Frost In Your Garden

In the Hilltops region, frost is an element you will need to contend with on varying scales every winter.

A frost will most likely occur when we have temperatures of about 2 degrees and below, with clear, cloudless skies at night. A frost can be very damaging to your garden, however the frost tolerance of the plants you have in your garden can reduce these affects and help your plants survive the frost. The best way to ensure your garden survives the frost is to make sure you are selecting plants that are frost hardy and be aware of which sections of your garden will be frostier and therefore, require some additional attention.

Why does frost affect your plants

When overnight temperatures drop below freezing, the water found in plant cells will freeze. This will cause damage to all the cell walls within the plant. Then, as the sun comes out, the plant starts to defrost at a rapid rate and this then kills the leaves and/or the stems of the plant. Those plants that get the first of the morning sun seem to suffer the most, as they defrost quicker than say the plants which are shaded from the morning sun. So get to know which plants face towards the morning sun – they will be the ones on the eastern side of your garden.

Plants most susceptible

Newly planted plants will not have totally acclimatized to your garden yet, so they may be burnt by frost the first year they are planted out. If there is new growth on these plants, these young, softer leaves can be more susceptible to frost burn than others. Also, plants that have large, soft leaves such as Canna Lilies will be more likely to be damaged from frost.

Plants that have been grown in hot houses or green houses should not be planted out in gardens during winter. Ideally, you should select plants that are suitable to your area and ones that have been grown as locally to your area as possible so that they will be hardened off to some frost.

Be aware of the microclimates within your garden

Every garden will be different. Your neighbour’s garden may not get any frost, whereas you do get frost. The key factors for this are the topography of the land and the amount of tree or shrub coverage you have throughout the garden. You will also find that even within the one garden space, some areas will suffer more than others.

Learning about your garden’s microclimate is very important so you can get to know where some plants will work better than others. For example, if you have a flat, open garden area, then the cool air will hit the ground level evenly and at the same temperature. You would expect the amount of frost damage would be the same across the total area. If you have pockets of your garden which sit at lower levels than other sections, then the cool air will hit the lower points and settle here. These lower locations will be more prone to frost.

I know here at the Garden at the Berkshires, we can have a very heavy frost on the flat, where even the water in the troughs have become frozen solid, but up on the hill where the garden is, there is no frost on any of the plants or in the ponds. That is a difference of about 50 metres with a height difference of about 40 metres. It is quite an interesting detail to understand and very important when it comes to growing plants.

You can also help reduce the effects of frost in the lower areas by planting out shrubs or trees that will increase the foliage coverage in the area and give more frost protection to the smaller plants. Another factor that will affect the microclimate in your garden is any structures in the garden, such as retaining walls, fences or even the wall of your house. During the daytime hours, they will absorb some heat from the sun shine so they can create little frost-free havens. Some plants will tolerate more frost than others

The foliage colour of a plant can increase the hardiness of plants to frost. A darker leaf such as purple or bronze coloured foliage can retain more heat and therefore, will be more tolerant to the frost. Also, plants which have downy or furry leaves can retain more moisture, which again offsets the effects of frost. This extra coverage can almost insulate the plants from the colder temperatures. Using plants that are suitable for your conditions is the easiest way to guarantee their survival.

Ways to help your plants survive frost

Covering plants – the easiest and the most common way to guard against frost is with the use of some type of covering. You could use hessian, sheets, plastic tarps etc. Then, depending on the location and the plant, you may need to secure them with some ties around stakes or use a framework to sit the coverage on. You can purchase frames that are covered in a plastic. These are then staked into the ground and secured with the plant at their centres. Ideally, the covers should not be heavy and they shouldn’t be touching the plant completely. Don’t forget to also remove the covers after the frost so that the plants don’t suffocate or fall victim to too much humidity build up underneath their frost blankets.

Mulching plants – mulch will act as an insulator to the soil. It already helps to retain the moisture in the soil and then during cold weather, this in turn will hold the heat in the soil.

Watering – a light watering the day or evening before frost is expected is a good way to offset the effects of frost. The water in the soil will raise the temperature and the humidity levels within the soil compared to a dry soil. Therefore, the warmer conditions will decrease the likelihood of frost damage. Another idea is to water your plants thoroughly in the early morning before sunrise so that any frost on the ground will melt away and not be melted by the morning sun. Alternatively, if you do not want to be up that early, set up a watering system on a timer to do this for you.

Cold frames for plants – some tender plants will require permanent protection over winter or they may even need to be brought indoors. You can build the sides out of wood, blocks or bricks and then use Perspex, a sheet of glass or even an old window as the top. You need a clear material so that the light can still get through to the plants. After the frost of the day, ideally you need to open them up so that the plants can get air ventilation to prevent too much humidity. Too much humidity can lead to fungal diseases.

Raised beds for plants – having a slightly above ground garden bed can help keep the soil warm. The colder air will sit at lower levels, so having the planting beds out of the ground will help prevent that colder air from hitting the plants. The raised beds for vegetable gardens are ideal so that they can survive the coldest days.

Apply a Seaweed Solution to your plants – using a seaweed emulsion will help plants develop an internal resistance to frost. Regularly using these as a foliar spray over your plants or on the roots will help mitigate the effects of frost, however you will need to use it over a period of time for it to be effective.

Don’t prune the frost damaged foliage – if your plants do get hit by a frost, the foliage will turn a blackish purple colour and more succulent leaved plants will turn to mush. It is best to leave this damaged foliage on the plants whilst the risk of a frost is still possible. Leaving the foliage on the plants will act as a barrier or protection to the rest of the plant for the remaining cool weather, so hopefully the crown of the plant is not damaged and once the warmer weather starts, it will reshoot. An example of this may be some species of soft wooded perennials or herbaceous plants. At this time of year, their flowers and foliage might start looking a bit tired and after a heavy frost they may look even worse, however you need to put up with the ugly foliage because most likely there are new shoots starting at the base of the foliage. This old foliage will be protecting all of those new baby shoots. No matter how tempting it is, leave the foliage on the plants.

Positive effects of frost

Despite all the damage a frost can have on your garden, there are some plants that actually need a frost for their fruit growth. Many fruit trees such as cherries, peaches and apricots, require a certain number of days at a certain temperature to actually bring on fruit set. This is called the chill hours or chill units required for fruit production. It is the number of hours at or below 7 degrees that the fruit will need for the tree to exit dormancy, create buds and flowers and then to set fruit normally.

Some vegetables such as carrots, beetroots, kale and parsnips actually benefit from the cooler temperatures, as it will enhance their flavours and make them taste sweeter. In the colder weather, these more starch-based plants will try to combat the effect of the water inside their plant cells from freezing. They change the starches in their plant cells to sucrose, they produce more sugars and this helps prevent the plant cells from freezing so that they can become more resistant to cold. Sucrose, of course, is a sugar so they in turn become much sweeter to eat.

A frost can also help destroy or suppress pests and disease. A very cold temperature might actually kill off some insects or fungal infections. It does not create ideal conditions for disease, pests and/or fungi to grow in. This will mean you have less work to do in your garden at managing these issues.

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