Sunday, April 22, 2012

Are we past the last frost date?

The problem with the last frost date is that it is different every year.  Some years the last frost is early, other years it comes late. So when planning to plant out frost-sensitive crops, you have to guess what the weather will do based on previous years.

Generally, you hear about the 'average last frost date'.  But I don't really like planning my planting dates based on that because it is the AVERAGE last date--meaning half of the time the last frost is before that date, and half of the time it is after.
"You mean to tell me that there is a 50% chance that I'll
endure freezing weather when you plant me out on the
average last frost date??" -Tomatoes
I'm a pretty meticulous planner, and my plants and I would really like to have more information than just the average last frost date. The best comprehensive resource for frost information I've found so far is the National Climatic Data Center website. They have information about spring (and fall) frost dates for all areas of the country. Click here for the pdf information on a bunch of North Carolina areas.

It includes 3 different temperatures (28, 32 and 36) and 3 different probability levels (10, 50 and 90). Below is the pertinent information for Chapel Hill, NC:

Better than your average last frost data for Chapel Hill, North Carolina.
I know what you're thinking: So how do you use this table? For example, let's consider the 90% probability level for the spring season at the 32-degree threshold.  The table shows the associated date is March 24. This means that nine times out of ten, a temperature as cold as or colder than 32 will occur later than March 24 during the spring season. Those aren't good odds if you are thinking about planting out tomatoes.

On the flip side, April 30 is the 10% probability level for 32 degrees. This means that 1 time out of 10, a temperature as cold or colder than 32 will occur later than April 30.  I'm willing to accept a 10% frost risk when planting out tomatoes. For more sensitive plants (like eggplants and squash) I would probably wait another week or two.

For the fall season, the probability level represents the chance of NOT having a temperature as cold or colder earlier than the computed date.

Given the above odds for various temperatures (and the projected forecast), I think I'll plant out my tomatoes sometime later this week!

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