5–6 minute read

Siting to minimize frost damage

Because good site selection is the best protection, consider the property’s microclimates before planting. The best quick way to choose a planting site is to visualize the flow of cold air and its possible buildup. If a site has good cold air drainage, then it is likely to minimize frost/freeze damage.

• Locate less hardy plants in the highest part of the yard. Frost pockets/cold spots are formed because cold, dense air flows by gravity to the lowest areas of the local landscape, where it collects. This causes temperatures to differ in relatively small areas, called microclimates.
• Protect plants from cold wind: use a tall evergreen hedge of trees or shrubs. Note that in the wrong spot, a dense hedge, wooded area or solid barrier such as a fence can create a cold air dam that will increase the likelihood of frost.
• Shade plants from direct winter sun. Early-morning sun is especially harmful. Plants that freeze and thaw slowly will be damaged the least. A shaded exposure will also delay spring growth, decreasing the risk of injury to new growth or flowers from late spring frosts. The south side of the house with no shade is the worst place for tender plants; the north side of the house is the best.

Cultural practices to minimize frost damage

Frost will not always cause plant damage. Damage occurs only if the internal temperature in various plant tissues becomes cold enough to break cell walls or disrupt cell constituents beyond repair. If this occurs, damage, wilting, and death of the affected tissues will follow.

How you address your plantings can offer significant protection:

Soil

Soil moisture and compaction can have a significant effect on minimum temperature.

• Avoid cultivating soil prior to a frost or freeze. Cultivation in late winter and early spring tends to increase freeze damage by lowering the air temperature several degrees relative to uncultivated soil.
  
• Water plants thoroughly the evening before a frost. A moist soil can hold four times more heat than a dry soil. It will also conduct heat to the soil surface faster, helping to prevent frost. In one study, the air temperature above a wet soil was 5 degrees higher than that above a dry soil and the difference was maintained until 6am the next day.

Groundcover

Vegetation reflects more solar radiation than bare ground or mulch during the day and also transpires moisture to cool itself. Thus, less stored heat is present. Mowing a vegetative cover (‘living mulch’) to heights shorter than two inches increases heat storage and can mitigate frost formation. However, the frost disadvantages of ground covers must be balanced against other benefits such as erosion control, dust reduction, and protection from soil compaction by rain.

If you use mulch rather than a groundcover, don’t mulch until there has been a hard freeze. Mulch is an insulator. Applied while the ground is warm, it extends plant activity, while applied after the ground is frozen, it maintains dormancy.

Fertilization

Encourage your plants to prepare for winter. Stop feeding plants quickly-available nitrogen in late summer, gradually reduce watering in autumn, and permit a few fruits to form (e.g., let roses make hips). Your plants are less likely to be damaged by an early cold snap.

Temporary measures

Covers

Covering plants can provide 2–5 degrees of protection. Try supporting covers on stakes or hoops because protection is reduced wherever the cover touches the plant. While any material can be used, woven agri-fabric is a better insulator then plastic or paper. The best time to apply covers is in the late afternoon after the wind has died down. Remove covers the next morning before the sun hits them.

Irrigation

While tricky, you can irrigate during the potential frost if winds are below 10mph, the air is not too dry, and the dew point is higher than 24°F. A single application at the coldest part of the night (generally just before sunrise) may suffice on 30–32° nights. On colder nights, you may need to apply overhead irrigation for an extended period of time, allowing actual ice formation on your plants and keeping the ice wet. Irrigation must continue until morning temperatures rise above 32°F and the ice melts. Note that once frost damage occurs, watering does not help.

Predicting whether frost will occur

Temperatures vary across Orange County and can be significantly colder in rural and low-lying areas, so it is useful to have a feel for when frost might occur despite the official forecast. A good indicator is the dew point, which is the temperature at which air is fully saturated with water (relative humidity of 100%). If the predicted low is close to the dew point, then the forecast is likely to be accurate. However, if there is a large difference (for example, a predicted low of 32° and a dew point of 17°), then the temperature is likely to drop below the forecasted level.