Starting seeds indoors is a must for northern gardeners who want to grow warm weather crops like tomatoes, but it can be a boon to any garden. Many plants do better when placed in the garden as transplants. See “When Should I Start Seeds? Printable Seed Starting Calendar” for a list of which plants to start inside and which to direct seed in the garden.
Sure, you can pick up transplants at your local garden center or big box parking lot, but starting seeds at home gives you access to many more varieties. It’s also fun! More on why I start seeds later in the post, but first, let’s get planting!
Starting Seeds Indoors – 11 Steps to Help You Plant Seeds with Confidence
1. Keep records and label all your seedlings
I keep a simple spreadsheet listing the variety planted, date planted, date when seedlings appear and some other basic information. You can print out your own copy from the post, “Simple Record Keeping for the Garden with Printable Seed Starting Chart”.
Mark your seedlings! I label all planting containers with craft sticks or other markers. It’s amazing how similar garden plants can look with they are tiny.
2. Choose the right seed starting mix
Potting mix should be fairly light but still hold enough water to keep seedlings moist.
Can you use garden soil for starting seeds indoors? I wouldn’t recommend it. Plain soil doesn’t drain well enough, and seeds will tend to rot or the seedlings will be more likely to get fungal problems. You might also introduce weed seeds or diseases from your garden.
If you’d like to make your own potting mix, try a mix of one third each:
- soil or compost
- sand, vermiculite or perlite
- peat moss or coir
If you’re worried about soil disease, pasteurize your soil or compost by heating it to 180 degrees in a warm oven and holding at that temperature for 30 minutes. If you have healthy compost or soil, this shouldn’t be needed. Overheating the soil will damage it, so heat with caution.
My current favorite potting soil is FoxFarm organic potting soil. It contains composted forest humus, sandy loam, sphagnum peat, earthworm castings, bat guano, and seagoing fish and crab meal.
3. Choose the Right Seed Starting Containers
Most of my seeds are started in old cell packs or other reused nursery pots. My friends know that I start seeds indoors (sometimes I sell or trade my extras), so they save their pots for me.
I like reusing old pots because they are sturdy, drain well, and fit neatly into the large seedling trays. I’ve seen plenty of ideas for homemade pots, but many of them seem too small (like eggshells) or too flimsy (like newspapers). Peat pots are often recommended because they “break down in the soil”, but in my experience they break down poorly and often stunt young plants.
Whatever you choose, make sure your pot drains well (punch holes in the bottom of old yogurt containers, for instance). Don’t reuse anything with potentially toxic residues (no old computer cases as planters, please). Don’t forget to place a shallow container under your seedling pots to catch the drips.
Many times I plant directly in the garden from the 1 inch cells, but sometimes I “pot up” seedlings indoors. When I want larger transplants, I start seeds inside early and then transplant them one or more times as they grow. You can learn more about this in the post “How to Grow Tomatoes from Seed”.
4. Provide plenty of light
Make sure your seedlings get adequate light. Indoor seedlings need 16 to 18 hours of sunlight each day. Turn off the lights at night so they can rest. (Plants need sleep, too!) I use a simple plugin timer to turn our grow lights on and off each day. Without enough light, indoor seedlings will stretch towards available light. This makes them tall and floppy.
Several years ago, my husband built me a really nifty seed starting rack with lights on adjustable chains. I park this in front of our south facing windows in the basement. (Get the plans for the seed rack here.)
Plants do best when lights are kept close, about 3-4 inches above the seedlings. LEDs or fluorescent lights allow you to keep lights close to the plants without overheating the delicate seedlings.
Do you need special plant lights to start seeds indoors? They’re great if you want to make the investment, but not absolutely necessary. I originally started with fluorescent shop lights with “plant bulbs”, and then switched to LED shop lights. Both work adequately, as long as they are kept the right distance from seedlings.
5. Keep Indoor Seeds and Seedlings at the Right Temperature
For germination (getting the seeds to sprout), a little warmth helps jump start the process. I’ve set seedling trays near the wood stove or on top of the refrigerator (plant lights aren’t usually needed until after seedlings emerge). Seed starting mats are convenient, because you don’t have to move your trays around the house. (One time I had a seedling tray near the stove upended by a toddler. Other times I’ve lost trays to cats. I’ve become more protective of my seed starting trays.) (For a detailed comparison of germination rates at different temperatures, check out the effect of soil temp on sown seeds.
Once seedlings are up, most grown happily along at around 60-70 F (15 – 21 C). Too hot or too cold will stunt seedling growth – or may even kill your seedlings.
Some seeds require stratification (chilling before planting), especially wildflower seeds. Double check planting recommendations if you’re working with a “new to you” plant.
6. Cover indoor seed trays to improve germination
Covering your seedlings with a plastic cover during germination will trap moisture and improve germination rates. Because I reuse commercial cellpacks for indoor seed starting, they fit neatly into planting trays with matching plastic dome covers. (Click here to order seed starting kits with trays, seedling containers, markers and covers.)
Once seedlings emerge, remove the cover to allow air circulation. It’s best to check your seedlings daily while you wait for them to sprout. If you see any signs of mold, remove the cover for a few hours to allow excess moisture to escape.
7. Plant Seeds at the Right Depth
Rule of thumb for planting: Plant seeds roughly three times as deep as the diameter of the seed. Seed packets should give specific instructions, but if you have loose seed, this is a good general rule. Some seeds need light to germinate, so they should be planted on the surface or only very lightly covered.
8. Provide Enough Water to Seedlings, but Not Too Much
Your indoor seed starting mix should be moist enough that it holds together when you squeeze a handful, but not so wet that water runs out when you squeeze it. Covering the trays should keep them moist enough until seedlings germinate
Once seedlings are up, I water gently every few days. (Warm rainwater is great if you have it.) Between watering, I allow the soil to dry somewhat, but not completely. Soil should never be soggy. Moss, fungal growth or seedlings that tip over dead with a rotten base (damping off) are signs of too much water.
9. Provide Good Air Circulation and Movement
You may have noticed large fans in commercial greenhouses. These fans provide air movement to keep foliage dry and prevent fungal diseases. They also help strengthen the plant stems, because the seedlings must hold themselves upright in the light breeze.
If you have only a small number of seedlings, gently stroking the plants several times per day can help strengthen the stems. With several dozen (or more) seedlings, a fan is much handier. We keep a standing fan on the same timer as the lights and have it oscillate across the seedling trays.
10. Feed and care for the young plants
I like to use foliar feeds such as liquid seaweed fertilizer to provide nutrients to the seedlings. Other great gentle feeding options include compost tea or worm poo tea. EM-1 Microbial Inoculant is added to the water to introduce beneficial bacteria to the soil.
If seedlings become crowded, transplant to larger pots or transition to the garden. If you have more than one seedling per container, you can transplant to individual containers or use a scissors to snip off all but the healthiest seedling at ground level.
If plants fail to thrive, check water, light, temp and fertilizer levels.
11. Harden off plants before transplanting to the garden
Indoor seedlings need time to adjust before going out in the garden. When you’re close to transplanting time, place seedlings outside in a protected area, out of direct sun. Start with a couple hours outside, and then increase the time each day. Within three to five days they should be ready for the garden.
Our Indoor Seed Starting Setup
Our basement was designed so that it could be used as an apartment for aging parents, so I have a kitchenette between my root cellar and the door to the attached greenhouse. Since we are currently without added housemates, it's my gardening area. I have water at the ready and my mess is contained quite nicely.
Once the seeds go in the planting containers by the sink, I set up seedling trays on my seed starting shelves. From the seed starting shelves, the seedlings graduate to one of our two greenhouses. Then the go to a cold frame or other semi-protected spot to harden off, and finally out to the garden.
Why I Plant so Many Different Vegetables, Fruits, Flowers and Herbs
I have a pretty good sized garden, about an acre or so. It's a bit of a crazy quilt. I like to plant a lot of different things – vegetables, flowers, herbs – all jumbled together. One year when I did a count, there were over 100 different varieties. Why so many? Other than my addiction to gardening, here are the main reasons I grow so many different types of plants in my home garden.
More Plant Variety Helps with Pest Control
If the critters are going to come after my plants, I want them to work for their lunch. I recommend the book Great Garden Companions for some ideas on how to use plant families and their companion plants to boost your garden production.
Small Farms and Gardens Contribute to Biodiversity
I believe one of the best ways to preserve biodiversity is to eat it. We are losing food plant varieties at an alarming rate. Even if I only plant a couple of plants of a particular variety, I'm still helping to keep that seed in production. You can help, too. (Love, love, love the book Heirloom Vegetables.)
If you look at the numbers, it's a pretty scary situation.
The world’s food supply depends on about 150 plant species. Of those 150, just 12 provide three-quarters of the world’s food. More than half of the world’s food energy comes from a limited number of varieties of three “mega-crops”: rice, wheat, and maize.
I've grown everything from sunchokes to mache to quinoa. There are hundreds of amazing food crops just waiting for the curious garden to explore.
Starting Seeds and Growing Heirloom Vegetables and Fruits is Fun!
Why grow only russet potatoes when you can grow potatoes that are purple, yellow and red? Why grow only red, ping pong ball tomatoes when you can grow tomatoes that pink, purple, green-striped, yellow, orange, white, turban-shaped, sausage-shaped – there are literally thousands of options.
My garden starts humming with pollinators early in spring as the companion plants lure in beneficial insects, and keeps humming until hard frost. The scents, the colors, the flavors – it's lovely!
If you're looking for ideas about where to buy seeds, you can check out my favorite seed sources. If you have specific questions about starting seeds, feel free to drop me a note and I'll see if I can help.
You may also enjoy:
- How to Start a Garden – 10 Steps to Gardening for Beginners
- My Favorite Seed Sources, Seed Storage and Germination – Printable Seed Longevity and Germination Charts
- Small Garden, Big Yields – 10 Tips for a Great Harvest
Originally published in 2012, updated in 2017.