Snoqualmie Valley Master Gardeners: Indoor Seed Propagation and Planting

This is the second guest post this spring from the Snoqualmie Valley Master Gardeners Association Intern, Beverly Morrow.

Now is the time to start indoor seed propagation for your summer vegetables and flowers. Not only will this save money, but you can also plant varieties you want to try, that local nurseries may not carry. Here’s what you need to get started:

grow light

Good quality seeds

In our maritime cool climate, plants such as tomatoes, cabbage, broccoli, eggplant, peppers, cucumbers, corn, basil, and flowers such as zinnia, marigolds, and petunias require a head start by being sown indoors before being planted outdoors. Read the seed packets for information about planting times and care.

Seed growing container

Greenhouse kits are made for starting seeds indoors and typically include a planting cell container, a tray to hold water, pellets of either coir (shredded coconut husk) or peat moss, and a clear plastic lid to cover the cells.

Seedling heating mat

The range of soil temperatures for most seed germination is between 70-80˚F., so a heating mat provides consistent temperatures and the seeds will sprout faster.

Supplemental lighting 

Use fluorescent or multi-spectrum light bulbs to provide light for your new seedlings.

Seed label supplies

Use plastic strips cut from butter or yogurt containers for labels.  With a black permanent marker, write the seed name, date planted and days to germinate (look on the seed package) on the label.
Spray bottle

As the seeds sprout they need to stay moist.

Larger transplant pots

About 12-14 days after sprouting or when the roots start growing out the bottom of the cell chamber, you’ll want to transplant the seedlings into larger pots.


Fertilize the seedlings after the first true leaves appear with a diluted fertilizer that has a ratio of about 5-1-1 (nitrogen, phosphorus, and potassium); fertilize once or twice a week.

When to plant the seeds

Most seed packets suggest starting seeds indoors 6-8 weeks before the last possible frost where you live. For the Snoqualmie and North Bend area, the average last frost is around April 29; a safe, last frost date is May 15.  So anytime from mid to late March is ideal.

Some seeds should not be started indoors because they sprout so easily outdoors once the soil temperature is 60-65˚ F. Don’t start beets, radishes, turnips, onions, potatoes, or carrots indoors or leafy greens like chard, lettuce, or spinach indoors. You can plant corn and beans directly in the warm spring soil near the middle/end of May with success if the soil is workable and warm.

Seed planting

If you’re using a greenhouse growing kit, fill each growing cell with a coir/peat pellet (flat side down to maintain their shape as they expand) or seed starting soil mix; water it until it is very wet. To save money use yogurt cups or plastic tubs (poke drainage holes in the bottoms) or pots made from newspapers (avoid colored ink); all work fine but are not as convenient and hassle free as the growing kits with pellets.

Place one seed in each cell, cover with a pinch of moist coir/peat mix material or soil at the depth recommended on the seed packet. Tap down the wet mix gently to surround the seed in the wet mix, then cover the tray with the plastic lid that is part of the seed kit. Place the trays on a heating mat, plug it in and leave it on 24/7. Seeds germinate in the dark so don’t use the grow light yet. Do not let the soil dry out! Mist the soil with the spray bottle.

What to do after the seeds sprout

Once the sprouts are 1/2 inch tall, move the cell tray under the grow lights. Take the clear lid off the tray, and place the grow light about 2 inches above the seedlings. Keep seedlings moist but not “standing in water” wet. Use a full spectrum or fluorescent bulb for light and hang the light so it can be raised up as the plants grow. Turn the light off the plants at night or after 16 hours of light per day (they need a dark cycle to grow).

Transplanting to bigger pots.

When roots start coming out the bottoms of the seed cells, it is time to transplant them. Be sure the mix is wet so it clings to the roots and prevents transplant shock.  Carefully remove each seedling with soil intact into a new 3-4 inch pot.  Plant the seedling about a ½ inch deeper than it was in the cell – this gives the stem more support. Place the new transplant back under the light. Be sure the soil is moist all around the new seedling. Each time the roots grow out the bottom of the transplant pots, transplant to size larger pot.

Two weeks before the outdoors planting date, take the seedlings outside for a few hours each day gradually exposing the plants to the outdoor environment. This is called “hardening off”. Bring them indoors at night, but continue to increase the outdoor exposure. When you’re ready to plant the seedlings in the garden, water them well and then water again after transplanting. Fertilize once a week and be sure to protect the new transplants from slugs and rabbits.

More information on vegetable gardening can be found on our website at

[The author is an intern in the WSU Extension Master Gardener Program. Extension programs and employment are available to all without discrimination. Evidence of noncompliance may be reported through your local Extension office.]

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