The Home Gardener – Understanding the Frost Phenomenon
I live in Melbourne and for the last 12 years, we have been in drought. When I started studying horticulture, frost was not a major problem except in the outer suburbs. I have always Mulching Melbourne lived by the sea and so I don’t have very much experience of frost. But this winter 2010, has been much colder and there have been light frosts at night. I am now beginning to gain some experience in coping with frost in my garden.
Frost can cause quite a bit of damage in your garden. Some plants are better able to survive the freezing temperatures where others such as succulents can become a soggy mess. Leathery leaved plant are better able to survive the damaging effects of the internal moisture freezing than the soft lush green leaved plants such as impatiens which are not able to withstand the freezing of the water in their cells, the walls rupture and they are not able to be repaired.
There are two types of frost, radiation and advective. Radiation frost occurs when there is a clear night and still conditions. This allows the radiant heat to rise to the upper level of the atmosphere and cold air collects near the ground. Radiate frosts are usually the first frosts of the season. Advective frosts are when a cold front sweeps into an area bringing with it windy gusts and clouds. But on overcast nights the clouds can also act as a blanket and keep temperatures warmer. The wind can help moderate frost occurring as it mixes the cool air with the warm air, thus creating a uniform temperature.
In the city, temperatures are higher because of the radiant heat from the brick and concrete buildings. But as you move out into the country, night temperatures fall. South facing areas are colder, more likely to experience frost and don’t receive as much winter sunshine as north facing aspects. Southerly aspects also take longer to warm up during the day and tend to cool down earlier in the evening.
Frost effects the internal structures of plant cells. As the temperature drops, the water inside the cells freezes. The cell walls can stretch to a point, but when they can no longer expand they can rupture. They can not be repaired. Once the cell walls are destroyed, the cell can’t operate all the complex processes they carry out and thus the leaf or sometimes the whole plant dies. The freezing doesn’t cause the damage, it is the thawing process. If they thaw rapidly, the cells expand and burst. If you can get out early in the morning before the sun’s up, you can water them gently before the sun hits them, you may be able to save your plants. It is when the sun hits them and they thaw quickly that the damage is done and is permanent. If you have overhead sprinklers, you can put them on early in the morning to keep the air temperature from dropping. If the plants cells are destroyed, watering won’t repair them. Watering during a frost works as follows:
Watering the plant helps to raise their internal temperature and the air around them which stops the cells liquids freezing. Also, heat is released as the water freezes which raises the surrounding air temperature. If it is going to be a frosty night, overhead watering will help increase the air temperature but if ice forms on the plants leaves, then you must continue the overhead watering until sunrise.
For the home gardener, what can we do? Well some solutions are as follows:
Put any soft delicate potted plants on the veranda or bring them inside
Temporarily put old sheets or a geotextile type material over the plants for the night.
Don’t use plastic, as moisture forms underneath it and freezes
Don’t forget to remove it in the morning
If you know a frost is coming, water your garden well. Wet soil holds more warmth than dry soil.
If you live in the country, lighting small fires, as the smoke (more than the flames) can help to heat up the air temperature
Choose indigenous plants as they will be adapted to your local climatic conditions
Mulch deeply around your plants (but not right up to the trunk)
Thickly apply animal manure around your plants, it heats up and raises the temperature.
Keep a diary, noting the beginning and ending of frosts, so you can get some sort of idea of when they start and finish.
Hardening off is a process of toughening up tender leaves of plants grown in ideal conditions such as hot houses. When plants are grown is ideal conditions they aren’t used to the outside climatic conditions and need to be ‘hardened off’. It toughens them up by thickening the cuticle on the leaves so that the leaves lose less water. This helps prevent transplant shock and sudden death from dramatic changes in temperature. The period needed for hardening off depends on what season it is and type of plants you are growing.
To harden up your plants you need to move them out of the “ideal conditions” and into the world, by leaving them longer each day adjusting to the hot or cold weather. Don’t put plants that have been used to hot house conditions in the full sun. Start them off in the shade for a few days then move them into the morning sun for a few hours, eventually leaving them all day in the sun but bring them in at night especially if the night time temperatures are still low. And finally, when they become acclimatised to the outside conditions, they can be left out all night in the open.
Plants by themselves are more likely to be damaged by frost than plants surrounded by other plants. By creating a micro-climate using plant material, this allows you to create an environment where the surrounding temperature is warmer. Remember plants respire at night and the energy released produces heat, which warms the surrounding air temperature. Also using structures such as brick walls, is a good idea because they absorb heat during the day and release it at night, thus raising the air temperature.