A.  Many gardening applications make reference to the "first killing frost" or the "last killing frost." These dates provide guidelines for when to plant various kinds of seeds or transplants, when to fertilize, and when to apply weed control products. You might think it would be easy to find the fall and spring "freeze safe dates" for any particular area, but it isn’t. If you dig for it, you can extract the information online from maps maintained by the National Oceanographic and Atmospheric Administration (NOAA).

In the fall, gardeners want to know when the first freeze is likely to occur. If you have freeze-tender tropical plants that you move indoors or to the garage before the onset of cool winter weather conditions, this date is the one by which you should routinely plan to relocate them.

According to NOAA, there is no more than a 10% probability in any given year that a freeze of 32º or colder (commonly known as a "light freeze" or "light frost") will occur earlier than November 15 in the northern part of Galveston County, or earlier than December 1 in the southern part of Galveston County.

In other words, if you plant a vegetable that cannot survive a light freeze, and you plant it so that you can harvest your last crop earlier than November 15 in Kemah or December 1 on Galveston Island, you have a 90% chance of not losing any of your vegetable to a killing frost. If you move your tender tropicals indoors by the above dates and locations, they are not likely to be freeze-nipped in the fall before you get them inside.

NOAA’s maps indicate that there is no more than a 10% chance of a temperature 28º or colder (commonly known as a "hard freeze" or "hard frost") occurring before December 1 in the southern part of Galveston County, and December 15 in the northern part of Galveston counties. If you have plants or crops that can survive a light freeze but not a hard freeze, this is the date by which you should plan to move them or harvest them.

Gardeners also need to know the average last freeze date in the spring. This date is a guide for applying pre-emergent weed control products, getting the tropicals out in the sun, and setting out freeze-tender transplants. NOAA’s maps indicate that there is at most a 10% probability of a temperature of 28º or less occurring after February 15 in the southern part of Galveston County, and after March 1 in the northern part of Galveston County.

So, for example, if a pre-emergent herbicide calls for application "2 weeks before the last killing freeze date," you may apply it around February 1 in the Galveston Island growing area and around February 15 in the Kemah growing area, with a 90% probability that your application will work as the directions suggest.

For setting out tender transplants, gardeners want to know when the last light freeze is likely. The NOAA maps indicate that there is at most a 10% probability of a temperature of 32º or less occurring after March 15 in the southern part of Galveston County and after April 1 in the northern part of Galveston County.

This means that if you live on Galveston Island, you can transplant your tomatoes after March 15 with reasonable assurance of avoiding frost damage, and if you live in Kemah, you transplant them after April 1 with reasonable assurance of avoiding frost damage.

"Reasonable assurance" in this context means that you have a 90% level of probability that your plants will not be destroyed by an unusually late freeze. Since we are dealing with Mother Nature and probabilities, experienced gardeners appreciate that the other side to the 90% level of probability of something not occurring is the 10% level that it will. In that case, be prepared to provide protection to tender plants or to start over!

For additional information, please visit NOAA's website.

Temperatures dropped to the 20ºF mark on March 5, 2002
at the Santa Fe residence of Master Gardener Chuck Myers.