People are often surprised that we get frost here in Queensland. Sure, the majority of Queensland is typically frost free, but here in the South East corner we can experience frost, with some inland areas around Charleville having 40-50 frost days every year. And certainly if you live in NSW, Victoria, Tasmania, and southern parts of SA and WA, you can expect a few frost days too. See the Australia frost map here.
I know our winters aren't really cold by any means, we don't get snow, in fact, we are more likely to have a beautiful sunny day after a frost, and can have a temperature increase of over 20 degC in one day! But this actually makes things trickier because we can ALMOST grow tropical plants, but it only takes one hard frost to knock them back until the weather warms up in spring. Fortunately there are a few tricks we can use to manage frost when we understand what it is and how it behaves.
frost on broccoli leaves
a pawpaw tree after frost
What is frost?
Frost is not just ambient air temperature below freezing. Frost occurs when the air temperature close to the ground cools below freezing AND below the dew point so that moisture in the air either condenses and then freezes, or freezes directly out of the air. The type of frost that we experience in a temperate or sub-tropical climate is typically caused by the land cooling overnight and cooling the air close to the ground. As cold air is denser than warm air it tends to sink and to flow downhill. This means that frost occurs lower down in the valley and can be trapped uphill of obstacles such as hedges or walls. Frost is unlikely on windy nights as the layer of cold air is disturbed. Dew point depends on temperature and humidity. More frost info here.
The important point is that we can expect cold temperatures to develop on clear still nights, but that if we understand how cold air moves and how frost forms from moisture in the air, we can take steps to prevent frost damage.
What happens to our gardens in a frost?
Some plants are well-adapted to frost, and some even benefit from frost conditions. Silverbeet, brassicas (broccoli, cabbage, kale etc), root crops, some lettuce, aliums (onions, garlic, leeks etc), herbs such as parsley, chervil, yarrow, mint, oregano and thyme, all survive frosts. Summer crops such as tomatoes, curcubits (squash, cucumber), capsicum, chilli, eggplant, beans, and herbs such as basil, ginger and sage, do not survive frost. Some of these will grow back when the weather warms up again, but many will be too badly damaged. Tropical plants that I try to grow, like sweet potato, rosella and paw paw also do not survive frost.
silverbeet with frosted leaves
How can we protect the sensitive plants?
First, find out if your area is prone to frosts, when you can expect the first and last frosts of the year and how cold its going to get. Apart from checking the meteorology data you should look for some local knowledge. We used to get frost around our house at Nanango, but our neighbour further up the hill from us hardly ever had frost and even then, always had a little frost-free pocket around the house and and water tanks just because their property was higher than ours.
You need to know if you are below the frost line (and then if you are too high, you can suffer from frosts due to altitude as well!). Properties in our area are advertised as "frost-free", but I didn't understand the significance when we were looking to buy. Our house at Kumbia is high enough to avoid most frosts.
Once you establish that frosts are likely, there are a few things you can do to keep your garden going through winter.
Plant appropriate varieties and understand which vegetables are going to survive and which will need extra protection or if they are just bad choices for your location.
Protect sensitive vegetables by planting under under cover of trees, close to water tanks or the house (which holds a little heat overnight and can prevent the temperature falling below freezing) or plant in pots that can be moved to safer ground.
Even though this sounds bizarre, if you water at night before a frost is likely, wet soil will hold more heat and may maintain temperature above freezing (although this can backfire if it does get cold enough to freeze the water on the ground and on leaves).
If you can circulate the air above your garden using a fan (or a helicopter or a million butterflies) you can mix the cold air near the ground with warmer air higher up - this is used in commercial orchards.
Cover your veges with fabric or straw/old leaves to try to maintain the air temperature around them. I keep reading to not use plastic, but personally I have had success with using clear plastic all around the plant, and including a bucket of water so the air under the tent is more humid, as this maintains a warmer temperature around the plants.
Consider where cold air will flow and make sure if can't get trapped uphill of solid fences or hedges in your garden, and also use this concept to direct air AROUND plants but stacking hay bales etc uphill of a sensitive plant.
Here are a few practical steps that I have taken in the past to help with frost in the garden:
Bella helping to trim the frost-sensitive arrowroot
I planted frost tolerant veges so I have something growing in the garden - this includes brassicas, peas, broadbeans, silverbeet, leeks and spring onions, and I won't get so upset when the beans and tomatoes die :)
plenty of frost-tolerant brassicas and asian greens this winter :)
I bought a small greenhouse for the sensitive plants that I want to keep - chillies, eggplant and avocado that took SO long to mature, I don't want to start from scratch next spring, so I hope to keep them all alive over winter, I also put away the greek basil and the ginger. We were planning to have the aquaponics greenhouse finished, but got carried away with other projects, so a $40 plastic greenhouse is sufficient to keep a few things safe from frost this winter.
a mini greenhouse to keep a few plants safe over winter
I cut back the frost sensitive perinnials - the beans, arrowroot and comfrey all die back from the frost, so I thought I may as well cut them back while they were green and use them for mulch before they die. This also reduced the shade around the garden (great for summer but unnecessary in winter). In the end I didn't get to use any of it for mulch as Bella and Molly decided to eat it all and I didn't mind. I didn't have the heart to cut back the giant paw paws though!
Arrowroot and comfrey trimmed
More arrowroot trimming
The paw paws have no idea what is coming!
Keep sprouting - even in the middle of winter if you can't get anything to grow outside, you can always sprout! Over summer I used lots of sprouts in salads, and in winter I like to grow the larger sprouts (mung beans and fenugreek in particular) to throw into stews and steamed veges.
sprouting - always something green to eat even when its cold outside
Changed watering routine - instead of watering in the afternoon after work using the hose with spray nozzle, I have rigged up a large sprinkler so I can water first thing in the morning, this means the plants won't have soaking wet leaves overnight, which can cause extra frost damage. Early morning water can also help to warm up frosted leaves before the sun hits them and causes them to heat too quickly.
I should be grateful that we don't have to deal with snow, so many blogs I read can't grow anything over winter, and that would be another challenge in itself! I guess I get frustrated because we are so close to having a sub-tropical climate here, but for a few cold nights over winter, however if you're prepared, you can at least keep something growing.
Actually I was trying to think of things to which I should be grateful to frost for and I did think of a few:
It does kill off a number of annoying weeds and bugs
At least I can grow veges that do like some chill time, like carrots, turnips, swedes and broadbeans, and fruit trees like apples and stonefruit (or at least I hope I can grow them!).