Plant covers, such as row covers, blankets, or cloches, serve as temporary shelters against environmental extremes that can damage or kill vulnerable plant tissues. This practice shields plants from damage caused by low temperatures, prevents sun-related cellular injury during intense heat, and offers a physical barrier against severe weather events. Understanding the specific threat dictates the type of cover and the necessary duration of protection.
Protection from Freezing Temperatures
Covering plants is necessary when air temperatures approach or drop below the freezing point of water, 32°F (0°C), especially for tender annuals and tropical varieties. A light frost, defined by temperatures between 29°F and 32°F, will typically kill sensitive foliage like that of tomatoes and peppers. Temperatures that drop below 28°F constitute a hard freeze and can cause widespread damage to most unacclimated plants, including many perennials.
Cold damage primarily occurs due to the formation of ice crystals within or outside the plant cells. Extracellular freezing, where ice forms in the spaces between cells, draws water out of the cells, causing them to dehydrate and collapse. Rapid cooling can also lead to fatal intracellular freezing, where ice crystals form inside the cell’s protoplasm. Covering plants helps trap the residual heat radiating from the soil overnight, raising the temperature immediately around the plant by a few degrees and slowing the rate of temperature drop.
Shielding Plants from Intense Summer Sun
While cold is a primary threat, plants also require covering to mitigate the effects of extreme summer heat and intense solar radiation. Sunscald, a common issue, occurs when temperatures rise above 90°F to 100°F, particularly affecting fruits like tomatoes, peppers, and apples. This injury is a form of cellular death caused by thermal stress, resulting in bleached or sunken spots on the sun-exposed parts of the fruit or leaves.
Newly transplanted seedlings and vegetables that have been recently pruned are particularly vulnerable because their tissues are not yet hardened to direct sunlight. During heat waves, covering plants with a light-colored material or a shade cloth becomes necessary to reduce the intensity of light and lower the ambient temperature. These covers work by diffusing the direct solar energy, minimizing water loss from the leaves, and preventing excessive heat accumulation.
Guarding Against Severe Weather Events
Covering plants also protects them from mechanical damage caused by sudden, high-energy weather phenomena. Heavy precipitation, such as hail, can shred leaves, break stems, and severely bruise fruit within minutes. Covering plants with a sturdy, temporary barrier like hardware cloth, thick row covers, or even overturned buckets can help absorb the physical impact of the hailstones.
High winds and torrential rain also pose a significant threat, especially to tall or newly established plants. Strong winds can cause physical stress, leading to broken branches and stripped foliage, while heavy rain can batter delicate plants and cause soil erosion around the roots. Temporary covers, along with staking and windbreaks, can slow the air movement and shield the plant structure from the direct force of the elements, preventing mechanical failure.
Duration and Timing: When to Remove Covers
Plant covers are temporary measures, and timely removal is necessary to prevent unintended harm. For protection against cold, covers must be removed once the temperature has risen above freezing and the sun is out, typically in the morning. Leaving insulating covers on during the day can cause the temperature underneath to rapidly increase, leading to overheating and heat stress, which can be as damaging as the initial cold.
Covers used for sun or hail protection should also be removed or adjusted when the immediate threat has passed. Prolonged covering restricts airflow and traps humidity, creating an environment for fungal diseases like powdery mildew or rot. For flowering and fruiting plants, covers must be lifted or temporarily opened during the day to allow access for pollinating insects.