SEATTLE — After a mild but a soaker of a winter, I’ve been asked a lot via email (firstname.lastname@example.org) and Twitter (@Q13Tim) about the last chance for frost. And I think for this year we’re done in Western Washington. (Sound the trumpets, cue the applause!)
Frost can kill young plants. Water expands when it freezes and that expansion can burst the cell walls. Young plants are particularly vulnerable. For much of the King County and Seattle neighborhoods, our overnight lows are much warmer than that of surrounding areas. Our latest freeze on the calendar recently happened in April, was back in 2008. Last year, we had an early spring and it happened in early March. Typically our last frost is in late March. This year was incredibly mild and our last frozen morning at SeaTac was in January.
The Sound Sound is our coldest area in Western Washington. Cold pock air pools in pockets around Joint Base Lewis-McChord. The official Nat’l Weather Service reporting station is in Olympia. The latest we’ve seen frost in the South Sound was May 19th, back in 2012. Typically it happens in early May. Our last frozen morning this year was at the end of last month. And while history says lower elevations could still see some frosty mornings, I’m doubting it for 2016.
The news is good up north. But, climate records are a bit spottier between Seattle and the Canadian border. In the last 5 years, last frost has been all over the spring calendar. With our mild but wet winter this year, our last frozen morning was in late February. So, up north I think the threat of frost is totally over.
But just because you can put plants in the ground, doesn’t mean you should plant everything now. Heat and sun loving plants like peppers and tomatoes can have their growth stunted in soil that’s colder than 55 degrees. This is the time of year when a soil thermometer comes in handy. So, you might want to wait on those until next month.