“My seed packets say to plant after danger of frost is over. When is that? Why don’t they just put the date?” This is a common concern for gardeners, and the answer is… It depends.
The only accurate way to answer the question is statistically, based on the history of frost dates from past years. One difficulty is that the “historical data” has to be from your garden, not from across the country, across the state, or even across town, and there probably is not a weather station in your garden!. How cold it gets in any one place depends on several factors - cloud cover, elevation, and topography (on a hill, or in a valley?). Temperature can vary considerably within just a few hundred yards. Usually the best you can do is use the history from a weather station that is located similarly to your garden.
There is another aspect of the historical data to be considered. Do you want to know the average date of the last frost in the spring (or the first in the fall), or do you want to know the latest recorded date when there has been frost? Average means that half the years have not had frost past this date, but half the years have had frost. You flip a coin about this year. Waiting past the last recorded frost may provide too short a growing season for the crops you are interested in growing, and even then, this year may yield a new record!
Another way to view the data is to ask, “at what date has the last frost occurred 90% of the time? Using this date you would (historically) not have frost 9 years out of 10. This might be considered a conservative planting date.
All this explains why the best answer may be to offer a table of places and percentages, and let you chose your own risk level. Good luck (and remember, Mother Nature hasn’t read this!)
Dates based on a temperature of 32.5°F
(a) The Last Frost Date is the last day that you are likely to have a frost (less than a 10% chance.)
(b) The Average Last Frost Date is the date, based on historical data, on which there is a 50% chance of frost.
(c) The Earliest Last Frost Date is the date by which you will almost certainly have a frost (90% chance of frost)
Dates are based on a temperature of 32.5°F.
(d) The First Frost Date is the first day that you are likely to have a frost (less than a 10% chance)
(e) The Average First Frost Date is the date, based on historical data, on which there is a 50% chance of frost.
(f) The Latest First Frost Date is the date by which you will almost certainly have a frost (90%)
Chart Source Information: Oregon Climate Service at Oregon State University (OCS). OCS is the state repository for weather and climate information. It is affiliated with OSU’s College of Oceanic and Atmospheric Sciences and the Oregon Climate Change Research Institute. Frost dates for additional cities may be found at: http://www.ocs.orst.edu/