For many gardeners, frost is an integral part of the natural cycle of seasons.
Late in 2017, I visited Alison Aplin’s garden ‘Timandra’, in the Clare Valley. The microclimate there is one of extremes, and Alison described stepping outside one morning to see what appeared to be a layer of white ash on everything from ground level to above her head. But the ash proved to be tiny crystals of ice – not fire, after all, but a spectacular frost.
In the subsequent winter, gardeners in the Adelaide Hills experienced some of the worst frosts in memory. In my cousin’s house, waterpipes froze and then burst. Elsewhere, hundreds of solar panels cracked. Plants that had survived in gardens for decades turned up their frozen toes.
But frost is a normal part of our weather. In many areas of Australia, it appears as regularly as the winter southerlies, or the summer northerlies. Frost is essential to certain plants (eg cyclamen, tulips, daffodils, blueberries and currants) to provide sufficient winter chilling to promote flower bud formation (and hence fruit), or to improve the flavour of food plants (eg kale). Cold and frost can intensify the colour and beauty of autumn foliage trees and shrubs, and many plants – especially northern hemisphere and alpine ones – need a winter chill or dormancy period each year if they are to survive long term. Jack Frost can be a charming and useful partner in the garden – he just needs to be handled correctly!
What is frost?
The frost we normally see is caused by water vapour freezing as tiny ice crystals on surfaces. Radiation frost is caused by heat lost (radiated) from the earth into the atmosphere. As the warm air rises, the cooler air higher up sinks to the ground. The greatest chance of radiation frost occurs after a cool, cloudy day (little heating of the soil to begin with) followed by a cold, calm clear night, when there is no blanket of cloud cover to hold in the heat or breeze to keep the air mixing.
Advective frost is frost caused by wind chill (these winds are sometimes known as "black southerlies"). They occur when very cold air masses sweep across southern Australia from even further south, and in south-eastern South Australia I have seen them kill the top two metres of a stand of young trees, while the lower three metres, sheltered by a hedge, remained lush and green!
White frosts occur when there’s sufficient water vapour in the air to condense and form (white) ice crystals. Black frosts occur when the air is very dry and cold: cold enough to freeze plant tissue, but too dry for ice crystals to form.
How does Frost affect Plants?
Frost freezes the cells of certain parts of certain plants, disrupting normal cell activity or causing them to dehydrate. Frost-affected leaves (usually on the most exposed outer parts of the plant) often turn black and/or shrivel before falling, leaving bare twigs behind. Young, soft and actively-growing plant parts are the most susceptible to frost damage.
Why only certain parts of certain plants? Because many plants have evolved to handle extremes of cold, including snow and frost. Deciduous trees and herbaceous perennials lose their leaves or above-ground parts during the cold season when snow and frost are likely to damage them. Others – such many cold temperate evergreen plants – have evolved thick, leathery or small leaves that discourage the formation of ice crystals on their surfaces, have thickened cell walls resistant to disruption, or have mechanisms for allowing ice to form in the spaces between the cells, rather than within the cells themselves. Last year, researchers discovered an anti-freeze protein gene in an Antarctic grass, with huge potential for reducing frost damage in commercial crops.
Managing Frost
The severity, frequency and timing of frost depends on your climatic region and the microclimate within your garden. Some parts of Tasmania can experience sudden late frosts into December, whereas other parts of Australia have regular, less severe frosts over a short period. In the latter regions, with a few precautions you can grow a wide range of non-frost hardy plants, but I believe it’s a waste of energy fighting nature in regions where frosts are very frequent and/or severe. Here, your best bet is to simply grow frost-resistant plants, at least until you have created a microclimate (eg in the shelter of established trees) for more sensitive plants to grow.
So, here are my Top Tips for Handling Jack Frost:
Sensible Plant Selection
Choose frost resistant plants, be they native or exotic. Many Myrtacea, Proteacea, conifers and Acacia are extremely frost hardy. Most Northern hemisphere deciduous trees are excellent too, as are herbaceous perennials that die back to an underground rootstock over winter. Some fruit and nut tree cultivars are specifically bred for frost-resistance; others are simply inherently more frost-resistant (eg cool climate cherries or apples compared to Mediterranean-climate citrus). You can choose late varieties rather than early ones if you have regular late frosts to avoid. Best of all, wander your neighbourhood to see what grows well in the gardens in your area – mature specimens will have weathered many frosts and you can be confident they’ll survive in your garden, too. Visit your local nursery.
Plant Location
North-facing stone or brick walls absorb heat during the day and radiate it back out at night, so that you can grow even cold-sensitive plants in the adjacent ground or in tubs. Walled gardens are practical in very cold areas for this reason.
Conversely, some cold-climate fruit trees may come into leaf too early if planted in a spot that’s too warm, so that the buds and flowers can be hit by a late frost. In this situation, choose your planting site carefully so that flowering is after the last spring frost.
Move tender tub specimens under the veranda or move them under trees during the frost seasons.
Garden Design
On sloping land, cool air flows to the lowest point. With a barrier such as a wall or fence in place, the cold air will pool there, creating a potential frost pocket. When designing in frost-prone areas, it’s therefore a good idea to leave breaks in hedges or fences to allow a continual throughflow of air from the top of the property to the bottom and beyond.
Conversely, wind protection is essential to protect from advective (wind chill) frost. The establishment of hedges, walls and mixed windbreak plantings should be first on your planting priority list if you live in an area with black southerlies.
If you have no luck at first, don’t despair. A bare new block is likely to experience more severe frosts at ground level than an established one with a canopy of trees. If new gardeners are patient, they will find that over time, they will be able to grow a wider range of plants as their garden establishes and the microclimate moderates. For example, you can grow more sensitive plants under a canopy of hardier trees for protection.
Hardening Plants
Many evergreen plants become hardened to frost in the same way as seedlings from a nursery can be hardened to sun by progressively increasing their exposure. This is a normal, natural process that occurs automatically in many evergreen frost hardy plants (eg conifers). If you’re aware of this process, you can manipulate the timing of your planting of new trees and shrubs so new growth can harden well before the first frost.
Protection
Over the years, numerous forms of protection have been used to keep plants alive during cold snaps. In Europe, active compost pits were used to heat glasshouses for a doubly protective whammy. Small fires were lit in orchards, and lights hung from tree branches. Giant fans and even helicopter flyovers have been used to circulate air, and plants covered with straw, cloth or cardboard for insulation or to catch extra heat. To me, this all seems like rather too much work for the home gardener. However, many plants are frost sensitive when very young, but become more resistant after a few years as their stems thicken up and their leaves harden. In this case, it’s feasible to protect plants with a square of fabric or shadecloth above them (not an enclosed tent as is used in overseas countries) on frosty nights, removed after the risk has passed, and only during their establishment period, before letting them fend for themselves in maturity.
Pruning
The timing of pruning is essential in frost-prone areas. Leave pruning until the last chance of autumn frost has passed. If any plants have been burned by frost, resist the urge to prune away the dead parts until all chance of further spring frosts have passed, because those dead straggly bits provide a degree of protection to the rest of the plant for as long as they are left in place. This applies especially to perennials such as salvias, which can be stimulated into early development that will simply be burned off by the next frost.
Feeding
Avoid excessive fertilising in autumn, where the lush growth produced is more susceptible to damage. Instead, let the growth rate slow for leaves to harden naturally.
Manipulating soil.
Firm, bare damp soil absorbs more heat during the day than one that has been mulched or is covered with vegetation. Of course, in Australia, mulch is essential during the hot, dry season, but Dr Peter May (Burnley College, University of Melbourne) suggests simply raking mulch away from plants in winter, and reapplying it in summer, for a best-of-both-worlds solution.
Some native frost tolerant plants:
Jack Frost may have a bit of a reputation but he really is not the villain he’s made out to be, and many, many plants and gardeners already enjoy the dance with him. Why not join them?
