Considering Australia’s mild winters, frost is more common than you’d think. Frost is formed when moisture freezes into tiny solid ice crystals, which happens on cold, calm and moist nights. There are two levels of frost experienced in Australia – mild and severe. Mild frost occurs at 0°C to -3°C and severe frost anywhere below -3°C.
Whether your plants will survive frost is completely dependent on their varieties. Some plants thrive, with certain flowers even blooming in the colder conditions. Whilst other plants are easily killed by a light frost – first rotting, then eventually dying.
Choosing the right plants for your location and climate is the first step to battling frost damage, but there are ways to protect the more sensitive species in your backyard.
Frost in Australia
Frost can occur in most parts of Australia, appearing as far North as Queensland’s Atherton Tableland’s. In Australia, frost is frustrating for gardeners but can be devastating for farmers. Every so often, a frost occurs that wipes out tens of millions of dollars worth of crops, so it’s no surprise that so much research goes into frost management.
Gardeners who are lucky enough to live on our coastline are mostly protected by the warm sea temperatures, but the further south you travel, the more common it is. South-Eastern Australia has experienced a rise in the incidence of severe frost, along with Tasmania, NSW and the ACT. Frost is also more frequent the closer you are to the high altitudes of the Great Dividing Range.
However, frost can be found almost anywhere with the right conditions and land formations. Freezing temperatures coupled with valleys, ground dips and creeks are the perfect combination to create frost pockets.
Frost in Australian Cities
For city dwellers, the threat of frost is extremely rare. This is because the heat created by close together dwellings, city traffic and a dense population is enough to ward off the temperatures needed to form frost. This phenomenon is known as the heat island effect. Pollution and cloud cover also plays a role in keeping our cities warm, preventing nighttime temperatures from cooling to the frost danger zone.
However, city frost still happens, especially in our southern capitals like Canberra, Melbourne and Hobart. Residents of East Coast areas not usually prone to frost will find East Coast lows bringing the perfect frost conditions. If you live in the city, keep an eye out for clear nights and predicted freezing temperatures, as this could mean frost the following morning. So, if the warning signs are there, it’s best to take action than see your beloved herb garden succumbing to the frost.
Frost and Your Plants
So, what exactly will happen to your plants if exposed to frost? It depends on the species. Some plants will wither at the site of a light frost, where others can withstand severe occurrences.
Frost affected plants are damaged plants, which can potentially rot and die. As the plant freezes, so does the water inside it. Then, as water freezes, it expands, and when the plant is quickly thawed, it results in tissue damage. If thawed slowly, frost damage is less severe or can be avoided altogether. Unfortunately, Australia’s cold winter nights and mild winter days create the perfect conditions for frost damage to occur.
On a cellular level, frost shrinks the plant’s cells as they freeze, forcing the expanding water into the spaces between and forming ice crystals. If the plant is thawed slowly, the water can be absorbed back into the cells by osmosis, but a quick thawing will leave cells without the water they need.
Frost damage commonly occurs on exposed leaves and new growth and almost looks as if the plant has been burned. Leaves will appear waterlogged or soggy, shrivel and eventually turn brown or black.
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Plants That are Most Affected by Frost
Recent shoots and flowers are most susceptible to frost due to their delicate nature. Some varieties of succulents are more affected by frost than others. Thanks to their soft, moisture-storing leaves, soft varieties of succulents do not fare well in the frost, and you will find their leaves are soggy and mushy in the morning after exposure. Succulents like aloe, with particularly high moisture levels, have been known to split in frost as the water inside them freezes and expands.
Tropical plants, and other varieties that thrive in warmer climates, will respond terribly to frost, leaving fruit crops barren and indigenous trees suffering. Frost that occurs in warmer climates which are usually frost-free, can be the most damaging. Temperate stone fruits are also at risk, as they flower before the frost risk over.
A plants’ origins will determine its tolerance (or intolerance) to frost. Plants that come from colder climates, like deciduous species, will stand up to frost better than tropical or indigenous species. Depending on the location and species, frost-tolerant plants protect themselves in a number of ways.
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It’s widely accepted that a plant is more tolerant to frost thanks to higher sugar levels. The sugar acts as a sort of anti-freeze, making it harder for the water content of the plant to freeze completely during a frost event.
Deciduous trees create and store a lot of food in their branches, roots and trunks through photosynthesis. The end result, or food source, contains an amino acid that has the same effect on freezing as sugar. Some species which originate in frigid climates even create an anti-freeze type protein to protect themselves during the winter months. Luckily, there is a long list of plant species available in Australia which are more frost tolerant.
Frost-Tolerant Species in Australia:
- Laurustinus (Viburnum tinus)
- Ornamental Pear (Pyrus calleryana)
- Maples, elms, and oaks or other winter dormant trees
- Creeping juniper (Juniperus horizontalis)
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- Swiss Chard
Benefits of Frost
Frost isn’t always a disaster and can have several benefits in Australian gardens. In fact, some plants need frost to reach their full growth potential. Deciduous fruit trees benefit from frost, intensifying the flavour of the fruit and leading to more vibrant coloured leaves. Starchy winter vegetables like parsnips and turnips also get a flavour boost from frost, as the cold snap turns their starches into sugar, creating a more delicious harvest. Sweet ice wine relies on a frost to create its thick, syrupy consistency, with the grapes needing to be frosted and thawed multiple times throughout the growing process.
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Frost can be helpful for gardeners too. Killing off weeds before they run wild in the summer months and disrupting disease and pest life cycles. It can also improve the quality of your soil.
How to Protect Your Plants From Frost
Several effective tactics can save your plants from frost or at least minimise the damage.
- Plant seedlings and cuttings in Styrofoam boxes instead of directly in the ground
- When frost approaches, cover or wrap plants in old bedsheets, curtains or towels etc.
- Gently tie the foliage of tufted plants together to shield the growing point
- Protect your low-lying plants by laying a pane of glass over the top, held up by two pots
- Relocate potted plants inside where possible or to a more sheltered area in the garden
- Use ice to your advantage – mist stone fruit flowers with water the night before a frost, which will freeze and form a protective barrier.
Words by Nell Matzen
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