Starting seeds indoors is a fun project that just requires a few materials and a little bit of know-how. It’s especially great for those of us who just can’t wait till spring to get our hands into the dirt!
Gardners typically start seeds indoors in order to:
1) Get a jump start on the season - eat the veggies you love sooner!
2) Grow very long season crops, such as leeks and onions, in shorter season areas (like here in Minnesota).
3) Grow cool weather crops in the spring before it gets too hot, which causes spicy/bitter flavors and/or bolting - a common hurdle with brassica crops.
4) Avoid some of the pest, disease and weather pitfalls that face vulnerable young seedlings.
While indoor seed starting is an optional task for most seeds, some crops, like the leeks and onions above, might require it in order to come to maturity during a shorter growing season. Tomatoes and peppers are also a no-brainer to start early so that you can enjoy harvesting from them for a much longer time period. On the other hand, some crops, like legumes and root vegetables, usually perform better with direct-sowing right into the garden. To help you sort out the differences, we’ve developed straightforward charts of seeds to start early versus those to sow directly into the garden, complete with details on when to sow and transplant (this resource was developed for zone 4 - average last frost date of late April to early May; you can adjust the dates forward or back depending on your own last frost date). You will see that we also reference “winter sowing” as an option for seed starting. You can learn more about that method here.
What you need
A few key ingredients will get you well on your way: seeds, containers, a planting medium, and a light source.
Seeds
Select seeds appropriate for indoor seed starting from a trusted source. While you can find seeds in almost any home garden store, we encourage you to look for sources that provide heirloom or open-pollinated seeds. Heirloom seeds are just open-pollinated seeds that have been around for many generations. Both heirloom and open-pollinated seeds:
1) Produce fruit with seeds that will reproduce “true to type,” meaning the seeds can be saved, planted again, and will produce the same fruit as the parent plant. Supporting the preservation of these open-pollinated varieties means food security and preserved biodiversity for our planet.
2) Oftentimes are suited to more specific climates and growing conditions than hybrid seeds, which are developed to have broad appeal. This can work in your favor or against you. Be sure to read variety descriptions to determine whether the seed will be a good fit for your area.
3) Offer more variety and flavor for your palate, not to mention rare and beautiful varieties in all shapes, sizes and colors!
A great source for open-pollinated seeds that is doing great work in the preservation and restoration of our food crops is Seed Savers Exchange.
Containers
Any container can be used, as long as there are holes for drainage (drill holes if they aren’t present). The larger the container, the less likely the need to "pot up" again before transplanting out into the garden. There’s no need to get fancy here. Save old yogurt containers, cottage cheese containers, take out containers and the like to use for this purpose. Just make sure whatever you use will hold its shape through the entire process - paper pots tend to work better for seeds that you’re starting 2-4 weeks prior to transplanting, whereas repurposed plastic containers can be used for seeds being started 6+ weeks before transplanting with no issue. If you’re a household that avoids plastic usage, feel free to ask your neighbors to save their recyclables for you for a few weeks - you’ll be amazed at how much you’ll get!
Planting Medium
Seed starting mixes such as coco coir provide the best conditions for germination, however will not support plants long term. Using an organic potting soil mix will eliminate the need for transplanting young seedlings, but germination may take longer or be less successful. Many people opt to start their seedlings in small containers or in a small-celled tray filled with a seed starting mix, and once the seeds have germinated and formed a couple sets of leaves, move them to larger pots with organic potting soil. You will find many options for seed starting setups and supplies online or at your local nursery or home improvement store.
How to Succeed
It’s really not difficult to succeed in starting seeds indoors - with just a few simple steps, your garden will be ready to go by May!
Assist Germination
Unless there are special instructions on your seed packet, warmth and moisture are all most seeds require for germination. Keep the soil moist but not saturated (covering with plastic can help retain moisture, but isn’t necessary), and put the seed containers in a warm place; around 70-75F is typically ideal. Check your seed packet to see the typical germination rate of seeds and when you can expect to see your seedlings start to emerge.
Encourage Growth
Once your seedlings start popping up, remove any plastic covering and move them to a spot with bright light and cooler temperatures, around 60-70F. While you can get away with less, seedlings need a minimum of 12 hours of light a day to grow thick, strong stems, and ideally 16 hours. If this can't be provided on a windowsill, consider supplementing with a grow light or even tube-style fluorescent lights - combining a “warm white” tube with a “cool white” in the same fixture will provide the same wide spectrum of light that grow lights do. Keep lights within a few inches of the seedlings and adjust the height of the lights as your plants grow. Water regularly (check every other day), preferably from the bottom to prevent disease and to avoid disturbing the soil. This is a great resource on how, when and why to bottom water your seedlings.
Thin and Pot Up as Needed
Once seedlings have two sets of leaves, it's time to thin and/or transplant. If you have started seeds in a seed starting mix, you can carefully separate seedlings, lift them out with a popsicle stick or teaspoon, and transplant them to a larger container filled with pre-moistened organic potting soil. Carefully handle seedlings by the leaves, not the stems. If you have started seeds in potting soil, select the strongest, healthiest looking seedling to keep and snip all others off at soil level. In either case, if the seedlings outgrow their pots before it's time to transplant them into the garden, they will need to be moved to a larger container. Signs to watch for are roots growing out of the drainage holes, or a plant that is wider and twice as tall as its pot.
Harden Off
1-2 weeks before transplanting to the garden, it’s time to start to acclimate your plants to life outdoors. Set the plants outside in a protected area, sheltered from sun and wind. Start with one hour outside, then two hours, gradually increasing the time outdoors, and also allowing some exposure to direct sun, starting with weaker morning or late afternoon sun. Water frequently during this time. Too much direct sun too soon can cause your seedlings to struggle, so make sure to take your time with this process!
Transplant Outdoors
Choose a cloudy day or transplant in the early evening to avoid the stress of the sun's heat. Before you start, water your plants and prepare the holes by digging them twice as wide and the same depth as the pot (except for tomatoes, which should be buried deep enough to cover the bottom set of leaves, which should be pinched off). Also add a scoop of compost and a quart of water to each hole. Compostable pots can be planted whole, but should be split open. Plants in plastic pots should be carefully removed by tipping the pot upside down and tapping the bottom. Gently place the plant in the hole, fill in the hole with soil, and gently tamp with your hands, forming a shallow basin around the stem of the plant to encourage water to run toward the root zone. Water plants carefully at the base with another quart of water. Keep an eye on new transplants for signs of dehydration, sun scorch, or pest damage, and cover or protect as necessary.
That’s all there is to it! If you’d like a handy, printable version of this post, you can check out our Indoor Seed Starting Guide. As you’re starting seeds for the first time, remember that gardening is a journey, and certainly not an exact science. What works for one gardener starting seeds may not work for another, so be patient, keep learning, and keep growing!