Timing, Color, and Laying Methods of Plastic Mulch for Strawberries, and Their Advantages

Plastic mulch covering is the simplest protected forcing cultivation model, requiring no major investment in facilities. It can also be effectively combined with overwintering cold protection. So when should strawberry plastic mulch be laid? Which color of plastic mulch should be used, and how should it be laid?

1. Types of plastic mulch

1. Black plastic mulch: The most widely used in strawberry production is high-pressure low-density polyethylene black plastic mulch, with a width of 0.5-0.8 m and a thickness of 0.008-0.01 mm. When used as ground cover, it can significantly lower soil temperature, has low light transmittance, can effectively prevent evaporation of soil moisture and inhibit weed growth, and can elongate the fruit size.

2. White plastic mulch: Black-and-white double-sided plastic mulch has one milky white side and one black side, with a thickness of 0.02-0.025 mm. The milky white side faces up and has a reflective cooling effect; the black side faces down and has a weed-killing effect. During high summer temperatures, its cooling and weed-suppressing effects are better than those of black plastic mulch.

3. Silver-gray plastic mulch: Its light transmittance is about 60%, and it can reflect ultraviolet rays. As ground cover, it has cooling, moisture-retaining and aphid-repelling effects, and can increase reflected light from the ground, which is beneficial for fruit coloring.

4. Red plastic mulch: Red plastic mulch can stimulate crop growth more than black plastic mulch, and plants will use more energy for photosynthesis in the aboveground parts. Red plastic mulch can transmit red light while blocking other light colors that are unfavorable for crop growth, making crops grow vigorously, which is beneficial for strawberry fruit coloration and improves coloring and sugar content.

5. Green plastic mulch: Green plastic mulch can suppress weeds and increase temperature. Green plastic mulch covering can reduce the amount of visible light transmitted for vigorous plant photosynthesis while increasing green light, thereby inhibiting chlorophyll formation in weed leaves, reducing photosynthesis of weeds under the plastic mulch, and achieving the purpose of inhibiting weed growth. Its warming effect on soil is stronger than that of black mulch, and it also promotes aboveground growth and improves quality in crops such as strawberries. However, green plastic mulch is relatively expensive, ages easily, and has a shortened service life.

2. Plastic mulch covering techniques

1. Mulching time: The suitable time for plastic mulch covering is 7-10 days after the film is closed for heat preservation, and it should be done before flower clusters emerge, so as to avoid breaking the flower clusters during the film-covering operation and affecting flowering and fruiting. After strawberries are planted in autumn, plastic mulch is generally laid during two periods: before overwintering and before sprouting in early spring. In cold regions, covering before overwintering is more favorable, and it is generally more suitable to do it when the daily average temperature drops to 3-5°C. Covering too early can easily cause strawberry regrowth

2. Thorough disinfection of the greenhouse

Before closing the plastic mulch, the strawberry greenhouse should be thoroughly disinfected to prevent and control various diseases, and it is best to disinfect after several consecutive sunny days. This disinfection should cover the entire greenhouse, and for a greenhouse of 400 sq m, 75-90 L of spray solution is required.

In addition to spraying chemicals, appropriately scatter sulfur powder on the strawberry bed surface, but it must not be excessive; otherwise, too much sulfur will cause soil acidification, reduced hardness, and severe occurrence of root rot, etc.

To make strawberry management easier after mulching, the following two things should be done well.

First, level the strawberry bed surface, break up clods, and gently remove the raised parts, trying not to expose strawberry roots. The bed surface should be flat and fine so that the plastic mulch can cling to the ground and remain flat;

Second, arrange the drip irrigation tape to ensure uniform dripping. If the bed surface is very high, dig a shallow ditch on the bed surface and straighten the drip irrigation tape. If the strawberry plants are lower than the bed surface, add a little soil if it is not serious; if it is relatively serious, use disinfected straw to pad underneath. When padding with straw, make it slightly higher, because the drip tape will press down when filled with water, and if the straw sinks, it will affect the drip irrigation effect. After the bed surface is arranged, straighten the drip irrigation tape and fix it in the center of the bed surface with U-shaped wire.

3. Laying the plastic mulch

Choose the width of the plastic mulch according to the width of the ridge; generally choose plastic mulch with a width of 90-120 cm.

When covering the film, spread the plastic mulch flat and cover it over the strawberry plants along the row, so that the film surface stretches without wrinkles. In solar greenhouse forcing cultivation, two pieces of plastic mulch are often used, with the overlap at the middle of the strawberry bed, and the overlap length kept at about 20 cm. The length of the plastic mulch exceeds the bed length by about 1 m, and both ends should have enough excess to bury into the soil, which is not only neat in appearance but also ensures that the plastic mulch is relatively tight. Compared with the traditional method of laying one whole piece of plastic mulch, this method is easier to operate and greatly reduces labor. It is not only less likely to break stems, but also makes operations such as intertillage, weeding, and topdressing easier.

Breaking the film and lifting seedlings: When covering the plastic mulch, break the film as you cover, and pull the seedlings out onto the film. When breaking the film to pull out the strawberry seedlings, keep the holes as small as possible to avoid overly large openings affecting the heat preservation and moisture conservation effect of the plastic mulch. Be careful to pull out the entire aboveground part of the strawberry; otherwise, the remaining strawberry leaves are very likely to rot and breed pathogenic microorganisms, affecting normal strawberry growth. Mulching should be done in windless weather and in the afternoon after strawberry leaves have softened, to avoid breaking leaves and petioles in the morning when strawberry leaves are relatively brittle during seedling pulling.

Method for laying plastic mulch late when flower clusters have already emerged: In production, it often happens that the best time for laying plastic mulch is missed for various reasons, resulting in flower clusters having already emerged. At this time, when laying the plastic mulch, if the method of breaking holes and lifting seedlings continues to be used, it is easy to break the flower stalks and affect strawberry growth. If the flower clusters have already emerged, when laying the plastic mulch, you can use scissors to cut the plastic mulch at the root area of the strawberry plant in half from the edge, cutting about 20 cm long, then wrap the strawberry root area from both sides with the plastic mulch, and finally fix the joint tightly, thus avoiding breaking the flower stalks.

3. Advantages of covering with plastic mulch

1. Effects on the soil environment

1) Increasing soil temperature.

Because transparent plastic mulch easily transmits shortwave radiation but not longwave radiation, and at the same time the plastic mulch reduces latent heat release from water evaporation, a large amount of sunlight passes through the plastic mulch during the day, causing the ground temperature to rise and continuously conduct downward, warming the lower soil layers. At night, longwave radiation from the soil does not easily pass through the plastic mulch, so the soil under open field conditions releases less heat; therefore, the ground temperature is higher than in the open field. The warming effect of plastic mulch covering varies with covering time, covering method, weather conditions, and type of plastic mulch.

2) Improving soil water retention capacity.

After covering with plastic mulch, soil moisture evaporation is small, so soil moisture can be kept stable for a longer time, avoiding sudden drying and wetting of the soil that affects normal crop growth.

3) Improving soil fertilizer retention capacity.

Because of plastic mulch covering, the temperature and humidity in the soil under the film are suitable, microbial activity is vigorous, and nutrients decompose quickly; therefore, the contents of available nitrogen, phosphorus, potassium and other nutrient elements are all higher than in the open field.

4) Improving the physical and chemical properties of the soil.

Because plastic mulch covering can avoid the impact of wind and rain on the soil surface, and reduce soil compaction caused by trampling during manual and mechanical operations such as intertillage, weeding, fertilizing, and watering, the soil bulk density, porosity, three-phase (gaseous, liquid, solid) ratio, and aggregate structure are all better than those of soil without plastic mulch.

5) Preventing salt accumulation on the soil surface.

Because plastic mulch covering cuts off the channel for exchange between water and the atmosphere, it greatly reduces soil moisture evaporation, thereby also reducing the salts brought to the soil surface with moisture, and can prevent soil salinization.

2. Effects on the near-ground microclimate:

Because plastic mulch has a reflective effect, it can increase light, which is beneficial to strawberry plant photosynthesis and increases product accumulation; plastic mulch covering can also reduce air humidity in the greenhouse and effectively prevent the occurrence of pests and diseases

3. Effects on strawberry growth and development:

Plastic mulch covering creates good growth conditions for strawberries, promotes strawberry root development, makes strawberries grow robustly, enhances their own resistance, accelerates strawberry growth and development, and advances each growth stage accordingly, thereby allowing earlier maturity and improving strawberry yield and quality;

4. Other effects: weed control; labor saving; water saving and drought resistance.