Danger of frost has passed for peninsula gardeners.

One of the challenges that any gardener faces is deciding when to plant seeds in the new year. Here in the San Francisco Bay Area, and in particular the peninsula, we are very fortunate to have extremely moderate weather and so it means that we can start the planting season very early. But how early?

The one danger that we face is that we can get frost in February. Frost is particularly damaging for certain types of seedlings, and so it’s important to consider. In fact early last year we had about a full week of evening frost. I remember this vividly because I had potatoes that were about to go to maturity and so I was out there every night covering them, and out there every morning uncovering them. I know that people’s memory of weather can be unreliable and so it got me wondering if there was good historical data out there. It turns out that there is.

A few years ago I really got into analyzing weather data. I discovered a UC website that had free, rich, downloadable set of climate data for various spots in California. And with the help of Excel I did a lot of analysis on it. At the time I was just doing it for fun but now as a gardener I can look back at the analysis that I did and pull out some very important information.

Frost danger climate chart SFO RWCHere is a chart of climate data from the year 2000 to the year 2010 for the San Francisco Airport and a spot in Redwood City. The red dots show the high temperature for the day and the blue dots show the low temperature for the day. The green dots are precipitation. For the purposes of deciding when it’s safe to plant, let’s focus on the blue dots.

Although technically 32 degrees Fahrenheit is freezing temperature, I personally start to get concerned when the temperature is below 34F. And so I have drawn a blue dotted line showing approximately where 34F it’s. And you can see that from around the middle of February onwards, the nighttime temperatures in this 10-year period have never dipped below 34F. So this gives me a fair amount of confidence with respect to going and putting seeds or seedlings in the ground.

I would say that mid February is a fairly safe guideline overall. Granted, it depends on what you’re planting, but I feel comfortable that I can plant from now going forward and I won’t have frost problems. Keep in mind that some vegetables need much higher temperatures than 32F in order to work, but the point here is that frost creates special problems for many food crops.

One might reasonably ask what happened to the data from 2011 to 2014. The problem I ran into is that the University of California website stopped published the data I was using. Fortunately somebody at the UC extension office was able to refer me to a data source from Utah State University which for some reason has California climate data. (Thank God.) And so if I wanted to I could work that data into my spreadsheet and update my chart.

In any case I just downloaded fresh data and did a quick spot check of weather data from 2011 to present. What I found is that in 2011 there was a late frost from 2/26 to 2/28. And in 2013 there was one night of frost on 2/20. To find the previous late February frost, I have to go back to 1996, and before that, 1990. So while historically there is a *chance* of frost in late February, it’s rare.

The key point I want to communicate here, besides mid-February being a safe time to start planting, is that there is wonderful free data available, if you know where to look, and we can learn a lot from it.