I'm in zone 6b outside of Boston and right now I'm in the situation I'm in every year: I have plants (mostly annuals) growing under grow lights, the weather is warming outside, but I'm never sure when it's safe for which plants to start the process of hardening off and then transplanting outside. I get that there are basically three types of plants in this regard: warm-season, half-hardy and hardy, and this year I have Eileen Powell's From Seed to Bloom as my guide.

We've had possibly the warmest April on record, or at least in the two decades I've lived here. Though the average last frost date for my region is April 28, I don't think it's dropped down to 32 degrees in the entire month. Here is New England we all love to make jokes about the unpredictability of the weather, but I suspect that we are done with frost for the season and that this year will turn out to have had the earliest last frost date in many years.

I know soil temperature is even more important than air temperature in determining planting dates, but I have no reason to think that soil temperature is not also warmer now than it typically is for this time of year. I do have a soil thermometer.

Eileen Powell's book gives instructions for each variety on when to plant seedlings outdoors, though there's no mention of soil temperatures. I guess what I'm wondering is for varieties where Powell says "After last frost," if I can go with my belief (backed up by an 10-day weather forecast on weather.com) that we're done with frosts and therefore start hardening-off and then planting some of those this early on April 19. I do think hardening off now would be fine.

I even see that for Salpiglossis, which I'm growing for the first time, Powell indicates under PLANTING SEEDLINGS OUTDOORS: two weeks before last frost. That sounds like one hardy flower variety, so I'm going to start there.

Curious to know what guides others use to make these decisions.