Fruit trees blossoming ridiculously early at 7,000 feet

I was walking downtown yesterday when I noticed the first fruit trees blossoming in Flagstaff. This is two weeks earlier than last year and one to two months earlier than they should be flowering to avoid spring frosts. While the blossoms are beautiful, it would be better for potential fruit development if the trees did not blossom until May. Freezing temperatures kill fruit blossoms and small fruit.

Adapted from Growing Food in the Southwest Mountains: A guide to high-altitude, semi-arid home permaculture gardens:

Our last spring frost was historically on average during the first half of June. However, climate change is pushing back the start of spring growth. My barberries have been flowering since January, and these hardy natives historically did not blossom until April.

As usual, the daily temperature extremes of our high elevation are confusing the trees. Daytime highs have ranged from the low
50s (°F) into the mid-60s. Nighttime lows have been averaging just above freezing. The lower picture of the San Francisco Peaks looking north from the railroad tracks in downtown Flagstaff on my way home shows that it is still winter. Flagstaff could experience arctic temperatures and snow storms through April, or even early May.

One characteristic that affects microclimate formation is thermal mass. A thermal mass is a dense material  like rock, brick or water (i.e., ponds and lakes) that stores solar radiation (sunlight and heat) during the day, and then radiates heat back into the air at night when the air temperature drops below that of the thermal mass. Thermal mass buffers day and night temperature extremes, reducing the range of temperature fluctuation. Fruit trees’ proximity to rock walls, concrete walls and sidewalks, and asphalt affect how soon they bloom as well as the depth of the nightly low temperatures. The crab apple in the top photo is surrounded by urban concrete and asphalt, which helps to explain it’s early blossoming. My two apple trees are still dormant. Of course, I chose late blooming cultivars on purpose.

Growing Food in the Southwest Mountains 4th Edition Book Cover Growing Food in the Southwest Mountains: A Guide to high-altitude, semi-arid home permaculture gardens
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