The Gardener's Almanac

Frost Pockets

What is a ‘frost pocket’? It is cold air trapped close to the ground. Cold air is denser than warm air and tends to flow downhill, i.e. into depressions or against fences, walls and hedges. It is in such conditions that cold air becomes trapped by the warmer air above it.

What is Frost?

As air cools at night, its ability to hold water vapour decreases, causing it to become saturated.

If the air continues to cool, excess vapour condenses, and forms as dew on the surface of plant leaves.

This dew then freezes if the temperature continues to drop to 0°C (32°F) or lower.

As mentioned above; as the cooling of the air occurs, heat accumulated in the soil radiates back into the atmosphere.

This loss of heat continues until just after dawn, i.e. until such times as the sun warms it up again, so this is why the sharpest frosts often occur in early morning.

Meteorologists normally record air temperatures at an average height of 1.5m (5ft) above ground level, whereas the ground temperature can be much lower i.e. 5°C (10°F) lower, thus forming a ‘frost pocket’.

Eliminating ‘frost pockets’ might be difficult, but in some cases it may be possible; e.g. if the condition is due to a slight hollow or slope, improve the situation by filling in the hollow or levelling out the slope, this will allow the cold air can flow over /through the garden.

To reduce the effect of solid barriers such as walls, fences or solid hedges, create an opening at the lowest point to allow the air to escape.

Impede the passage of cold air flowing into the garden from higher ground by erecting a slatted fence, hedge or line of small trees / shrubs at the top of the incline.

Do’s & Don’ts; if you have a ‘Frost pocket’ in your garden.

Find out the average dates of the first and last frosts in your area (click here)

Listen carefully to weather forecasts.

If a frost is forecast cover susceptible low-growing plants with some form of insulation such as agro-fleece this can keep the temperature beneath a few degrees warmer.

Some plants are particularly vulnerable to late-spring frosts if positioned where they receive the early-morning sun, the rapid warming of the plant can damage plant tissue.

Do not dig or hoe the soil if frost is expected.

Tests have shown that hoeing 20mm (¾”) deep can lower the surface temperature overnight by 2°-3°C (3°-5°F) so check weather forecasts prior to cultivating, especially in spring.

Moist soil can hold as much as four times the heat as dry ground e.g. air temperatures just above recently soaked soil are as much as 3°C (5°F) higher than over dry ground, meaning, if frost is forecast, watering a seedbed during the day then covering with fleece or cloches will help protect young / tender plants.

Select tallish hardy plants that start into growth or flower late, as these tend to hold their flowering and fruiting shoots above the cold air near the soil.

More tender plants thrive higher up the slope, where air is warmer, and drainage is generally better.

Delay pruning until cold weather is past this will avoid the moist open pruning wounds being affected by frost.

Avoid using high-nitrogen fertilizers from mid-July onwards to avoid soft lush growth.

Potash feeds applied in late summer can help ripen the stems and make them less susceptible to frost damage.

Continue watering regularly in a dry autumn, especially evergreens which can suffer badly from frost damage on dry, light soils.

Note; Most of the above procedures can be used in other areas that; although not necessarily frost pockets, are still subject to frosty conditions.