What measures are there to prevent frost damage during the flowering period of walnut trees?
1. Choose suitable sites and varieties. Refer to the results of local agroclimatic zoning, rationally plan the industry layout, and choose favorable terrain that is not prone to cold air accumulation, to resist and avoid the harm of low-temperature frost. Generally, cold air tends to gather in valleys or low-lying areas, often causing frost, so building an orchard in low-lying areas should be avoided as much as possible; instead, choose gently sloping areas and establish a good windbreak forest for the orchard.
At the same time, increase investment in frost damage research, breed cold-resistant, cold-tolerant, high-yield walnut varieties, and expand the planting area. Selecting varieties with strong frost resistance, a later flowering period, and a short growth period is an important way to avoid late frost. In addition, increasing the application of organic fertilizer, carrying out proper pest and disease control, and maintaining a reasonable crop load can strengthen tree vigor and raise the tree's nutritional level, thereby improving cold resistance.
2. Smoking. This is a traditional frost prevention measure. After smoking, a smoke screen can form around the tree body, containing large amounts of carbon dioxide and water vapor, which can effectively prevent the loss of heat from the orchard site, prevent the temperature in the orchard from dropping, and keep the tree body in a stable temperature environment; at the same time, smoke particles absorb moisture, causing water vapor to condense into liquid and release latent heat, thereby preventing the formation of frost.
The materials usually used for smoking piles are mainly crop straw, dead branches and fallen leaves, and weeds. These materials need a certain degree of moisture, and a thin layer of soil can also be spread over the straw, leaving openings for ignition and smoke release. According to the frost time forecast by the meteorological department, they can be ignited to produce smoke and protect the walnut orchard from frost damage. Generally, each smoke pile uses 30-50 kg of material, and 4-6 smoke-producing piles per 667 sq m are sufficient. This method produces a large amount of smoke, is simple and easy to carry out, and has good results. Frost often occurs from 3 to 5 a.m., so during the walnut flowering period, weather forecasts should be carefully monitored and smoke piles set up in advance. Assign dedicated personnel to be on duty to observe air temperature, especially temperature changes in low-lying areas. When the temperature drops to -1.5°C and is still continuing to fall, it can be ignited. Through smoking, the orchard temperature can be increased by more than 2°C, preventing frost from occurring.
Special smoke-generating agents can also be used for smoking. The method is to mix 20%-30% ammonium nitrate, 50% sawdust, 10% waste diesel oil, and 10% fine coal powder, then roll them into tube shapes with waste newspaper for easy carrying. Sufficient quantities should be prepared to allow continuous smoking. When the temperature drops to 2°C, light the prepared smoke-generating agent and distribute it evenly in the orchard, which can effectively reduce or prevent late frost damage.
3. Ground irrigation and tree spraying. Irrigation before bud break can lower soil temperature, delay bud break, and avoid frost damage. Before late frost arrives, in areas with good water conservancy conditions, timely irrigate the ground according to weather forecasts, or spray water directly on the tree body. Water has a high specific heat. When the air temperature is below 0°C, irrigation or spraying can increase the air humidity of the walnut orchard and slow the rate of temperature drop in the orchard. Spraying the tree body can also delay the flowering period and reduce the severity of frost damage. If a 0.3%-0.5% sucrose aqueous solution is sprayed, the effect is even better.
When water contains salt and becomes a salt solution, its freezing point decreases. When frost occurs, spraying salt water on the tree body can prevent water vapor in the air from forming frost on the branches, thus avoiding frost damage to branches and flower buds. According to experiments, from the dormant period to the germination period, the commonly used concentration of salt water solution is 0.5%-2%. The concentration can be higher during the dormant period and should be lower during the germination period, otherwise salt damage is likely to occur.
4. Whitewashing the trunk and spraying lime milk on branches. Whitewashing the trunk in winter can reduce the absorption of solar radiant heat by the aboveground part of the tree, causing the tree temperature to rise more slowly, thereby delaying bud break and the flowering period, avoiding late frost damage. It also has the effects of resisting bacteria, killing insect eggs and larvae, and preventing sunscald. Preparation method of whitewash: fully and evenly stir 5 kg quicklime, 0.5 kg sulfur, 0.1 kg rapeseed oil, 0.25 kg salt, and 20 kg water, then brush it onto the trunk.
In combination with whitewashing the trunk, spraying lime milk on the tree branches can effectively reflect sunlight, lower tree temperature, and delay the flowering period by 5-6 days, thereby avoiding frost. The formula for lime milk is 50 kg water plus 10 kg quicklime. After stirring evenly, add 100 g diesel oil as an adhesive to increase its adhesion to the branches.
5. Covering the tree body. This method is more suitable for scattered young walnut trees. That is, before frost arrives, cover young trees with straw curtains, plastic sheets, etc., or tie straw bundles around young trees to protect them and avoid late frost damage. For orchards with initial fruiting and orchards that are difficult to cover, straw barriers can be erected around the orchard and between rows to block incoming cold air and retain released ground heat.
6. Spraying chemical agents. Spraying a 0.005% potassium naphthaleneacetate solution on the canopy before bud break, spraying a 0.5% calcium chloride solution during the germination period, and spraying 200-times high-ester film before flowering can delay the walnut flowering period by 5-7 days and effectively prevent late frost damage during the flowering period. Before frost arrives, spraying antifreeze and fruit-preserving agents on the walnut tree body can also prevent low-temperature frost damage and protect flowers and fruits.