For skiers, February may be the peak of winter, but maritime gardeners are moving on to spring. In Seattle, we have a wonderfully early last frost date, especially considering how far north we are. Seattle’s actual last frost date depends on the year, of course, but many books and websites publish dates based on averages over a couple of years. At least, that’s how I hope they get to their conclusions- some people may just guess. Last frost date is sometimes reported as the day when there is less than a 10% chance of temperatures below 28 degrees until fall. Or, sometimes it’s below 32 degrees. What day folks come up also depends on which years they’re taking their data from. Usually in science the more data you have the better, but with a changing climate and increasingly good measurement and record keeping methods, going back too far may skew the results. In any case, I like to think of Seattle’s last frost date as April 1st, because it’s a day I can remember, and it lines up pretty well with what the National Climatic Data Center reports, for both the 28 degree and 32 degree measures.
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Seed packets often say things like “sow indoors 6-8 weeks before last frost date.” Guess what- that’s now! If keeping pots of dirt on your windowsills or tending them under grow lights for 6-8 weeks seems like a hassle, it is. I find it fun to watch the plants grow for about two weeks after they sprout, and then I get sick of watering them. This can be helped by a) having children and telling them that the plants are pets and they’ll suffer horribly if they aren’t watered, b) building a gardening robot (arduino prototyping board + tiny sprinkler heads), or c) buying seedlings from a nursery in 6-8 weeks. I’ll do a little bit of option c this year, and maybe even get around to making option b happen, but I’m still going to start lots of seeds indoors, just because it’s so much cheaper than buying seedlings.
Some seed packets may say “soil temperature for germination 55-75 degrees F”. Note that they say soil temperature, not air temperature. For example, as I’m writing this the AgWeatherNet reports a 54 degree air temperature in Seattle, and 45 degree soil temperatures, which means I’m taking a chance if I put my peas in the ground today, because my seed packet says they germinate between 50 and 77 degrees. Pea seeds are cheap, though, so I’ll probably plant a few now but save some seeds for later, in case the first batch don’t sprout.
If this whole seed starting business doesn’t sound fun to you, seedlings for edible plants are already showing up in nurseries, and there will be a flood of local plant sales starting in mid-march. One of the earliest sales for edibles is the Seattle Tilth March Edible Plant Sale, on March 17th.
References
[1] National Climatic Data Center
[2] AgWeatherNet, The Washington Agricultural Weather Network Version 2.0
