When a sudden drop in overnight temperatures is forecasted, the safety of outdoor plants becomes an immediate concern. Protecting tender garden favorites from cold shock requires a swift assessment of the weather severity and your plants’ specific vulnerabilities. This guide helps you determine which flowers need shelter tonight and how to keep them safe.
Identifying the Critical Temperature Thresholds
The level of protection required depends on the difference between a frost and a freeze, and the forecasted duration of the cold. A light frost occurs when the air temperature dips into the 33–36°F range, causing ice crystals to form on plant surfaces. This can damage only the most sensitive leaves and flowers. The true danger begins with a light freeze, defined by temperatures between 29°F and 32°F, which is enough to kill tender annuals and warm-weather vegetables.
Temperatures falling below 25°F constitute a moderate or hard freeze, which is widely destructive to most non-dormant vegetation. The air temperature is not the only factor; the duration of the freezing event significantly affects the extent of the damage. For tropical plants, chilling injury begins when temperatures drop below 50°F, causing discoloration and wilting, even before freezing occurs.
Meteorological conditions like the dew point and wind also influence the risk. A higher dew point indicates more moisture in the air, which can slow the rate of temperature drop. Strong, dry wind can accelerate moisture loss from leaves, a process called desiccation, which exacerbates cold damage. Clear, calm nights are especially prone to radiation frost, where heat radiates quickly from the soil into the atmosphere, causing temperatures to drop rapidly near the ground.
Which Plants Need Immediate Shelter
The most vulnerable plants lack the biological mechanisms for cold acclimation, the process by which hardy plants prepare for winter. Tender annuals, such as impatiens, petunias, and tomatoes, are highly susceptible due to the high water content in their cells. When this water freezes, it expands and ruptures the cell membranes. This leads to the characteristic watersoaked, blackened, and mushy appearance of cold-damaged tissue.
Tropical and subtropical plants, including many common houseplants and succulents, have never developed the ability to manage freezing. They cannot produce natural antifreeze compounds, like sugars and proteins, that lower the freezing point inside their cells. These species are the first priority for shelter, as they can suffer irreversible damage even from chilling temperatures above freezing.
Newly planted seedlings and soft new growth are also extremely susceptible to cold shock. This tender new tissue has not yet had time to “harden off” and is largely unprotected. Container plants are at greater risk than their in-ground counterparts because their entire root system is exposed to cold air, rather than being insulated by the earth. Roots in pots can freeze much faster and suffer damage at temperatures where established in-ground roots would be safe.
Quick Fixes When Moving Isn’t Possible
If container plants are too large or too numerous to move indoors, grouping them tightly together against a warm, south-facing wall can offer a temporary microclimate. This strategy captures heat radiating from the wall and shields the plants from cold wind. For any plant that must remain outside, a protective cover is the best defense, acting as a blanket to trap heat rising from the soil.
Ideal covering materials include old bed sheets, blankets, burlap, or commercial frost cloth. These materials provide insulation while allowing for some air exchange. The cover must be draped loosely over the plant, ensuring it extends all the way to the ground to effectively trap the thermal energy radiating from the earth. Never use thin plastic sheeting directly on foliage; where the plastic touches damp leaves, it can concentrate the cold and cause freeze damage.
Before sunset, thoroughly watering the ground around the plants is a simple yet effective step. Moist soil holds and releases significantly more heat throughout the night than dry soil, potentially raising the temperature under a cover by several degrees. Ensure the foliage itself is dry, however, as moisture on the leaves can create a layer of ice and increase the risk of cellular damage. For container roots, wrapping the pots in bubble wrap or burying them temporarily in mulch provides added insulation for the root ball.