Starting annual seeds indoors works best 8 to 10 weeks before your last spring frost. NC State Extension puts the window right there, and it holds for most of the flowers you would ever grow from a packet. Seed starting timing decides more about your summer bed than the brand of seed or the tray you buy. Sow too late and the first blooms show up in August. Sow too early and you spend weeks babysitting tall, floppy seedlings with nowhere to plant them.
Not one seed in that zinnia flat came up. I had sown it in April, well ahead of schedule, and the mix never got past cool to the touch. Germination often fails below 60°F (16°C) soil temperature, so the seed sat in wet mix while I checked it every morning for three weeks. The whole flat went in the compost. I sowed a fresh one in May and had seedlings in six days.
Two clocks run at the same time here, and mixing them up is where most schedules fall apart. The indoor clock runs backward. You look up your frost date, count back, and that is the day you fill your flats. The outdoor clock runs forward from that same date, and seed you put straight in the ground goes in 2 to 3 weeks after the frost has passed. Soil temperature gets a vote in that second one. A cold, wet spring pushes your outdoor sowing later no matter what the calendar page says.
Keep the temperature numbers in their own lanes, because they answer two different questions. One of them is a germination floor, and it describes the soil rather than the air above it. The rest describe how well a plant performs once it is up and growing. Cool-season annuals sulk in real heat. Warm-season ones want warm nights as well as warm afternoons, and without those nights they stall out and stay short.
Tender annuals get a harder line than the rest of the tray. They came from the tropics, so they go outside only after all danger of frost has passed. One light frost is enough to finish them. A warm stretch in April will tempt you every single year. Ignore it, because the thing you are waiting on is frost danger and not the reading on a nice afternoon.
Look up the average last frost date for your area before you open a single packet.
Subtract eight to ten weeks from that date and mark your indoor sowing day on the calendar.
Hold direct-sown seed until the frost date is a couple of weeks behind you, then check that the ground has warmed.
Keep tropical annuals under cover until every chance of frost is gone for the year.
If the window for starting annual seeds indoors closes before you get around to it, you have not lost the season. Direct sowing still works once the ground warms, and it skips the indoor stage. You can also take stem cuttings of 4 to 6 inches (10 to 15 cm) in spring or early summer, and those root in about 3 to 4 weeks. That is quicker than most people expect from a piece of a plant.
Seed pays off with annuals in a way it does not with everything in the garden. Annuals bloom the same season you sow them. Perennials grown from seed often need at least two years before they flower, so that packet buys you patience instead of color. Look up your local frost date first, mark the countdown on a calendar, and put the flats under lights on that day.
Read the full article: Annual vs Perennial: What Actually Comes Back
