Covering plants with plastic sheeting creates a controlled microclimate, offering distinct advantages, especially in unpredictable weather. This method is highly effective for plant protection and season extension. However, success relies on careful management to prevent risks associated with improper use, such as overheating and excessive moisture retention.
Primary Reasons for Using Plastic Covers
The primary motivation for covering plants with plastic is to provide a thermal buffer against colder temperatures, especially frost. Plastic acts as a physical barrier, trapping heat radiated from the soil and plants underneath. This greenhouse effect slows the rate of heat loss, maintaining temperatures several degrees warmer than the ambient air, protecting plant cells from damaging freeze events.
This localized warming effect is instrumental in extending the growing season into the early spring or late fall. By warming the soil, plastic covers allow for earlier planting of cool-season crops or help warm-season vegetables, such as tomatoes and peppers, mature successfully. Beyond temperature regulation, the plastic also serves as a physical shield against mechanical damage from harsh elements, including strong winds, heavy rain, or hail. Furthermore, a continuous plastic barrier can prevent common garden pests from reaching the foliage, reducing the need for chemical interventions.
Managing Temperature and Humidity Risks
While plastic is excellent at trapping heat, solar gain can quickly become detrimental to plant health. On a sunny day, the temperature inside a covered space can rapidly increase due to trapped solar radiation, leading to overheating. Temperatures exceeding the plant’s optimal range cause heat stress, leading to wilt, sunscald, and lethal damage if prolonged. This danger necessitates constant vigilance to ensure adequate ventilation.
Proper air exchange prevents the accumulation of excessive heat and moisture, requiring the structure to be opened during the warmest part of the day. Poor ventilation creates a stagnant, high-humidity environment, which is an ideal breeding ground for many plant pathogens. Fungal diseases like powdery mildew, botrytis rot, and damping off thrive when water remains on leaf surfaces. Additionally, if the plastic sheeting touches the plant foliage, the trapped moisture and cold air can lead to contact damage and rot.
Practical Applications and Material Choices
Application Methods
The method of application varies widely, from temporary cloches placed over individual plants to larger, semi-permanent structures like hoop houses or row covers. For temporary frost protection, a simple barrier is used. For season extension, a supporting frame is recommended to ensure the plastic does not contact the plant leaves and allows for proper air circulation.
Material Selection
The type of plastic selected must be matched to the grower’s goal. Clear plastic film allows for maximum light transmission and solar heating, making it the preferred choice for early-season warming. Conversely, white or opaque plastic diffuses the light, offering a cooling and shading effect beneficial in warmer climates or during intense summer sun.
The thickness of the material is measured in “mil,” where one mil equals one-thousandth of an inch. A 6-mil polyethylene film is the standard for durability and light transmission in multi-season use. Thinner 3-mil material is often used for short-term overwintering. Professional-grade plastics feature additives, such as UV stabilizers to prevent breakdown from sunlight and anti-condensate treatments to reduce moisture droplets inside the cover.