How and When to Start Seeds Indoors

Learn when to start seeds indoors, what plants you should pick, how to care for them, and how and when to transplant them outdoors.

It’s easy to cure an early case of garden fever: follow these tips on how and when to start seeds indoors. Growing your own seedlings also saves money and gives you the opportunity to enjoy unusual heirloom varieties.

“It’s kind of fun to start things indoors,” says Richard Jauron, a recently retired horticulturist at Iowa State University. “If you’re looking for a specific variety, you may not find it locally at a greenhouse, so you can buy the seed and start it yourself.”

Selecting Seeds

Deciding what to grow from seed is up to you, but Richard notes some things aren’t worth the bother. Begonias, for example, have tiny seeds that are hard to germinate and slow to mature. It’s much easier just to buy those plants at the garden center.

Fortunately, there are plenty of easy and readily available candidates, including flowers such as marigolds, zinnias, petunias and impatiens, and vegetables such as tomatoes, peppers, cabbage, kale and broccoli.

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Teach children how flowers and veggies grow by planting seeds.

When to Start Seeds Indoors

Determining when to start seeds indoors depends on where you live. Those who live in hot climates—Zones 8 or 9, for example—may be able start seeds as early as December. Northern gardeners, like those who grow in Zones 3 or 4, should wait until March or April to start most seeds, depending on the plant’s germination period.

Don’t fret if you have to wait a little longer. Extra time offers the perfect opportunity to order seeds—preferably as early as possible to get the best selection. Check out our favorite seed catalogs. Seed packets often include recommendations on when to plant indoors based on your average last frost date.

Richard cautions not to start growing too early. “Something like cucumbers or squash germinate and grow quickly, so start these inside three to four weeks before they can go outdoors,” he says. Tomatoes need five to six weeks indoors; peppers require seven to eight weeks. Here’s more expert advice for growing tomatoes indoors.

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Seedlings in peat pots

How to Get Started Growing Seeds Indoors

Richard starts vining vegetables in peat pots, which are biodegradable cells made of compressed peat moss that can be set directly in the ground without disturbing roots. For most other plants, he recommends plastic trays that have been washed with warm, soapy water and disinfected with a 10% bleach solution to prevent spreading disease. You can also make your own newspaper pots.

Fill the tray with a commercial seed-starting mix of peat moss and vermiculite, and then sow the seed at the depth recommended on the packet. “For the seed to germinate, it needs good seed-to-soil contact,” Richard says. “And make sure they’re not too crowded.” Properly spaced seedlings are easier to transplant later.

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Boost Germination With Heat

When you start seeds indoors, warmth and humidity aid germination. “You need to keep the seed consistently moist,” Richard says. After sowing, he waters the tray before stretching clear plastic food wrap across the top and securing it with tape. (Some trays are sold with clear plastic domes to create this greenhouse effect.) He then places it by a heating vent, or you can also use a heating mat specifically for plants.

“As soon as the seedlings come up, take off the plastic, put the trays under lights and let the germination medium dry out—not completely dry out but just enough to dry out somewhat—then water again,” Richard says.

Light is very important. “If you put the seedlings in a window, even a sunny window, they’re not going to get enough light, so they stretch and get very spindly,” Richard says. “I typically use an ordinary fluorescent light with two 40-watt tubes. The light needs to be very close to the seedlings when they come up, typically within 4 to 6 inches.”

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When to Move Seedlings Outdoors

Once the seedlings develop a second set of true leaves, Richard transplants them into cell packs filled with potting mix. “I just use a knife and dig them up really carefully by hand,” he says.

When it’s time to take plants outside (see the seed packet for guidance), start by acclimating them to the conditions. This process, called hardening off, can be done by placing plants outside in the shade for a few days, gradually moving them to increase the amount of light they receive every few days.

“I like planting in the morning or in the evening, not during the heat of the day when the plants might suffer a little bit,” Richard says. “Water them well and they should be OK.” If seedlings appear a bit pale, he suggests adding a water-soluble fertilizer when watering, and wait for your plants to flourish. Learn the best time to water plants.

Top Plants to Start from Seed

These popular blooms and veggies are easy to grow start from seed.

Flowers

  • Aster
  • Bachelor’s button
  • Calendula
  • Cosmos
  • Phlox
  • Salvia
  • Sunflower

Vegetables

  • Beans
  • Broccoli
  • Cauliflower
  • Cucumber
  • Eggplant
  • Lettuce
  • Squash
  • Tomatoes
  • Peppers

Need to organize your seeds? Check out 5 useful seed storage boxes and containers.