The luscious peach you savor on a hot summer day traces its success to fall and winter nights when the temperature drops below 45 degrees. 

Our stone fruit trees – peaches, nectarines, apricots, plums, pluots and especially cherries – need to go dormant each year. Trees go dormant so they can survive harsh winter weather. And the time trees spend in dormancy impacts fruit production during the growing season. To go dormant, trees need to clock hundreds of chill hours. This is the cumulative number of hours that range between 32 and 45 degrees that trees need to produce fruit, experts say. 

Cherries, for example, need 800 chill hours to set a good crop. This is a number that’s becoming more difficult to hit as the planet gets warmer. But so far, so good in 2023! As of January 22nd, we’ve had 695 chill hours in Brentwood. That’s 108 more hours than we had during the same period in 2022 or an 18.4% increase. 

What happens in the chill?

We track chill hours from November 1 to February 28/29. During the fall, shorter days and cooler temperatures stimulate tree hormones that inhibit growth. These hormones keep the trees dormant through the winter. Some experts compare the cumulative chill time to the tree’s internal ledger. Chill duration and distribution throughout the seasons are both important to balancing the ledger. 

As the days grow longer and temperatures rise, the tree starts to grow again, producing the buds and blooms that will become the fruit we love.

Warm winters hurt fruit

Farmer Al says chill hours have been declining over the last ten years. Trees take a big hit when this happens. Fewer chill hours can affect crops in different ways:

• Trees leaf out late in the season, blossoming can be prolonged, buds may deteriorate or drop, and trees produce fewer flowers. Fruit forms from flowers. When there are fewer flowers, we have less fruit.
• Inconsistent chill affects the quality and size of the fruit. It also affects ripening patterns. Farm crews don’t have a uniform crop to work with, and that makes their jobs more difficult. 

We’re adapting

Chill hours are one of those things that we simply cannot control. So, we adapt! Farmer Al is diversifying our crops, and he’s planting more trees, like mulberries, that don’t require as much chill. And horticulturalists are developing new varieties of stone fruit that don’t need as much chill. For example, Red Tioga cherries only need 500 chill hours, whereas Bing cherries need 800. As our Bing cherry trees move past their prime, we are replacing them with Tioga cherry trees.  

Through our regenerative farming methods, we’re doing all we can to offset climate change and chart a better future. We’re also finding new ways to work with what Mother Nature is giving us today.

More to come on the new varieties as we plant them.