Burnt Branch Blossoms

Ah, the sweet cherry blossoms are bursting forth!

You have to look closely in this photo, but the branch closest to the camera suffered in the great fire of 2017 that took out my mulberry tree and beekeeping shed. This cherry tree was growing next to the shed and, post-fire, had black limbs. I trimmed those off, and left the upright branch that seemed to be alive. They are blooming and leafing out, just more slowly than the rest of the tree–it’s about a week behind the non-fire scorched limbs. But why?

My theory is that the heat of the fire reset this limbs sense of chill hours. So most fruit trees like apples, cherries, pears, etc., need exposure to cold temperatures–that is, they need to know that winter happened. A chill hour is an hour of temperatures below 45 degrees. So when you buy a fruit tree, you need to look at the tag and make sure your area has the same number of chill hours recommended on that tree’s tag. In the East Bay, we get between 600-800 chill hours, depending on where exactly you live. (If you live in California, you can look up your chill hours here.) There are varieties of apples that need, for example, 1000 chill hours in order to stimulate blooming and to set fruit, so it would be a poor fruit producer here, but in Minnesota, look out!

But what about my slow-to-bloom cherry limb? It got the same number of chilling hours as the other branches. I’m wondering if the idea that extra warm temperatures can cancel out some chill hours is at work. There is a different model that UC Davis uses, called the Dynamic Model, which uses something called Chill Portions instead of hours. Here’s what they say: “The model calculates chilling accumulation as ‘chill portions’ (CP), using a range of temperatures from ~35-55°F (some temperatures are more effective than others), and also accounts for chill cancellation by fluctuating warm temperatures.”

Maybe my conflagration was like a super warm temperature that reset the tree’s chill hours? Send any theories my way….

P.S. Note that my cherry tree is pruned in an open center pruning style. Not ideal for cherry trees–that’s more of a peach thing. According to Stella Otto’s the Backyard Orchardist, cherries should be on a modified central leader. I wish someone told me that 7 years ago!