degree of freeze damage to citrus, the following measures can be taken for winter citrus freeze protection.

1. Choose a suitable microclimate environment for orchard establishment: For planting citrus on hilly and sloping land, choose south-facing areas that are sheltered from wind and exposed to sun; for establishing an orchard on flat land, windbreak belts should be built on the east, north, and west sides of the citrus orchard. Windbreaks can use evergreen tree species such as camphor and privet.

2. Choose citrus varieties with strong cold resistance: Citrus belongs to the Rutaceae family——the Citrus subgenus of evergreen fruit trees. There are many varieties, and the differences in cold resistance among varieties are also very large. Among the citrus varieties currently planted locally, their cold resistance from strong to weak is as follows: Satsuma mandarin, Nanfeng mandarin, ponkan, navel orange, hybrid citrus (a cross between mandarin and orange), and pomelo. When choosing varieties for planting, navel orange, hybrid citrus, and pomelo should not be developed on a large scale locally.

3. Apply base fertilizer at the right time: Locally, the base fertilizer for citrus should be applied between late October and mid-November. Too early is likely to promote the sprouting of late autumn shoots, consume nutrients, and reduce the cold resistance of the tree; too late is not conducive to root wound healing because the ground temperature is low.

4. Irrigate the orchard: In mid-to-late November, begin irrigating the citrus orchard. When there is no rain in winter, keep the orchard soil moist. According to the weather forecast, irrigate thoroughly once more 3-5 days before the cold wave arrives.

5. Whitewash the main trunk and mound soil: Use 30-40% lime solution to whitewash the main trunk, and mound soil around the graft union between the aboveground plant and the rootstock, with the soil mound more than 30 cm high. Whitewashing and mounding soil can reduce the severity of damage from snowmelt cooling and the occurrence of post-freeze bark splitting disease.

6. Spray an anti-transpirant and heat-retention agent: 5-7 days before the cold air arrives, spray a 200-fold anti-transpirant and heat-retention agent over the entire tree canopy once. Immediately repeat the spray once after the cold wave passes.

7. Cover the tree canopy: When the minimum temperature drops below zero, cover the tree canopy with non-woven fabric or plastic film. After the cold wave passes and the weather turns clear, the covering can be removed during the day when the temperature is above 5°C, and then put back on at night to prevent frost. If conditions permit, a simple plastic greenhouse can be built before late November for insulation.

8. Remove snow from the tree canopy: When heavy snow occurs, promptly shake the tree canopy and branches to knock off the accumulated snow and avoid branches being broken by the weight of the snow.

9. Spread quicklime on the ground: On clear nights after snow, spread 50-100 kg of granular quicklime per mu in the citrus orchard. By decomposing and releasing gas, quicklime can raise the temperature in the orchard and reduce the harm from radiative cooling.

10. Spray fertilizer and water on the leaves: After the cold wave passes, choose a sunny midday (between 10 a.m. and 3 p.m.) to spray clean water once every 1-2 days, for 5-6 times in a row. About every 7 days, spray a solution of 0.3% urea plus 0.2% monopotassium phosphate once, for 3-4 times in a row. This reduces dehydration and leaf drop in citrus after freezing and restores tree vigor through foliar fertilization.