Timing of Vegetable seed starting
After trial & error I finally figured out what to plant when in our unique maritime climate. The last frost date doesn't matter so much as the soil temperatures & for warm weather vegetables air temperature, too!
Vegetable Crops are cool or warm weather preferable.
SHV = semi-hardy vegetables are less frost tolerant
HV = hardy vegetables will likely survive a frost
WWV = warm weather vegetables not tolerant of frost or cooler weather
MGT = minimum germination temperature
MST = minimum soil temperature
HV = broccoli, cabbage, kohlrabi, onions, lettuce, peas, radishes, spinach and turnips (MGT = 35F)
SHV=beets, spinach, carrots, cauliflower, parsley, parsnips, potatoes and Swiss chard (MGT = 40F)
WWV = beans, celery, corn, cucumbers, New Zealand spinach, all squash, melons, eggplant, pepper & tomato (MST = 55-60F)
I'm always eager to plant early to get a head start, but have learned to hold off a bit with some vegetables (beets, cabbage family, onions, leeks, Swiss chard) because of vernalization, premature flowering when exposed to 40-50 degree temps when seedlings are big enough to do so. Planting later & having smaller seedlings during our cool spring prevents it. I decided to start many things from seed rather than purchased transplants because of the bolting. There's nothing you can do to stop it when those starts are held too long in small pots & too mature for our weather. Save your $ & energy to start your own. It's easy if you winter sow them.
A useful book is Backyard Bounty by Linda Gilkeson where she explains specifics for how & when to plant in the western British Columbia, WA, & OR climate. She's not a winter sower, but a winter sower could use her suggestions for after germination.
She also publishes articles on the link below & will be at the Mother Earth Fair in Puyallup the 1st weekend of June.
Here is a link that might be useful: Timing Seed Starting by Linda Gilkeson