The decision to uncover plants during the day balances the need for light and air against the risk of ongoing environmental threats. While covers offer necessary protection from cold, wind, and pests, keeping them in place when conditions improve can quickly harm the plants they are meant to save. Managing this transition correctly is important for garden health. This guidance provides a framework for deciding whether to remove protective covers based on current weather conditions and plant requirements.
Why Uncovering is Essential for Plant Health
Removing protective covers is necessary to allow plants to perform photosynthesis, the process by which they convert light energy into chemical energy for growth. Even translucent coverings reduce the amount of light reaching the foliage, and prolonged shading can lead to etiolation, where the plant develops pale, weak, and elongated stems as it stretches to find light. Plants require sufficient direct or diffuse light exposure during the day to ensure strong, healthy growth.
A primary risk of leaving covers on during daylight hours is the potential for overheating. Solar radiation passing through the cover is trapped as heat, quickly raising the temperature beneath the material far above the ambient air temperature. This rapid heat buildup can cause severe heat stress, scalding the leaves or stems. When a plant’s temperature exceeds its optimal range, typically \(68^{\circ} \text{F}\) to \(86^{\circ} \text{F}\) for most crops, its biological functions, including photosynthesis, can shut down.
Air circulation is another requirement hampered by continuous coverage. The stagnant, humid conditions created under a cover are ideal for the development of fungal diseases, such as powdery mildew or Botrytis blight, which thrive in still, moist air. Uncovering the plants allows for fresh air exchange, which dries the foliage and reduces the humidity pockets that encourage fungal spore germination and spread.
Conditions That Require Keeping Plants Covered
There are specific environmental conditions where the risk of damage from the elements outweighs the need for daytime light and air. The most common necessity for keeping plants covered is the persistence of dangerously low temperatures. If the daytime high is forecasted to remain at or below \(32^{\circ} \text{F}\), or if the temperature is expected to hover in the mid-30s for cold-sensitive plants, the cover should stay on. For tender annuals, temperatures below \(40^{\circ} \text{F}\) can cause stress, making it safer to retain the thermal barrier.
High winds are another reason to leave the protective layer in place, even if temperatures are mild. Strong gusts can cause significant physical damage, snapping fragile stems, tearing leaves, or even uprooting smaller plants. Covers, especially those secured to the ground, act as a windbreak, shielding plants from physical trauma. If wind speeds are predicted to exceed \(20\) to \(25\) miles per hour, maintaining the cover prevents mechanical damage.
Temporary coverage may also be necessary to mitigate the effects of intense, unseasonable spring sun. Newly transplanted seedlings, which have not yet developed a protective cuticle layer, can suffer from sun scald and shock when exposed to sudden high-intensity light. A light shade cloth, which permits air movement but filters the harshest rays, might be used during the sunniest part of the day. This protection allows the plant time to acclimate without the thermal risks of a solid, insulating cover.
Practical Guidance for Timing the Removal
The removal process should be gradual, particularly if plants have been covered for an extended period. This transition slowly acclimates the plants to the harsher outdoor conditions of direct sunlight, fluctuating temperatures, and wind. Begin by uncovering the plants for just one to two hours on the first day, ideally in a shaded or protected location, and progressively increase the duration of exposure over seven to ten days.
The optimal time of day to remove the cover is mid-morning, after the ambient temperature has begun to rise and any lingering frost has melted. Waiting until the morning air has stabilized helps prevent cold shock as the cover is lifted. Conversely, the protective layer should be replaced in the late afternoon or early evening, well before the sun sets and the temperature begins to drop toward the forecasted overnight low.
If a frost cover is left on during a sunny day, the temperature beneath the material must be monitored closely to prevent overheating. Using a simple thermometer placed near the plants allows you to track the temperature and remove the cover immediately if it rises too high. Monitoring is particularly important when using materials like plastic, which trap heat efficiently and quickly cause damage if left in direct sun.