Monday, May 18, 2009

Happy Last Frost Day (Sort Of)

Predicting the last frost day is always a gamble. As another food growing blogger writes, "tiny farming is in the end about a very natural, basic and satisfying form of gambling". For the Calgary area, the average last frost day is May 20. Since this year is below average in general, it is possible, if not likely, that the last frost day could be a week or two later.

Statistically the average last frost date represents a 50% chance that no frost will occur after that date. If you'd like to decrease (or increase) the risk of your plant investments, you can use the following Freezing Date Adjustment Factor (below):

According to this factor you can expect the chance of frost in Calgary after June 4 to be reduced to 10% (15 days after May 20). Note that the graph does not provide a 0% chance of frost after any particular date! According to Alberta government records, the Calgary area has seen as low as -2 C in July at least once in the last century.

The freedom from frost in this part of the world is no guarantee at any time. For the Middle Earth Garden this is not a large concern. Large parts of the garden can be covered if a frost is predicted. But of course frost is not always predictable. Hence the gamble. My investment is small, but for larger commercial growers this risk can be serious. This is where frost insurance comes in. For this garden, frost insurance is the local grocery store!


Anonymous said...

I don't know how to contact you, but I saw your post on another site about chickens and petitions. I would definitely sign a petition! How do I do that?


Middle Earth said...

Hi Karen,

I don't know of any formal petition, I know an electronic petition went around on the Calgary Urban Agriculture Google Group. There are other discussions there about starting a petition:

Anonymous said...

So when is the first frost day?

Middle Earth said...

The average first frost day, measured at the Calgary International Airport, is September 14. The same Freezing Date Adjustment Factor applies.

For example, after August 30 (15 days before SEP 14) there is a 10% chance of frost, compared with 50% chance after September 14.

The average last frost day is likely earlier at higher elevations i.e. west side of Calgary. However, I do not have stats for this. The Calgary Int'l Airport is about 1084m above sea level. Some areas on the west end of Calgary can be 200m+ higher than this.

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