More on Protecting Plants from Cold Weather

Measuring Frost:

When it is said "tonight's low is expected to be 6 degrees", this in fact relates to a point 1.3 metres above the ground where the temperature is measured. But at night the ground surface can actually be 2-3°C colder than this.

Moisture freezing on the ground is called a "ground frost”. A 3 degree frost means the temperature at ground level fell to -3˚C.

An "air frost" is when the air 1.3 metres above the ground falls below zero.

Slight or light frost is 0° to -2°C.

Moderate frost is -3° to -5°C.

Severe frost is -6° to -8°C

Extreme frost is lower than -9°C.

Frost Pockets:

Cold air is denser than warm air and flows like water to lower-lying areas. These lower areas can be several degrees colder than nearby higher spots. If a forecast predicts night time temperatures of around 5-6˚C, the lower lying areas could get temperatures closer to freezing. A pocket forms where the downward flow of air is blocked by a fence.

Some areas of the garden will always be colder than others, so avoid planting these spots with sensitive plants.

Warmer Positions:

Stone and brick walls, large rocks and ponds and streams all absorb heat during the day and radiate it out slowly at night, slightly warming the immediate environment. Plants growing nearby will suffer less damage from frost. Trees, and overhanging eaves, also offer some protection and plants underneath can remain untouched, while just a few metres away more exposed plants may be frost damaged.

It is possible to create a microclimate with raised beds exposed to as much sun as possible.

Frost Covers:

Cover plants with a layer of insulating material to help prevent or reduce damage in the short term. Cover plants just before or very soon after the sun goes down to prevent as much heatless as possible.

Cloth:

Drape directly over plants, or form a tent with stakes. The material lets light air and moisture pass through so it can be left covering the plant for several days without harming the plant.

Other Covering Material:

Bubble Wrap provides good protection but as it is airtight it needs to be removed when the sun comes up to prevent overheating. It can produce a lot of condensation leading to disease problems.

Newspaper, cardboard, old sheets and sacking can be used to trap heat rising from the plants and soil. Form a tent over the plant with stakes and remove during the day to let light in.

Organic Mulch:

Mulch can help protect plant roots and soil from frost by creating air spaces that provide insulation and also maintaining a more stable temperature. For best results, apply a 5–10 cm layer of mulch around the base of plants before the first frosts are due. Good examples are pea straw, shredded leaves, pine needles, or bark.

Cloches:

Use when the soil is cold, the weather bad, or to protect plants from birds and bugs.

If placed in position a week before planting or sowing, the soil can be warmed by around 10˚C.

A cloche provides some protection from very light frosts but a single layer of plastic loses heat quickly at night and doesn’t provide much protection unless combined with frost cloth or bubble wrap. Cloches can also heat up very quickly during the day unless they are opened for ventilation. And because they shield plants from rain, plants can suffer from being to dry.

Alternatives: - glass jars, cut-off plastic drink bottles and upturned plant pots can be used to cover individual plants. They'll protect from light frosts, but, depending on type, will need removing the next day to allow light in and prevent overheating.

Cold Frames:

Essentially a mini greenhouse, used to protect plants and extend the growing season. Traditionally made from wood but can be made from plastic or lightweight metal with airtight sides and a transparent, hinged lid.

The sides are generally sloped and the roof opens for venting.

Cold frames have no bottom and can be placed directly in the garden onto soil or a bed of sand.

Place the cold frame in a sunny position to collect and hold solar heat, keeping temperatures warm at night to protect against frost.

Mound mulch, straw or a planting mix around the sides for insulation. If you are growing tall plants in it, set the frame on bricks to gain height, and then insulate around the bottom.

In below zero temperatures, cover the cold frame with a tarp or plastic sheet.  And in warmer weather, vent the structure during the day.