Seed Starting 101 Average Last Frost

There are lots of opinions among gardeners about what date to use for "last frost"My when calculating when to plant out.

I use May 15th. The official "average" last frost is May 5th. That means that 50% of the time, the last frost was AFTER that date. May 15th has a 90% confidence level...that means 10% of the time, the last frost was AFTER that date.

I worked at Sturtz & Copeland in the bedding plant department for several summers. Generally, people who bought basil plants (very sensitive to temperature) in early May, bought it once or twice more before the end of May because it kept freezing and dying.

Here's a pretty good simplification of the data:

Find Your Last Frost Date
paraphrasing Carl Wilson, Colorado State University Extension, Denver County April 1999 http://www.ext.colostate.edu/Pubs/columngw/grfrost.html

Last spring frost date confusion - People quote different last spring frost dates for the same area depending on their risk tolerance. Last spring frost dates are important to home Gardeners to help them plan when to set out frost-sensitive annual flower, vegetable and even tropical container plants.

In Boulder the last spring frost date at a 50 percent confidence level is May 5th . If you want to be 80 percent statistically confident it's May 9th and 90 percent confident, May 15th. All dates are based on 47 years of data. The latest last frost date for Boulder was June 3, 1951.

Place your bet - Once you know your last spring frost date information, you are ready to place your bet for beating the last spring frost and maximizing the growing season. Simply plant out your tender annuals and see what happens!

ed note: I like this...the official CSU extension advice is "Place your bet!" They UNDERSTAND Colorado weather.

NOAA points out that temperature is usually recorded at 3 meters above ground...that means the poor plants on the ground are probably even colder.
http://www.cdc.noaa.gov/boulder/lastfrost.html