The Grounded Orchardist

So, chill hours. What are they? How many chill hours do your fruit trees need? Do chill hours affect any other fruit? Here are my thoughts.

What are chill hours?

Chill hours are a very important part of fruit growing, a necessity actually! A chill hour is roughly an hour of 0 (32)-6.5 (43.7) degrees.

Different varieties need a different amount of chill hours some need 1000 plus and some might need 600. Without hitting the crucial amount needed the variety will struggle or not fruit at all.

Minus figures do not count. I don’t fully understand why and cannot seem to find the answer. Perhaps the trees temporarily shut down in freezing figures therefore it’s not classed as dormancy. If you know why, please feel free to get in touch.

Why are chill hours important?

I think it’s fair to say. We are only really now beginning to understand the importance of chill hours. It’s clear something has changed as we noticeably start to lose them with climate change.

If a tree does not get its required chill hours all sorts of problems can arise such as…

• The tree being more vulnerable to attack from pests and disease due to being weakened.
• The tree not blooming at blossom time.
• The tree blooms but the blossom drops off before pollination.
• Erratic blooming.

Remember, a strong tree is your best defence against pest and disease attack and that’s every season! You don’t want the tree fighting to survive, you want it thriving!

A few thoughts on what I have noticed

Our orchard has a weather station, and I painstakingly poured over the data from over the last few years and saw a continuing trend. We are losing 300 chill hours each year in our little piece of Oxfordshire! That’s not good news for a fruit grower.

When I started Orcharding over a decade ago, fruit growing was relatively predictable. The effects of the weather were predictable within a two week window either side. Now the weather is so erratic it’s completely unpredictable from one season to the next.

Do Chill hours only affect apples?

No, a lot of fruit and nut trees need a certain amount of chill hours. This necessity includes even varieties from warmer climates, such as peaches and apricots.

Soft fruit also needs a certain amount of chill hours. This is a particular worry with Blackcurrants in the UK at the moment, for instance.

Citrus trees bend the rules a little as they can fruit in winter and are not deciduous. Meaning they still collect a few nutrients by photosynthesis so conserving energy is not so important. They do slow down dramatically here in the UK. I still have to feed them in winter because there is not enough sunlight!

Is there anything we can do?

If this trend continues you could think about planting low chill varieties. New fruit tree varieties are being bred all the time to counteract the myriad of issues affecting trees. Apple breeders around the world aim to grow varieties suitable for the changing climate. Hopefully, they will create a more resilient tree for our climate. Or you could choose varieties from France which are widely used to match the change in climate. We have many French varieties and I have to say they are noticeably doing better and better each year.

Unfortunately, most nurseries and shops do not include the chill hour requirements on their varieties. Who can blame them? If you want to factor this into your orchard or garden, then do your research.

In a nutshell…

If the tree does not get enough chill hours, it hasn’t had enough dormancy. Without sufficient dormancy, it cannot conserve enough energy to use for fruiting. Remember, fruiting takes a lot of energy! We all perform better after a good sleep!

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