Last Frost Date...Or Is It?

Yesterday was a really big day in the farming world. April 15th, in addition to being tax day, is the supposed last frost date for this area. I'm not sure where this particular date came from, but it's a fact of conventional wisdom, old wives' tales, and folklore that April 15th is the last date on which frost can occur. After this date, you can start setting out those plants that don't like the cold - plants like tomatoes, squash, eggplant, etc.

Curious about the true last frost date for around here, I went to the National Climatic Data Center where I found the average last frost dates recorded in Santa Rosa from 1971-2000 (http://cdo.ncdc.noaa.gov/cliwwwmatenormals/clim20supp1/states/CA.pdf). According to NCDC data, there is only a ten percent chance that the temperature will drop to 28 degrees after February 19th, a ten percent chance the temp will drop to 32 degrees after April 5th, and a ten percent chance the temp will drop to 36 degrees after May 1st. So really, the frost date varies depending on what degree of frost you're tracking. And although the frost dates are usually fairly good predictors of the temperature trends, strange things, including frost, can happen long after April 15th.

Mid to late April is always a good goal for when to begin planting summer veggies, but it's always a smart idea to provide these often delicate plants with a little protection from the elements. Covering your young plants with floating row cover or a light frost blanket certainly won't hurt them and will actually keep them warm and help them get off to a healthy and fast start. It's also a good idea to get used to reading the dew point as a predictor of how cold it will actually get in the early morning frost periods. The lower the dew point and the predicted temperature, the more likely the temperature will drop and stay low for a longer period of time. A high dew point, on the other hand, coupled with a low temperature, makes it less likely for a significant and devastating frost to occur.

Ultimately, the best way to protect your plants against frost and to determine when it's safe to start planting your heat loving plants is to be observant of what the weather seems to be doing. If it feels like it's going to frost, it probably will. If it hasn't gotten close to frosting in weeks and the night time temperatures have been staying safely high, then it probably won't frost. And it's always better to be safe and protect your plants, than to risk it and lose your young plants.