The first day of winter marked the first significant frost this season at Sophie’s Patch and there have been more since then. Prior to this there had been some minor frosts in lower areas of the garden but last weeks finished off my summer crops. Frost can wreak absolute havoc on plants that are not adapted to cope like many of the succulents I grow on my back deck, so now is the time to be proactive.
When does frost occur?
In the Adelaide Hills we are blessed with a cooler climate than on the plains – this gives up relief from the stifling summer heat, it gives us the spectacular displays of autumn foliage that is just drawing to a close, and it also allows us to readily grow a whole range of plants that require cooler temperatures, such as cherries and camellias. Frost helps the autumn tones, it can intensify the flavour of winter vegetables and improve fruit set on cool climate fruit trees such as cherries, and yet it can also cause challenges of its own. Not all areas of the hills experience frost, but for those gardening in low lying areas, such as Mt Barker, the Onkaparinga valley or around Mylor to name just a few examples, it can present a challenge.
Frost occurs on clear still nights when the temperature drops below one degree. If there is no wind, cold air settles to the ground and pools to the lowest level such as the bottom of a valley. These cold temperatures cause moisture to freeze and expand, including the moisture inside a plant’s foliage. The expansion causes the cells to burst however we don’t usually notice the damage until it thaws is because the ice holds the plant rigid.
Frost damage can be worse in particular areas of a garden. Cold air, being heavier than warm air, tends to collect at the lowest point. It can actually get trapped by a fence or hedge at the bottom of a garden so makes sure to leave a narrow exit for this cold air to escape. The canopy of evergreen trees and shrubs often protects the plants growing underneath and frost damage will not be as severe here. This is often seen with Cliveas, those wonderful shade loving, waterwise perennials with dark green strappy leaves and bright orange flowers. Out in the open they are frost tender and can actually be burnt off completely, however under an evergreen plant in the same garden, they may thrive.
Seasonal variances, such as early autumn rains, can also help to cause frost damage in plants that are usually frost tolerant. For example early autumn rains and mild weather can cause plants to put on a spurt of growth, all of which was tender and can then be badly burnt by frost. This can even occur on plants that are indigenous to an area and well suited to their environment.
The obvious solution for gardeners in frosty areas is to choose plants that are frost tolerant, however understand that there are degrees of frost tolerance is vital. A plant that survives frost to -2 or -3 degrees like those we would get at my previous garden at Ashton may be killed by the -5 degrees we get at Mt Barker Springs. Unfortunately plant labels may not reflect this. Choose plants that come from places within Australia and overseas that regularly experience low temperatures and frost. Just as there are adaptations that help plants cope with heat and drought there are a number of adaptations that help plants cope with frost. Deciduous trees and shrubs have lost their leaves over winter, and in this dormant state are unaffected by frost. Herbaceous perennials have died down and are safely under the ground. Other adaptations include the downward facing foliage of conifers and any plant with tough leathery leaves.
So what do I do to prevent frost damage in my garden?
Firstly I move tender plants if I can. If I am growing plants like succulents, lemon grass or ginger in pots, I simply move the containers under the eaves of my house and that protects them. Similarly if I want to grow plants that aren’t suited to my climate, I try to place them where there is a favourable microclimates. My frost tender night-scented Jessamine (Cestrum nocturnum), Plumbago and Iochroma are positioned against the stone walls of my house and they have survived really well with the protection that affords them.
Treating my plants with a seaweed based plant tonic seaweed solution can also help increase a plant’s frost tolerance by 2-5 degrees, which can mean the difference between life and death. As well as this I spray tender plants with an anti-transpirant spray such as Droughtshield which puts a protective polymer film over the leaf and prevents frost damage for up to a month. In New Zealand this same product is sold as Frostshield. Also, the simple act of washing the frost off the foliage of sensitive plants before the sun hits will help to reduce the damage.
Some trees and shrubs need to grow up high enough that their new foliage is above the frost line, and this may only be matter of a couple of metres. This is the case with Jacarandas which I am trying to establish on my front lawn. The main tree has grown well as it is closer to the paving which must increase the local temperature slightly. Over the past few years young plants further away from the paving which haven’t been covered have been killed or at least severely knocked by frost so this year I have them wrapped with frost cloth.
Some plants may be frost hardy once established but can still be frost tender when young. So if you live in a very cold frosty area, unless you are planting known frost hardy plants, delay planting until the risk of frost is over (usually mid spring) even though autumn is often a better planting time from a waterwise point of view. If there are only a few possibly sensitive plants you want to plant, and you believe that they will only be frost tender for the first year or so until they get established, you can cover them at night when frost is predicted. Place a cloche like structure over the frost tender plant.
In early spring when I am planting small summer vegies which dislike our late frosts I use old glass flagons which have had their bases cut off. You can also create a protective cocoon using some sort of frame covered in frost cloth (available in a roll from good hardware stores or nurseries), clear plastic or hessian, or even an upturned plastic pot, but be sure to remove solid covers like pots in the morning. Frost cloth works the best as it becomes the surface on which the moisture freezes rather than the plant. It can also remain on the plant for long periods – during the day and night, providing an insulating layer while allowing about 70% light transmission as well as rain penetration. Draping frost cloth over small shrubs and trees and securing it to the ground with bricks or such, traps heat that has collected in the soil during the day and releases it slowly at night, preventing frost formation. Holding the material down away from the trunk allows more warm air to be trapped.
Finally if you do have some plants damaged by frost this year, do not prune them back until the last frost has passed as even the dead foliage protects the plant underneath, and the act of pruning stimulates new growth which will be tender and very susceptible to frost damage.