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I understand that these are not ideal conditions for a greenhouse

I understand that these are not ideal conditions for a greenhouse

We’ve now had our April snow for the year.  The date is now the 7th of April and there is a pretty significat accumulation of snow on the ground.

This return of winter weather really highlights why it is important to pay attention to the average last frost date for your area.  This spring we have had a few days where the temperature hit double digits (in celsius) during the day and managed to stay above freezing over night.  But gardeners who jumped the gun and planted anything but the hardiest plants will find their plantings killed by this blast of winter.

Along with average last frost date the other important metric for the spring gardener is growing degree days (GDD).  As the wikipedia article explains GDD can be calculated by adding a particular day’s minimum temperature to its maximum, dividing the result by two, and (in most cases) subtracting ten from this.  (Any temperatures below ten are set to ten and the maximum is capped at thirty.)  Basically this means that, speaking broadly, once daytime highs creep above ten degrees celsius plants will start to think that it is time for a new year’s growth.

The Weather Network’s online lawn and garden forecast includes daily information for GDD and though it currently predicts zero GDD over the next week we have already had half a dozen days with values between 1 and 2.5.  The wikipedia article provides a table of how many GDDs, accumulated since the winter low, it takes for certain garden events to occur (like trees flowering).  It is this genetic requirement for a minimum amount of warm weather that helps trees and other plants avoid exposing delicate flowers and new growth to April winter weather.