| Frost dates are based on a sliding scale of probability calculated from historic norms. Many stations calculate the growing season from 50% probability of spring frost to 50% probability of autumn frost. Many gardeners, though, prefer to wait until frost liability declines to 20% or even 10%. In some areas, this in only a week or so later. In others, it may be more than a month later. You can get a graph showing local frost liability from the Western Regional Climate Center. Select the station that's closest to you and click on either "Spring Freeze Probabilities" or "Freeze-Free Probabilities." The date at which the orange line on the graph intersects the probability parameter that you're comfortable with -- 50%, 10%, etc. -- is the last frost date for that confidence level based on historic norms. Bear in mind a few things: 1. Even if you find a weather station that is very near you, the microclimate in your planting area may vary considerably in its frost liability. 2. This is an El Nino winter during a cold segment of the Pacific Decadal Oscillation, so you probably don't need to be too conservative about planting-out dates. 3. There are no guarantees. These dates are probabilistic only. Are you feeling lucky? |
Here is a link that might be useful: Western Regional Climate Center
