General Guidelines for When to Plant Seeds Indoors In general, you should start planting seeds indoors 6 to 8 weeks before the average last frost date in your area. The right time to start seeds indoors varies significantly depending on the crop and ranges from 2 to 16 weeks before transplanting. Use this calculator to determine when to start planting seeds indoors and then transplant them outdoors, based on the frost free date in your area. Each plant has a different recommendation for when to start planting seeds indoors.
The general rule of thumb is that most annual vegetables should be started indoors six weeks before the frost end date in your area. Most seed packs also list this information, saying something like “start seeds indoors six weeks before the frost end date. To know when to start planting indoors, you first need to know when the average last frost date is. That's the day in your area that's most likely to be the last day a night frost occurs every year.
Understand that this is not a fixed date, just a good starting point. Start planting 6 weeks before the last frost date. For annuals and perennials, you can extend it to 8 weeks. Plants that should be started earlier are broccoli, cabbage, cauliflower, and lettuce.
Sow seeds indoors 10 weeks before the last frost date. If there is no information on the seed package, you can start all your seeds quite safely about 6 weeks before planting them outdoors. Take note of which plants are too big or too small at planting time, and then you can make adjustments next year based on your notes. For detailed instructions on starting 500 varieties of annual and perennial flowers, I highly recommend Eileen Powell's book, From Seed to Bloom (Storey 199).
This allows optimal space for growing strong seedlings that can go from the seed tray directly to the garden or flower beds. Use this initial seed table to know when to start vegetable and herb seeds indoors and then transplant them outdoors, based on the frost date in your area. Cucumbers are normally planted directly in the garden, but the seeds don't germinate until the soil is warm enough, giving it a late start. Most seeds will germinate faster with a little heat, but some seeds will stop germinating if they get too hot.
Even if you buy a ready-made commercial seed mix, adding one cup of molds for every 4 cups of mix will do wonders for your seeds. However, each seed package will contain the information needed to determine when to start planting seeds indoors. Seeds are available from many sources, from your local building supply store to garden centers and mail-order catalogs. Most gardeners don't grow trees and shrubs from seed, so I won't mention them, except to say that they can generally be treated as perennials.
If you start seeds in the basement or somewhere cold, make sure the temperature isn't too cold for the seeds to germinate. One of the most rewarding experiences for a gardener is starting their own indoor vegetable and flower plants from seed.