Frost in the organic garden

Frost – an enemy of great beauty! Nothing is more stunning than the white, sparkling glitter of a sunny frosty day. But nothing is more deadly.

Frost kills plants, as the water inside the plant cells expands when it freezes. This breaks the cell walls, which means they can no longer carry the plant's nutrient juices (sap), causing it to die.

We look at why and how frost happens. And some of the ways that you can protect your growing area.

What causes frost?

Frost is normally formed on still, clear and cold nights. The cool air causes water vapour in the air to condense and form droplets on the ground. This forms dew. When the temperature of the ground or surface is below 0 °C the dewey moisture freezes into ice crystals - known as the frost point.

There are 3 different types of frost:

Ground Frost the formation of ice on the ground, as well as objects and trees, whose surfaces have a temperature below the freezing point of water. This can also be a ‘white’ frost.

Air frost this is when the air temperature is below freezing point of water at a height of at least one metre above the ground. This can kill plant stems, flowers and fruits. Sometimes, however, the ground cools quicker than the air, so you can have a ground frost without an air frost.

Hoar frost is composed of tiny ice crystals and is formed by the same process as dew, but when the temperature of the surface is already below freezing point. This gives a 'feathery' variety of frost. Fog often prevents the formation of hoar frost as it reduces the potential for radiational cooling of surfaces.

Plant Hardiness

Certain plants are more frost hardy than others. This means their stems and leaves can survive a frost - like evergreeen bushes and trees - or they can survive a whole winter, but their stems, leaves or flowers may not survive a frost - such as deciduous trees and bushes, and perennial flowers like lupins, roses, and lavender. This hardiness may be due to their cell construction which can adapt to freezing and thawing. The sap may also contain a higher sugar content so the sap freezes less easily. Some plants can draw water from their cells, into the spaces in between the cells, where it will do less damage if it freezes.

Unfortunately, even hardy plants and tough evergreens can be damaged by prolonged spells of severe cold when soil becomes frozen. Roots are unable to take up water and plants die from lack of moisture. Conversely, some plants – such as the Mediterranean herbs and lavender – can tolerate frost, but not persistent wetness round their roots. They need free draining soil.

Most flowering annuals and vegetables are called 'tender' because they do not survive a frost.

See the RHS Frost Hardiness classifications

Symptoms of frost damage

In autumn, the above ground parts of non-hardy plants will blacken, wilt and shrivel with the first frost. However, it is the late spring frosts that cause the greatest damage. The new tender growth is very vulnerable, and if killed by frost the plant may not recover.

The shoots of a plant will appear blackened and scorched. Often dying back to the main stem. Buds and blossom, especially on fruit trees, will become brown and drop off. Small fruits will also be damaged.

How to prevent frost damage

These are some of the ways you can prevent and minimize frost damage:

  • Choose plants that are reliably hardy in the area where you live.
  • Plant tender specimens in a sheltered spot, under large trees and shrubs or against walls, to give them some heat and protection during the winter.
  • Avoid east-facing sites for tender early flowering plants. The first warmth of the sun can cause a quick thaw. The plant cannot acclimatise and the cell walls can rupture.
  • Leave the old growth of plants unpruned over the winter months. This will help to protect the central crown of the plant and take the brunt of any frost damage. If plants are cut back hard in autumn any new growth could be damaged by frost.
  • Don’t put your pot plants, which you have kept inside over winter, outside too early. Wait until the risk of frost is passed.
  • Cold air and frost always descend to the lowest point in a garden so avoid planting tender plants in obvious frost pockets

Frost pockets As cold air is heavier than warm air it will flow downhill, collecting in valley bottoms and hollows, termed frost pockets. These pockets may be natural, or man-made. Similarly, a dense line of shrubs on a slope will cause cold air to build up against it and create a frost pocket. Create gaps or remove the lower foliage to allow the cold air to pass through.

How to protect plants from frost

Perennials are plants that should last a long time – unlike annuals, which grow and die within a year.

Here are some ideas to protect your perennials, either from one-off frosts, or throughout the whole winter:

  • Tender plants (such as pelargoniums and fuschias) should be grown in pots so that they can be moved inside during winter. Take cuttings of those that cannot be grown in pots and overwinter these in a warm greenhouse, ready for planting in spring.
  • Plants growing in the open ground in early spring can be protected with horticultural fleece. However, to be absolutely sure, sandwich a layer of bracken leaves or straw between or under the fleece to provide insulation against the frost.
  • Tender summer-flowering bulbs, corms and any perennial plants that die back should be covered with a thick mulch of manure, straw or old leaves to prevent the soil from freezing. In the spring, remove this mulch (it can trap the coldness in the soil) and protect new shoots with a loose layer of straw or a cloche. Cloches can be made of glass or plastic.
  • An upturned bucket, plant pot or even a cardboard box can be used in an emergency. Just remember to remove it later in the day, once the threat of frost has passed, to allow light to the plants.
  • Evergreen plants will benefit from a thick layer of mulch around their bases to keep the soil frost-free. This will allow them to take up moisture during periods of cold weather and stop them from becoming dehydrated.
  • Protect the crowns of tree ferns and insulate their trunks by wrapping them in layers of fleece or hessian stuffed with straw. Cordylines and palms should be treated similarly, by tying their leaves into bunches, to protect their crowns.
  • Outdoor pots and containers should be frost-proof to prevent them cracking. Place them on 'pot feet' to prevent waterlogging and contact with the icy ground. Using a light, free-draining compost with added perlite will also help with this. You can also insulate them with a layer of bubble wrap or hessian to prevent the root ball freezing inside.
  • Protect plants from the morning sun, which can damage growth if the plant defrosts too quickly. If you can't move the plants, try covering them with a layer of black plastic to block out the sun.
  • Newly-planted specimens will often lift themselves proud of the soil surface if there is a hard frost straight after planting. Check them regularly and re-firm the ground around them to ensure their roots are always in contact with the soil.

Damaged plants

If your plants do get frosted this doesn't necessarily mean the end for them, some plants will recover given time. However there are ways of minimising the damage:

  • Cut back frosted growth in spring to a healthy new bud. This prevents further die back and encourages plants to produce fresh, new shoots.
  • Give them a liquid feed, such as Comfrey tea, to encourage new growth.
  • Dig up small, tender plants and take them into the greenhouse. Many will quickly produce new growth and recover, provided they are not subjected to prolonged periods of heavy frost, wet or cold.

Fruit damage

Frosts are very damaging early in the year when plants begin to put on new growth, particularly in the case of fruit. Fruit plants are relatively hardy when dormant but vulnerable when they begin to grow in spring. The further advanced the growth, the greater the damage will be, and the higher the temperature at which it will occur. A pink bud will be damaged at -3°C (26.6°F), a full bloom at -2°C (28.4°F), and a fruitlet will be damaged at -1°C (30.2°F). Later flowering varieties are obviously a good choice in frosty areas.

Frosty terms

Frost is classified as slight, - 0.1° to -3.5°C; moderate, -3.6° to -6.4°C; severe, -6.5° to -11.5°C; and very severe, below -11.5°°C.