When To Start Garden Seeds Indoors

Seedlings in Pots

Getty Images/Jordan Lye

Savvy gardeners start their own garden seeds at home rather than purchasing transplants at the store. There are many benefits to starting your own seeds including cost savings and the wider selection of cultivars available. Purchasing seed and starting your own seedlings is sometimes the only way to add rare plants to your collection. Perhaps the most important benefit of starting your own seeds is the ability to time sowing according to your expected planting date so that transplants are ready when you need them. You can use the information provided on seed packets along with a little knowledge about your local climate, as described below, to determine when to start garden seeds indoors. 

Which Seeds Can Be Started Indoors

Many vegetable, herb, and flower seeds can be started indoors, but not all seeds benefit from an early start. Some plants, like cilantro and carrots, do not tolerate transplanting and should be direct sown outdoors when soil temperatures warm. Other plants benefit from starting indoors while the temperatures are still cold outside. This is particularly true of crops from warm climates, like tomatoes and peppers, that require a long growing season to produce a crop. Read seed packets and catalog descriptions carefully to determine whether they should be direct sown or can be started indoors.  

  • Plants commonly started indoors: cabbage, broccoli, cauliflower, Brussels sprouts, tomato, eggplant, pepper, basil, chives
  • Direct sow these into the garden: spinach, carrots, beets, peas, beans, corn, kale, chard, dill, cilantro
  • Can be started indoors in pots or direct sown: cucumbers, melons, pumpkins, squash

Know Your Last Frost Date

Timing for indoor seed sowing is based on the average frost-free date for your area. The frost-free date is just as it sounds, the average date of the last light freeze in spring. Note that frost dates are averages based on historical climate data. Actual conditions can vary from year to year, with the last frost occurring before or after the frost-free date. However, frost-free dates are useful for timing seed sowing. You can find the average frost-free date for your area with a quick internet search or by asking at your local garden center or Master Gardener organization.

When To Start Garden Seeds Indoors

Different plants vary in the amount of time it takes for seeds to germinate and for seedlings to develop to the proper stage for transplanting into the garden. Seed packets and catalogues typically list indoor sowing time as a specified number of weeks before the first average frost-free date for your area, such as 6 to 8 weeks for tomatoes. This corresponds to the amount of time different plants need to develop before transplanting. Armed with this information, you can time your seed-sowing according to local conditions. The following generalized list can be helpful in timing indoor seed sowing; however, seed packets will contain more individualized recommendations. 

  • Thyme: 14-16 weeks before frost-free date
  • Onions, chives, lavender, rosemary, mint: 10-12 weeks before frost-free date
  • Peppers: 8-10 weeks before frost-free date
  • Tomato, eggplant, basil, lemon balm: 6-8 weeks before frost-free date
  • Broccoli, Brussels sprouts, cauliflower, cabbage, lettuce, chamomile: 4-6 weeks before frost-free date
  • Melons, cucumbers, squash, pumpkins: 4 weeks before frost-free date
  • Flowers: refer to seed packet

Seed Starting Tips

See packets are your best friend when it comes to starting seeds. They are packed with the information you need for success. Here are a few pieces of information to look for, in addition to those mentioned above.

Germination Rate

Also called the days to emergence, this is how long it takes before a seedling emerges from the seed. Seeds from different plant species germinate at different rates. When sowing, group seeds with similar germination rates together in planting trays to best manage the changing water needs of seeds verses seedlings. 

Sowing Depth

Seeds have different light requirements for germination. Some need to be on the soil surface to germinate, others buried in the darkness below. Seed packets and catalogues typically provide seed depth in inches. When in doubt use this rule of thumb—sow seeds to a depth of approximately three times the diameter of the seed.

Temperature for Germination

Most seeds will germinate well at a temperature between 70˚F and 80˚F held constant during day and night. A small heating pad can be used beneath the container to provide extra warmth for the seed bed. 

Soil Temperature for Transplanting

While the average frost-free date is a good estimate of planting time for the purposes of starting seeds, it is important to watch the weather and check soil conditions to determine when it is safe to transplant seedlings into the garden. Late freezes can delay planting. Soil temperature is also important, as it directly impacts plant growth. Some plants are sensitive to cold soils and are planted much later than the frost-free date. 

Remember to begin hardening off your seedlings about a week before transplanting into the garden. Hardening off involves slowly acclimating plants to outdoor light and wind conditions to prepare them for life in the garden. This is done by bringing plants outdoors for a few hours at a time, beginning in heavy shade and increasing both the amount and duration of sun exposure over the period of several days.

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