Updated: Oct 18, 2021
There is something magical about the big blue skies and frosts in inland Australia. I love the cool crisp morning, where the grass is covered in an icy-glean but the sun is shining bright. Some plants love frosts, and need it for reproduction. However, timing is everything!
What is frost and why worry about plants?
“Frost is a deposit of soft white ice crystals or frozen dew drops on objects near the ground; formed when the surface temperature falls below freezing point” (definition sourced from BOM).
Temperate food plants, like berries, pome (apple and pear) and stonefruits, need it to get very cold before they will grow fruit. This is called chill factor or chill hours. Frost is just part of that cold that some plants need to be productive. And we have the perfect climate here for fruit plants that needs chilling-our temperatures in Castlemaine can get as low as -6.3°C (using data measured from 1966-2018). But timing is everything. We have to hope we get the cold at the right time of the year, and not a late spring frost that can damage new growth, flowers and fruit.
Some plants I am establishing don’t like frosts–I have my newly planted quince and fig covered up on cold nights. And, I have unfortunately had some losses this year related to late frosts and early budding, particularly in my raspberries and boysenberries.
I recall a friend saying in early September that Castlemaine and surrounds wouldn’t get any more frosts because budding had started. And then we did. Several frosts over September, October and I am expecting one tonight in November. I wondered, is this a symptom of climate change? What is actually happening here?
A raspberry hit by frosts at budding, just as leaves unfurl (left) and a healthy raspberry (right).
How does climate change affect frosts?
I heard at a Connecting Country talk on weeds and climate change that our frosts are supposed to change. The presenter said that they would initially get worse. What does worse mean?
The GRDC and CSIRO are doing research on frost and climate change, and what it means for food production. GRDC have a great webpage on frost research that says Australia will get either more frosts in general or more late frosts, depending on location. With the increase in temperature with climate change, they have said that food plant production will also start earlier, which makes them more prone to frost damage. This evidence is backed up by work at CSIRO, where they have found that the frost season has already become longer. There has also been research worldwide, including by ETH in Zurich and University of Maryland, into frost seasons and earlier plant production as a result of climate change.
We have a three-fold problem-more frosts and more late frosts, plus earlier plant development (e.g. budding, blossom etc). The combination of earlier plant development and late frosts is a potential reason my raspberries got wiped out-they budded early, and then there were successive late frosts that killed the new growth. Frosts are going to be an increasing problem for people that like to grow their own food!
The biggest concern for farmers, be it large scale or backyard, is crop failure-timing planting with rainfall, soil moisture and growth, with the increased risk of frosts hitting at the wrong stage in plant development. You can read more about this here and here, or a climate change surprise lecture here. We also know that repeated frosts mean that plants won’t have leaves to photosynthesise, so they can’t create and store energy. During establishment, this means that the plants are likely to die.
Yet, like all good science, this theory is not yet settled. There is also some research by a different group of scientists who agree extreme frost events (increased number of nights that may cause a late spring frosts) are linked to climate change. However, they also expect climate changes such as increased temperature might counteract the effect of extreme frosts. In Germany, they found that cherry blossoming started early at two research sites, but the frequency of frost damage has been decreasing over the 21st century. In Western Australia, the agricultural department state that frost prevalence has also changed and is dependent on biophysical location. There has been an increase in frost period inland and a decrease in coastal areas.
Whilst impact of frost and climate change is really going to depend on your location, there are still things you can do to protect your plants from these expected, and already occurring, changes.
What can I do about this in my backyard?
I am pretty concerned about this shifting pattern in early plant development stages (e.g. budding and blossoming) and later frost events for food production and establishing plants. Living inland and in an area prone to frosts, I need to think about late frosts and early plant development if I want to get fruit and more importantly to establish new plants. Here are some things I am going to consider:
Planting for frost protection. Raspberries grow wild in forests in Europe, so why not plant them under deciduous trees here in Australia! I am not falling for the ‘sunny position, North-South’ alignment again, which was the information I sourced for planting raspberries. Next year, I am going to try planting under my established elm tree. You can also use architecture as shelter (e.g. walls, tanks etc) that are north or west facing and retain heat overnight protecting the plants in front of them.
Plant species that are more frost tolerant and don’t worry about frosts at all e.g. rhubarb, tuber species, spinach, carrots etc.
Potassium fertiliser. There is evidence available from Murdoch University including via GRDC to suggest a potassium fertiliser can help reduce frost induced stress by thickening plant cell walls. And it helps with drought stress too!
Gently water plants and ground before a likely frost event so that the ground stays warmer and moisture is released back into the atmosphere. This reduces the likelihood of frost settling on your plants.
Mulch seems to be a conundrum! I need mulch to keep moisture in soil in my dry hot climate and protect from weeds. However, mulch can also aid in frost settling-frost will settle on mulch, but bare soil will warm up and release more moisture overnight, reducing likelihood of frost. You can move mulch during the day before a frost, but that seems like hard work. Other suggestions are to only mulch after frosts have finished – this doesn’t work for the earlier growing seasons and later frosts though. It may be easier to mulch as normal and use another frost protection method.
Permanent frost cloth for frost season. I have seen people put in a little frost cloth stand above and/or around their plants for the duration of the frost season. The plants still get filtered light, but are also protected the whole time.
Put out frost protection over plants the night before e.g. sheets, frost cloth etc.
Use BOM's Frost Potential maps to get an idea of whether a frost is likely, and to prepare for protecting sensitive plants.
Make my own frost record. The local meteorology unit won’t know if you are going to get frost in your backyard on a specific day. For me, if BOM says we will get a minimum of 3 overnight, I know that we will probably get a frost. To understand frost damage, I am recording the temperature and indicators of frosts at my house (e.g. clear night, rain events etc), budding dates, and preparing for frosts on nights when it is likely to happen at my house.
Knowing that these changes are already happening, and considering them in garden design is a really great way to protect your plants and food from the changing climate.