This autumn we harvested summer beans until almost the end of April - an unexpected bonus harvest because the frosts took so long to arrive this year. This may be part of an emerging trend. Did you know that Canberra’s winter nights are forecast to get warmer over the coming years? This will mean fewer and fewer frosty nights over the coming decades. Could this eventually change what we can grow here in Canberra?
The data is already showing that Canberra’s climate, like most places around the world, is warming up. If you have been out and about in your garden you may have already started noticing changes over the seasons. Remember that amazing heatwave back in January? We had 4 days of 40 degrees centigrade - breaking all previous records for January.
So it seems sensible to share what we understand about likely changes to the climate here in Canberra and what this means for you and your garden, and how you can best prepare for what is coming.
There’s actually quite a lot to know about this subject so I will write this as a series of articles focusing on one element of Canberra’s climate system at a time, to help you think about ways to make your garden as resilient as possible to what is ahead.
While the weather is still cool, I figured a good place to start was to look at frost patterns, given that this is familiar and relevant to us right now.
A few years ago the ACT and NSW governments funded research to see what changes could be expected in the climate of Canberra and surrounds if the world were to continue on its business as usual trajectory and not limit emissions of greenhouse gases. You can find the report here.
What I hope to do with this series of articles is translate this big and very general picture of the future into something tangible, garden-focused and practical for you. Climate change is often portrayed in terms of sea level rise as if that was the only impact, when in fact pretty much all locations in the world will experience a range of different impacts. One of the things the study looked at was changes to the amount of frosty nights.
The low down is that we’re looking at potentially major reductions in frost days over the next 50 years and beyond.
A reduction to 27 cold nights could mean Canberra’s winter temperatures in 2070 will be rather similar to winter in outer-suburban Melbourne today. Ok, so 2070 is a long way off. I will be, let’s see, oh my, 97!! (So there’s a good chance I won’t be hanging out in Canberra’s balmy winters then.) But, our now 7-year-old daughter will be a mere spring chicken of 58. I expect she will be interested in what’s in store for her by then.
On first glance, the idea of having warmer winters in Canberra sounds pretty good - after all, it can get uncomfortably cold here. However, the reality is a bit more complicated.
Changes to frost patterns in Canberra: the good and the bad, and what to do about it.
The good news
A reduction in the total number of frost days will likely mean a growing season that gradually gets longer each year. This means that over time we might be able to plant out our summer vegetables earlier and continue harvesting squash and zucchini later. If you’re into growing your own food, as we are, then I can only encourage you to take advantage of this situation. Plants that require relatively long growing seasons, like capsicum and watermelons, will become more viable here than they currently are.
You might recall the Canberra gardening lore that you wait until after the Melbourne cup to plant out your tomatoes, and make sure you get your broad beans in before ANZAC day? Already, by last year, we had planted out our tomatoes in the middle of October (crossing our fingers there wouldn’t be a sudden, late frost) and that proved a very successful strategy - we got a total yield of about 70kg of tomatoes which is enough to last our family all year (fresh tomatoes in summer and autumn, bottled passata for winter stews and soups). While there will always be year-to-year variation, expect that in the coming decades the last frost date will eventually be in September (or even late August), and the first frost dates will continue to move from April through to May and possibly later.
You might also have noticed that there have been a greater number of sunny, bright days over the past few winters than previously - a result of static high pressure systems sitting over Canberra. The modelling I referred to earlier also suggests that there will be a reduction in rainfall over winter.
Any reduction in cloudy or rainy days can also be used to your advantage. If you have the means, consider putting in a window in any north facing wall of your of your house where it is practical and viable to do so.
We have north facing windows in our living areas (one of the reasons we bought our current house). On a sunny winter day we can usually turn any heating off by mid-morning and keep it off until around 3.30pm, because the heat from the sun is so strong. It’s way better than any heater - I think I read somewhere that sunlight gives you something like 3000 Watts of heating per square metre. And of course, it’s lovely to stand in.
Unfortunately, we have those 1960s aluminium sliding windows that leak heat terribly, so as soon as the sun goes, the heat just escapes back out. We do our best with thick curtains to trap at least some of the indoor air in the evenings and are saving our pennies so that one day we can replace the windows with double or triple glazed ones that will keep us a lot warmer. One permaculture principle is to ‘catch and store energy’ - we’ve gotten as far as catching the solar energy, the next step is to work on ways to store it! Oh and in case you’re wondering, there is very little you can do to retrofit aluminium sliding windows - I have looked into this. I even sticky-taped some of them shut one winter to stop the draughts but that just means we have funny marks all around the edges of the windows now…
Of course, if you have the roof space and it's affordable for you, putting on PV panels is a no-brainer, and a greater number of sunny winter days means you will capture more solar energy at the time of year when total energy demand is greatest, which is a win-win. Also consider converting your heating to reverse-cycle air conditioning to enable it to run on your solar powered electricity. Even without generous feed-in-tariffs, solar PV systems pay themselves off in only a few years.
Before leaving the subject of capturing solar energy - I wanted to share another idea. We all know that not a lot of food growing happens outdoors in the winter months in Canberra: growth slows or even stops. An increase in sunny winter days means that if you do have good sun inside your house, you can grow a range of sprouts and microgreens all winter long, just with the light from the sun. We’ve just successfully experimented with kale and pea shoots - both delicious and they didn’t get too leggy at all. A greater amount of sunny days means more direct sunlight on indoor plants that may need it. Similarly, you can start off your summer crops from seed indoors from late August, ready for transplanting in October or November. I started many of our tomatoes from seed last year using this method and just a bit of space on the window sills.
Marginal species may become more successful
Citrus trees are quite marginal in Canberra's climate - yet there's a lemon tree in loads of back yards here, demonstrating that it's perfectly possible to grow plants that are not officially supposed to thrive here. While cold hardy varieties of lemon, lime, mandarin and cumquats have been staples here, if you have the right conditions in your garden, you might even want to try expanding into grapefruits and oranges too as winters get milder. Other marginal species, such as avocados, may become more tenable here too - I know a few people who are growing avocados though success rates vary. However, there's a caveat in there - we need to be very careful about where we plant these species, to take advantage of warmer microclimates and places in our gardens that get less severe frosts (there's more detail on some techniques you can try in the following section).
The not-so-good news - and some techniques to try
While the absolute number of frosty nights may decrease, the quality of the remaining frosty nights is also important to consider. With high pressure systems giving us a great many sunny and dry winter days, the flip side of this is that clear skies at night can mean quite severe frosts. So while the number of frosty nights might go down, we may still get some particularly savage frosts when they do occur.
Do you remember there was a particularly cold week during last winter (July 2018) when we got a series of very cold nights? One of my friends, an avid cyclist, recounted how his bike computer recorded a temperature of minus 9 in a very foggy frost hollow on his way to work from Tuggeranong. Brrr!
Just a few weeks ago, at a garden consultation in Nicholls, I learned that those same cold nights were enough to kill a very healthy lemon tree in my client’s garden, which had been growing well since 2006. It was a Meyer lemon, widely regarded as the most cold-tolerant variety of lemon. What was intriguing was that a range of other, much younger, citrus plants, which were placed on the western side of a metal shed were unaffected. What was the reason why the lemon tree had died and these other younger trees had survived the same conditions? The amount of water going to the plants had been the same.
We puzzled over the situation and it seemed that the critical difference was that the trees beside the shed did not receive any sun until at least the middle of the day, while the (now departed) lemon had received sun all day, from the early morning onwards.
While there isn’t a whole lot of research on it, it seems that rapid thawing may exacerbate frost damage. There’s certainly plenty of anecdotal advice around suggesting that gradually thawing reduces damage to the plant tissue while rapid thawing (such as when a frosty leaf is hit by morning rays of sun) causes much more tissue damage and should be avoided.
We can conclude that the risks that frosts pose to marginal perennial plants in Canberra (which include pretty much all citrus trees), are therefore going to still be present as the climate warms. Indeed, more frequent clear skies may increase the risk of severe frost when it does occur - this recent ABC news article documents a severe frost in NSW caused by the current drought and clear skies.
Overall winters are likely to become warmer, with higher daytime average temperatures, but this may in turn help to reduce the number of foggy mornings, or lifting the fog earlier than it used to. (I’m sure I’ve noticed a reduction in foggy mornings over the years - I wonder whether anyone else has too?) This may increase the likelihood of rapid thawing of frost affected plants and hence make them more vulnerable to frost damage, in spite of a declining number of frosty nights overall.
This may be a passing situation towards less severe frosts in decades from now, but given that this is already an issue right now, here is a list of ideas for keeping citrus and other frost sensitive plants alive over winter that move beyond just using a bit of frost cloth:
- Make use of thermal mass: even with the most severe frosts, there may be parts of your garden, closest to the house, that avoid the frost. If sunlight hits brick, rocks or concrete, it absorbs the heat and radiates that heat back out overnight, keeping the immediate area a little warmer than the surrounding garden. Placing your citrus trees in this environment will minimise the amount of frost they are exposed to overall. Ponds may also provide useful thermal mass, if they are close enough to buildings to not freeze over.
- Use energy leaks: If your home is as badly insulated as ours is, then there is probably heat from inside the house leaking outside via doors, windows and cracks - strategically locating a potted lemon or lime near these could at least make use of this escaping energy, giving you some comfort that your energy bills are funding more than just your indoor comfort! But be aware that if you improve your energy efficiency, such as via gap sealing, putting in double glazing or wall insulation, you may need to relocate outdoor plants previously reliant on this escaping heat!
- Use shading from large trees and other overhanging structures where appropriate: large evergreen trees or pergola roofs tend to act like umbrellas and trap a bubble of warmer air beneath them. Frost falls to the ground and will go around and over these structures (like rain over an umbrella). Plants beneath them are less affected by frost.
- Try to ensure your plant is in a place that stays shaded at least all morning long in winter: that means west facing locations are good.
- Use raised beds or planters: frost rolls downhill so it is always coldest at ground level, and on undulating ground, the bottom of any slope is where the frost develops in what are known as ‘frost hollows’. We have one warrigal greens plant that has survived the winter because it is in a raised planter and sheltered by another plant as well… go little plant! Not long now before it’s warm enough to start growing like crazy again!
- Do as many of the above as possible: If you can do a combination, or even all of the above, you might find that you can get away with growing an increasingly wide range of warm climate and sub-tropical plants in Canberra. We are experimenting with avocado trees - currently we have 6 baby trees (which are highly vulnerable to frost) growing in pots, under the eaves of the house, on a raised deck, on the west side of the house, close to the back door (which leaks heat as it’s not well insulated yet), and we cover them with a fleece blanket when it’s a cold night - so if anywhere is going to keep them happy it’s there! (And now winter is officially over, I can report that all trees are still doing very well.)
While I’ll reserve a full discussion of rainfall changes to a separate post, it’s helpful to note in passing that the climate models for the ACT do predict a shift in rainfall patterns over time so that Canberra’s winters will become drier than previously. Of course, this doesn’t mean every single year will be drier, but overall, the number of dry winters will increase relative to wet ones. You may have noticed that this winter seems quite dry - in fact, according to the Bureau of Meteorology, rainfall in the ACT for June and July 2019 has been well below average while temperatures have been higher than average. The winter we have just had may well be a taste of what will become the new normal in coming years.
In addition, with temperatures higher than normal and a lot of sunny days, there has been a lot of evaporation too - far more than usual for this time of year. I read somewhere that for every increase in temperature by 1 degree centigrade that means a 7% increase in evaporation. So, it’s no wonder that things are looking pretty dry out there right now - they are indeed very dry.
What this means for gardeners here in Canberra is that you may well have to start watering your plants from late winter onwards. We have begun watering our vegetable crops again. We grow a lot of produce in wicking beds, which although they are very water wise, still need to be watered every now and then. Rather soberingly, this year is the first where we have had to water our wicking beds over winter because there has not been enough rain to keep them moist, as in previous years.
There’s a much bigger conversation to be had about how to make your property as water resilient as possible, but I’ll reserve that for another post. For now, however, it’s something to be very aware of, and one consideration may be that mulching of soil in vegetable beds will need to happen earlier as the soil is already warm enough by the end of winter.
A longer growing season does also mean that some of our little garden friends are going to hang out a bit longer than they used to. Cabbage white moths/butterflies, for example, are still busy throughout April and into May and I’ve seen caterpillars still happily munching through June and even July.
In our garden we netted the brassicas with a fine net to keep these butterflies out so they could not lay eggs. This worked well until July when we discovered that the nets provided sufficient buffering to the cold that lots of aphids were having a great time sucking the sap out of the kale, thanks to the gentler microclimate under the nets.
Taking the nets off has seen a major reduction in aphids and the kale was recovering well until the weather warmed so much that the aphids are back anyway. But the butterflies are only just getting back to action. What this little example illustrates however, is the need to strike an ongoing balance between managing different garden pests - and as the winter gets shorter there may become a time when this little technique is no longer successful, and we will no longer be able to take advantage of so many frosts to rein in the numbers of pest species. Our aim is to garden without using any insecticides, because the beneficial species in the garden are more vulnerable to chemicals than the pests, so our strategy is to keep boosting habitat for predatory species, but we may need to get out there with a cloth every so often to wipe off some pest populations if we have to! And we may just give up on growing some plants and focus on more resilient ones (for example, our dwarf curly leaf kale seems to have been designed as perfect aphid habitat, while the purple sprouting broccoli next to it is almost completely bug free.
Reduction in chilling hours
Some trees need a certain amount of cold weather (called chilling hours) to go fully dormant, and facilitate good bud burst in spring. If there aren’t enough chilling hours, then the tree doesn’t flower properly and then doesn’t set as much (or any) fruit. It’s already becoming a problem in quite a few regions of Australia - according to the ABC cherry growers in SW Western Australia, for example, have been experiencing problems with cherry varieties not getting enough chill over the past decade, leading them to look for varieties with lower chill requirements or give up entirely.
And in South Australia, growers have been removing pistachios, apples and walnut trees because the summers have become too hot and winters insufficiently cold for the trees to produce properly.
What does all of this mean for Canberra, given that we currently have the coldest winter temperatures of all Australia’s major cities? While significant disruption to chill hours is not as imminent here as it is to growers in other parts of the country, it is still sensible to bear in mind when planning your next generations of trees. While 2070 seems a long way off, remember that plenty of fruit trees can live for a very long time. Apples, apricots, pomegranates, hazelnuts and walnuts can all live well into their 100s so if you are planning on establishing a tree that your grandchildren will harvest, it is advisable to consider how conditions might change over the course of their lifespan and plan accordingly. Here are two related strategies:
- If you have the space, plant several varieties of each fruit tree type (e.g. different types of cherry, apple, plum, pear, peach etc). Different varieties have different chilling requirements - some need lower and some need higher numbers of chilling hours. With a variety, you can capitalise on high chill varieties that will likely produce well over the coming 10-20 years but you have some insurance with other, lower chill requirement varieties that will continue to produce decades after that.
- Prioritise getting varieties with low or medium chill requirements that are known to do well in Canberra already - particularly if our are short of space. For example, among cherries, lapins has a much lower chill requirement than stella, and it is already successfully grown here. If you only have space to plant one cherry and it’s going to be there for your grandkids, maybe choose the lapins variety to be on the safe side… remember nurseries and online retailers will have information on the different chilling requirements of different fruit trees, so make use of their knowledge and advice when selecting varieties for the long term.
Some concluding thoughts
While this may mean we are encouraged to try more marginal species in Canberra, there is still the chance of severe frost which means we need to have strategies in place to protect such marginal species from severe frosts when they do happen. Permaculture is about being creative and adaptive in a changing environment, so it's important to take advantage of opportunities like increasing winter solar gain and a longer growing season.
In the latter case, growing as much as you can is also really critical, because the more biomass and shade you have in your garden, the more you can moderate and manage the increasingly fierce summers we will also encounter (managing summer heat will be covered in detail in another post). Pests are also likely to become more of a problem over winter and some high chill requirement fruit trees, like some varieties of cherries and apples, may eventually not be tenable to grow here, depending on how much warming eventuates. However, planning ahead by selecting lower chill requirement varieties will buffer against this somewhat, at least for the next 50 or more years.
I realise that climate change is a difficult and challenging topic that gives rise to a lot of emotions, and I don’t want to end on a depressing note. I hope that by providing this information I can help you see we are not entirely helpless in facing an uncertain future, and the more knowledge and tools we have at our disposal, the better prepared we are to face it. What we do in our gardens is only a tiny element in a much bigger picture, and it's never going to compensate for not reducing emissions, yet it can be remarkably healing and empowering to feel that we can do something, and we are not just 'sitting ducks'. Whether you're an activist or a skeptic, we invite you to engage more fully with the natural world that begins at your back door, experience some of the wonder at the amazing dance of life on this planet, grow some of your own food, and begin the journey to learn how to effectively cooperate with Nature to the benefit of both ourselves and ecosystems more broadly.
Stay tuned for upcoming articles on changes to temperature, rainfall patterns, evaporation and bushfire risk and what you can do to adapt and manage risks.
Wishing you well in your journeys to productive and resilient gardens in these changing times.
Canberra Permaculture Design and Education