How a changing climate is impacting fruit trees
There's plenty of advice in magazines and on social media about creating drought resilient gardens, harvesting water or even coping with floods. But there’s another problem that I feel is just as worrying - and getting very little airtime. And that’s how our fruit trees are responding to milder winters and earlier springs.
Climate change affects fruit trees in many ways. Flowering is a key time of year, and the trees depend on insects to pollinate their blossom. But there’s such a narrow window of opportunity – just a couple of weeks for insects to find and pollinate the flowers before they die.
We’re all aware spring is arriving earlier and, on average, our fruit trees are opening their buds more than two weeks earlier compared with the 1980s. With premature springs, the chances of a mismatch between the appearance of the blossom and the activity of insect pollinators increases. There’s also the chance that cross pollination between different cultivars doesn’t occur – plus, there’s the added risk that buds that have opened too early get frosted.
Waiting for chill hour 🔗
Fruit trees, like many other deciduous trees and shrubs, have a mechanism to stop them coming out of their winter dormancy too soon: opening their flowers and being at risk of frost damage.
They must experience a certain number of chill hours before opening their buds. A chill hour is an hour of time when the air temperature is below 7°C between 1 October and 1 April. This mechanism is critical to successful fruiting, and it means the trees need to experience five to six weeks of chilly weather, something that was quite common in the past. But as we experience global warming, our winters are becoming milder and cold weather with frosts is becoming increasingly infrequent, so the number of chill hours is declining.
This has been particularly evident in southern England. Recent research by Carlota González Noguer of the University of Reading shows there’s been a significant decline in chilling in just the last few years. Similarly, Paula Fleming, the orchard gardener and tutor at Le Manoir aux Quat’Saisons in Oxfordshire, estimates their trees are getting around 300 fewer chill hours than before.
Why is a chill so important? 🔗
Chill ensures the plant remains dormant through winter, conserves its energy resources and doesn’t open its buds too early and risk frost damage. The production of buds and leaves is a result of a complex system that’s reliant on the correct signals.
Get the wrong signals and the plant becomes confused, so insufficient chill can result in the poor development of fruit buds, delayed flowering, or buds may open too early. It can also cause floral abnormalities, such as reduced fruit set and clusters of smaller malformed fruits and even flowering at the same time as fruiting.
The number of chill hours that are required for apples, for example, varies from 200 to 1,000+ hours. Those cultivars typical of warmer climes needing 200-500 hours, while those from more northerly regions requiring 800-1,000. Plums and damsons generally require 1,000 hours or more, while medlars, quince, pears and mulberry need far less.
Obviously, there’s nothing we can do about the lack of chill hours, so we have to adapt. Fruit trees have a long life, and a tree that we plant this autumn will be maturing in 10 to 15 years’ time. Hopefully, it will continue to produce fruits for many years beyond - but will it be able to cope with the climate of 2040?
The choice of cultivars that we plant now is critical. Many of our favourite heritage cultivars were developed 200 to 300 years ago as the country was experiencing the ‘Little Ice Age’, when it was considerably cooler and wetter. I had a look through Apple Varieties of Somerset – a Guide to the Origins of Somerset Apples by June Small of Charlton Orchards. This document identifies 156 cultivars that are linked to Somerset, the vast majority of which were developed in the 19th century, when the weather was particularly wet and cold.
What cultivars should you plant this autumn? 🔗
With the climate changing quickly, we need to be savvy about our fruit tree choices. If you live in the south, for example, you definitely don’t want to choose cultivars with a high chill hour requirement, but finding information about chill requirements is difficult. Fruit tree suppliers give plenty of information on pollination requirements, etc, but nothing on chill. Given the fact most of our heritage apples date back at least 100 years or more, I think we have to assume most will have a chill requirement that suited the climate of the time.
There are some low chill cultivars available in the UK, but the selection is limited, and include ‘Braeburn’, ‘Bramley’, ‘Lord Lambourne’, ‘Saturn’, ‘Spartan’, ‘Crispin’ and ‘Gala Galaxy’. Some commercial fruit growers in Kent are looking to the future and are planting more international cultivars, such as ‘Granny Smith’, ‘Golden Delicious’, ‘Gala’ and ‘Fuji’. But it’s not straightforward as some of these cultivars, such as Granny Smith, come from Australia. Granny Smith has a low chill requirement of just 200 or so hours, but it needs a long growing season and lots of sunny weather.
You may have to look further afield. Foresters are recommended to look 200 miles south to regions with a climate that we may be experiencing in the future, and the same applies to fruit trees. If you live in northwest Wales, you might choose cultivars typical of Cornwall and Devon. I live in Somerset, so I’ll be looking 200 miles south to Normandy and Brittany.
In my own ‘climate change orchard,’ I’ve opted for apple, pear and plum varieties that are French in origin alongside medlars and quince (200-400 chill hours). I’ve also planted a couple of Asian pears (Nashi) that have grown well and are starting to fruit and I’m considering some Japanese plums (Prunus salicina). These small fruit trees have a wonderful blossom, dark red fruits and lower chill requirement than European plums. They are hardy but the flowers open early and may be at risk of frost, so I’ll need to find a sheltered position.
With our changeable climate, it’s difficult to know what to do. But diversity is always best, so I recommend you choose a wide range of apple cultivars to ‘hedge your bets’ and include other species with a lower chill requirement, such as pears, medlars and quince