Should I water plants before an expected frost?
Written by
Benjamin Miller
Reviewed by
Prof. Samuel Fitzgerald, Ph.D.Watering plants ahead of frost requires precise timing and a suitable method. Always use tepid water at 60-70°F (16 to 21°C) and only in the late afternoon. Moist soil retains the day's heat more effectively than dry soil. This means that a microclimate is created around the roots. Never wet the leaves; ice will form and can present dangers.
The physics of watering your plants involves the release of heat energy when water freezes. Water molecules release 80 calories per gram of water when they change from liquid water to solid ice. The energy from water warms the soil around the roots. Well-watered earth will behave warmer than dry earth on cold nights. I measure the soil temperature to take advantage of this layering process.
Timing Protocol
- Water 3-4 hours before sunset
- Complete watering before temperatures drop below 45°F
- Allow surface drying before nightfall
Water Specifications
- Use 60-70°F (16-21°C) water temperature
- Apply 1 gallon per square foot of soil
- Avoid runoff with slow soaking
Plant-Specific Rules
- Container plants: water until drainage occurs
- Seedlings: light watering only
- Succulents: minimal pre-frost hydration
Soil conditions restrict water uptake. Sandy soils require more frequent irrigation than clay soils. A moisture probe is a good measure of the depth of moisture absorption. When soil is adequately watered, it is moist at a depth of 4-6 inches. Saturated soil holds more moisture, which freezes faster. I use my finger to test when/if I need irrigation.
It is beneficial to couple any watering with other forms of protection. You can even water the plants and then cover them to trap the rising warmth. Use water jugs near the plants to add some additional thermal mass. Any watering and covering will provide different degrees of frost protection during critical periods of extreme cold.
Promptly intervene if trees or shrubs are improperly watered. If frozen on the foliage, wait until sunrise and gently brush off the ice crystals. Plants under water stress are most often damaged by injury from drying winds; protect them with windbreaks. Resume normal watering only after the temperature has stabilized above freezing for 48 hours.
Read the full article: How to Protect Plants from Frost Effectively
