Weather Blog

It's past March 10 -- safe to plant now?

It's past March 10 -- safe to plant now?
As we tread deeper into March and spring looms larger and larger on the horizon, plenty are asking if the threat of overnight freezes has passed and it's OK to start planting.

For the city of Seattle, the average last frost over the 30 year period between 1970 and 2000 is March 10. (As in, the last average date the temperature drops to 32 degrees or below.) While that is a good rule of (green) thumb, it's not a guarantee-- the record low for May 1 is 28, set in 1954 and the record lows for late March and April are peppered with temperatures in the low 30s. But by now the odds of a freeze are pretty low.

But if you want to be really safe, the average last low temperature of 36 degrees or under in Seattle is April 11.

Now, Seattle's date is earlier than most other cities around here because the urban areas create their own heat at night and thus tend to have warmer temperatures overnight than some of the outlying areas. So if you live away from the city, your vulnerability to a frost extends later than in the metro area.

You can find out your average first and last frost at this National Weather Service site

Just check the box that says "first/last dates" then find your city on the list in the middle column, and set the right column to "min temp < 32 degrees".

You can also look at these maps for more help in figuring out a date to start planting.

By the way, the average first frost in Seattle is November 11.

Happy planting!