For most of the year, seasonal change is gradual and persistent, but in April the shift from summer to winter becomes obvious. Dawn air has a crispness that didn’t exist a month ago and as the polar jet stream moves north, the first big cold front of the season will push deep into the continent. In cold areas, the first frosts are due within weeks.
Plants respond to this impending cold in various ways. Some see it as a chance to flower and set seed in anticipation of winter rains. Lots of native plants, particularly those from the southern states, fall into this category. Others either cease growing or slow their growth right down until conditions are again favourable in spring. Deciduous plants herald the looming winter by changing colour and shedding their foliage.
This creeping dormancy is a boon for “leaf peepers”. These people are the train spotters of the gardening community and travel to areas in the world renowned for autumn colour. The north eastern United States is prime leaf peeper country, but we’ve got some decent areas here in Australia, including Bright in Victoria, the Blue Mountains in NSW, and of course, here in Toowoomba and the around the Downs. One of my favourite local spots for a bit of leaf peeping is Davidson Arboretum at Highfields.
But for those of us who like to grow temperate fruit, the onset of “doona weather” is a bit like welcoming back a much loved visiting friend. A lot depends on the individual variety, but it’s fair to say that without an appropriate period of cold weather, deciduous fruit trees get all confused. If temperatures are too warm, some will fail to go into dormancy, others will go dormant but not know when to wake up in spring, others will wake progressively or sporadically.
This need for a tree to experience cold temperatures is known as a chilling requirement, measured according to a scale of chill units or chilling hours. You may have come across the terms high chill and low chill – these refer to whether a tree requires lots of cold weather while dormant or very little. The simplest way of measuring the chilling hours for your garden is to add up the total number of hours below seven degrees Celsius accumulated during winter. Here in my garden on the western fall at Hampton we get around 1000 hours of chill in an average winter. Stanthorpe has similar chilling hours but Toowoomba has somewhere in the vicinity of 600 – 800 hours. Various locations on the Downs will have anywhere between about 400 hours of chill to 1100.
Once you have a rough idea of the chill units for your area, it’s possible to make more informed choices about the fruit varieties it might be possible to grow. Requirements vary widely according to the species of fruit and the variety, but here’s a very general guide: cherries, apples, European plums and pears need high chill conditions (800 hours plus); apricots, peaches and quinces need medium chilling hours (400 to 800); Japanese plums, persimmons, mulberries and figs are considered to be low chill (100 to 400). It’s nowhere near that clear-cut though (some apples are low chill for example), so check with your nursery prior to purchase to make sure you’re not being sold a dud.
Plant dormancy is a tricky beast to pin down. So many factors can come into play that despite the best efforts of researchers, there is still uncertainty and many growers manage to turn conventional theory on its head. We know one thing for certain – there’s more to dormancy than simply cold temperatures.
Day length plays a role, which might explain why there are very few deciduous trees endemic to equatorial climates. For apples and some other plants, extended winter rainfall can mimic chilling hours. It’s not fully understood why, but scientists believe that a substance gets washed off the trees, making them think they’ve experienced a cold winter. Apple growers in highland California experienced this phenomenon in 1995 after an unusually warm and wet winter. Farmers were petrified that fruit set would be terrible because of the lack of chill, but on the contrary, bumper crops set new records.
The lesson I take from this lack of certainty is that it’s okay, even advantageous, to experiment. Unless you’re a commercial grower and success is a financial imperative, the best advice I can offer is to get hold of a few trees and try them out. Sure, you might make some mistakes. But you’ll avoid the biggest mistake of all – a refusal to get stuck in and have a go.
First published in the Toowoomba Chronicle 10th April, 2010. Photo by Don Sutherland via flickr.com
{ 2 comments… read them below or add one }
I’m a displaced Canadian attempting to garden near Kingscliff, NSW (coastal northern part of the state). How would I get the local average of chilling hours in the area? Thanks in advance.
Hi Lynette,
Thanks for your question. I imagine it must be quite a stark contrast between Kingscliff and Canada. In terms of chilling hours, the difference would be huge. As a guess, Canada would receive at least 2000 hours. Kingscliff would be somewhere in the vicinity of 100 – 300 hours, depending on whether you are right on the coast or slightly inland. One way to get a more accurate estimate is to attempt to add up the total number of hours below 7 degrees Celsius your site experiences during autumn, winter and spring.
The range of deciduous fruit you could grow is reasonably small. Try Japanese plums, lower chill nectarines and peaches, persimmons, mulberries, almonds, and low chill apples and pears. Remember too that chilling is only one factor required for deciduous trees to fruit properly. There are other growers around the world growing apples successfully in warm climates, for example, so it would be worth experimenting. You can always graft a tree over to something more suitable if a variety isn’t working.
Cheers,
Justin