One of the biggest mistakes gardeners make is starting seedlings too soon.
I know gardeners in Los Angeles who harvest tomatoes in January, but north of USDA Zone 6 you shouldn’t even think of starting tomatoes until February.
In USDA Zone 4 we start ours in two batches: A few in mid-March, and a larger group around the first of April. If the weather breaks early, the March planting is worth it; but if spring is sow, the early seedlings just get tossed out, as they are too big when planting season arrives.
That’s why it’s a good idea to make two sowings of seed, a week or two apart. If something happens to one set, you’ve got the backup. With two plantings, a week before and a week after the theoretically ideal planting date, you’re ready either way.
TIMING CHART FOR STARTING SEEDLINGS
Start 10 to 12 weeks before the frost-free date.
Start 6 to 8 weeks before the frost-free date.
Start 2 to 4 weeks before the frost-free date.