Why Cherry Trees Need 700 Chill Hours

With the recent bout of unseasonally hot weather this August in Victoria, cherry growers have been anxiously reviewing the forecasts for normal cold weather. The problem with hot weather in winter is that cherry trees require a certain number of chill hours during winter dormancy to properly break bud, and produce flowers in the springtime. Chill hours play a crucial role in the flowering and fruiting of cherry trees.

Let’s look technically at what a chill hour is. A single chill hour is an hour during which the temperature of the air surrounding the dormant tree is within the appropriate chill range for that variety. The chill accumulation is then the total number of chill hours that a tree needs to spend at that temperature during its chill period.

Sweet cherries have high chilling requirements compared to other fruit trees. Most varieties in Victoria need between 700-800 chill hours each year. A chill hour is a consistent temperature below 7 degrees Celsius. Think of it like a good night sleep - the type of sleep that rejuvenates you, and prepares you for the day ahead. That is precisely what chill hours do for our cherry trees.

If our trees don’t have sufficient chill hours, they will likely have a delayed or uneven bud break, and the number of blooms will be reduced. This is basically because chill hours serve as a signal for the tree to exit dormancy, and begin its growth cycle.

The proper accumulation of the necessary chill hours promotes a uniform and healthy flower bud formation, and a timely opening of the flowers in the spring. Earlier flowering can lead to unfavourable pollination conditions, because if the temperature is too cold, or its raining, it prevents the bees from visiting all of the flowers to pollinate them. That results in reduced fruit set and lower yields.

Chill hours also directly impact fruit set and the overall quality of our cherries. Insufficient chilling results in poor fruit development, potentially reducing fruit size and decreasing overall yield. It can also affect the flavour and sweetness of the cherries.

The amount of chill hours that trees require will depend on the variety. Many of the older varieties such as Bing, Sylvia and Summit require a minimum of 700 chill hours. However, some of the newer varieties known as low-chill cherries which include Stella, Lapin and the Royals varieties need 500 or fewer hours.

Whilst there are various models for calculating the chill range and the chill accumulation during winter, we at Yarra Valley Cherries are fortunate in that the chill hours are calculated by the Government Agency at Coldstream, so we just need to go online to check where we are up to.

We are in good shape, because the chill hours this year are already at 1210 hours, so we are set for a normal bud burst and fruit set.

Whilst we appear to be fine for cherry season 2024, a likely effect of climate change is a delay in the beginning of chill accumulation in autumn, the fulfilment of chilling requirement, and thus the time at which the trees will become receptive to heat during spring. Since bloom and leaf emergence result principally from chilling over winter and then heat in springtime, later and slower chilling accumulation could lead to delayed blooming and leafing.

Andrew Fairley

Managing Director - Yarra Valley Cherries