crabapple_frozen.jpgIt’s no secret the weather was incredibly warm (and dry!) in March in our area this year. People have been thrilled to see plants greening up and blooming earlier than usual. In fact, many of our flowering fruit trees, such as peaches and apples (including crabapples), are blooming well over two weeks early this year.

The problem is, the earlier these trees start blooming, the more vulnerable they are to spring frosts and freezes that often occur in early-to-mid April. And tonight, we are predicted to get down to 27 degrees (or lower!) in the Pikes Peak region. I used to think that as long as my tree had finished blooming before a major frost like this hit, it was okay and going to have fruit. Last year proved me wrong. And this year, my tree is just starting its bloom with this frost coming.This past weekend, I attended the 2012 Peak to Prairie Landscape Symposium, and Joel Reich, the Horticulture Agent for Boulder County and one of Colorado’s top authorities on fruit production, gave a terrific talk on apple trees for our area. He shared some interesting, very specific information from Washington State University on what temperatures will kill the buds on apple trees in the spring. I will summarize, but if you’d like to see all of the data, including for other types of fruit trees, Utah State Extension has kindly pulled it all together on this fact sheet (PDF): http://extension.usu.edu/files/publications/factsheet/pub__5191779.pdf

The Washington State University information looks at various stages of the buds and what temperatures (in Fahrenheit) will kill 10% and what temperatures will kill 90% of the buds at that stage. Most of the trees we are concerned with will be at the first pink/loose cluster stage through full bloom, so I’ll only cover the numbers for around those stages. Earlier stages are covered in the previously mentioned fact sheet.

For apple and crabapple trees, here are the killing temperatures (trees must be exposed for at least 30 minutes) for each stage:

Tight cluster (buds still tight, but definitely present):
27 deg (10% killed); 21 deg (90% killed)

First pink (buds are now pink, still fairly tight):
28 deg (10%); 24 deg (90%)

Full pink (buds now all pink and loose):
28 deg (10%); 25 deg (90%)

First bloom (first flower has opened):
28 deg (10%); 25 deg (90%)

Full bloom and post-bloom:
28 deg (10%); 25 deg (90%)

The numbers are very similar for cherry trees. And peach trees have maybe just a degree or so of increased frost survival (in full bloom, 90% of buds are killed at 24 deg).

If your trees are small enough, Joel suggests that you can add a few degrees of protection for them by wrapping bubble wrap around them. Even better, wrap the bubble wrap around the drip line and then put a couple of large buckets of warm water inside the wrap. For those of us with large trees, well we can really only cross our fingers and hope. Commercial growers have all sorts of other tricks they try, but they aren’t practical for the home grower.

Submitted by Carey Harrington, Certified Colorado Gardener and Colorado Native Plant Master

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