Frost occurs when temperatures drop to \(32^\circ\text{F}\) (\(0^\circ\text{C}\)) or below, posing a significant threat to tender flowers and foliage. Damage results from the freezing of water within plant tissues, which ruptures cell walls and membranes. This process forms ice crystals that mechanically disrupt the internal structure of the cells, leading to their death. Protecting vulnerable flowers requires specific preparation and proper covering techniques to ensure their survival.
Preparing for a Frost Event
Closely monitor local weather forecasts, especially when nighttime temperatures are predicted to dip near or below \(32^\circ\text{F}\) (\(0^\circ\text{C}\)). Clear, calm nights are dangerous because they allow heat to escape rapidly from the earth’s surface, creating ideal conditions for frost formation. Once a cold snap is confirmed, deeply water the soil late in the afternoon before the expected freeze.
Moist soil holds heat better than dry soil because water has a high specific heat capacity, allowing it to absorb warmth during the day and release it slowly overnight. This slow release of stored warmth creates a localized buffering effect, keeping the root zone and surrounding air warmer. A well-hydrated plant is also more resilient to the stress caused by cold injury.
Choosing Effective Protective Materials
Effective protective materials act as an insulating barrier while allowing for air exchange to prevent excessive moisture buildup. Commercial frost cloths or floating row covers, often made from lightweight polypropylene fabric, are highly effective because they trap ground heat while remaining breathable. Old bed sheets, blankets, or burlap can also provide sufficient insulation for a light to moderate frost.
Heavier materials, such as thick blankets or tarps, offer greater protection but may damage delicate flower structures. Plastic sheeting can be used, but it must never come into direct contact with the foliage. Any leaf touching the plastic during a freeze can transfer cold directly, worsening the damage. The cover should be secured to the ground to trap residual earth heat underneath.
Step-by-Step Covering Techniques
The covering process requires correct execution to maximize protection. A support structure, such as stakes, cages, or hoops, should be placed around the flowers to ensure the covering does not rest directly on the foliage. Direct contact transfers cold to the leaves and negates the insulating effect of the trapped air layer. This framework creates a protective tent that establishes a warmer microclimate.
The material should be draped over the structure just before sunset, when the ground begins to lose its stored warmth. The cover must extend all the way to the ground on all sides of the plant. This contact with the soil seals in the heat radiating from the earth and prevents cold air from seeping underneath the protective layer.
The edges of the cover must be securely anchored to the ground using heavy objects like bricks or mounds of soil. A secure seal prevents wind from lifting the material and stripping away the thin layer of warmer air. Ensure the cover is taut but not straining the support structure to prevent collapse from overnight ice or snow.
For larger flower beds, multiple layers of lightweight fabric offer superior insulation compared to a single, heavy layer, provided the weight does not crush the plants. A double-layer system, such as a fabric sheet covered with a non-touching plastic tarp, offers enhanced thermal protection against severe cold.
Removing Covers and Post-Frost Assessment
The timing of cover removal is important to prevent subsequent damage. Covers must be removed promptly in the morning as soon as temperatures rise above freezing and the sun begins to warm the area. Leaving covers on too long can cause the temperature beneath the material to rise excessively, potentially “cooking” the plants due to the greenhouse effect.
Overheating causes rapid dehydration and stress to vulnerable tissues. Once the covers are off, assess the flowers for signs of damage, which often appear as blackened, water-soaked, or wilted foliage. If damage has occurred, resist the urge to immediately prune off the affected parts.
The damaged foliage provides a layer of natural insulation, especially if subsequent cold snaps are expected. Wait to prune the damaged areas until the threat of frost has completely passed and new growth emerges in the spring. This ensures you only remove the truly dead material, allowing the plant to recover using its full stored energy reserves.