I started my tomatoes back around the first of March....it says 6-8 weeks in most instructions, and I usually plant near the first of May. Now, many locals say to wait until May 15 for tomatoes, but most years earlier is fine. However, this year for the first time I had a seed heating mat and boy did it make a difference. I started about 50 tomato seeds and they almost all came up. No problem, even though I'll only plant about 30 and some of them I'll buy because I didn't have the Black Krim and Park's Whopper seeds. Then all those tomatoes started growing, and I've had way leggy plants for the last two weeks, and it's still two weeks before I should put them out. So I took the biggest ones and hardened them off a bit, then I planted them beside a south facing brick wall and covered them with a cattle panel covered with plastic. Kind of a mini greenhouse. Anyway, it got down to about 41 last night and that's the predicted low for the 10 day forecast. Who knows? Maybe I'll have really early tomatoes. On the other hand, I've planted early before and even though I didn't lose plants to cool weather they never really took off until it warmed up, so tomatoes I planted later mostly caught up anyway.
Chuck