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How to Start Seeds Indoors: A Step-by-Step Guide

Written by MasterClass

Last updated: Apr 29, 2020 • 6 min read

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Ron Finley Teaches Gardening

Sowing seeds indoors protects young plants from most pests and extends the growing season, providing an earlier harvest.

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Ron Finley Teaches GardeningRon Finley Teaches Gardening

Community activist and self-taught gardener Ron Finley shows you how to garden in any space, nurture your plants, and grow your own food.

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Which Seeds Should Be Started Indoors?

In general, start seeds for long-season crops indoors. These types of plants include tomatoes, broccoli, kale, eggplant, okra, peppers, cabbage, and pumpkins. Starting long-season crops indoors allows you to begin growing seedlings in late winter as you wait for the weather to warm.

Take into account how a plant reacts when transplanted. Root vegetables like radishes, beets, turnips, and carrots generally do not fare well when their roots are disturbed, and some types of plants simply don't react well to the stressful change of scenery.

How to Start Seeds Indoors: A Step-by-Step Guide

To provide optimal conditions for seed germination, start your seeds indoors before the growing season.

  1. Soak certain seeds. Seeds with tough exteriors, like pumpkins, squash, parsley, and chard, benefit from exposure to moisture before they are planted. This gives the seed a head start in the germination process. Place your seeds in a bowl of warm water overnight for approximately eight to 12 hours. Another way to speed up the germination process is the paper towel method: Dampen a paper towel, place your seeds on the towel and fold it in half, place the paper towel in a plastic bag, zip up the bag and leave 1 inch open for air circulation, and place the bag in a warm location until the seeds sprout. Plant the seeds into the soil once they germinate, and be extra careful to not break the delicate sprouts.
  2. Choose a container with drainage holes. Drainage prevents soil from becoming waterlogged, so make sure the container you select has holes that allow excess water to escape. Store-bought seed starting trays come equipped with pre-cut drainage holes and a humidity dome. Peat pots are made from peat moss—they have excellent drainage, and you can plant them directly into the ground outside once the seedlings have matured; this reduces root trauma when you transplant your young plants. You can also use tiny regular pots or even egg cartons, as long as you poke your own drainage holes in the bottom. Don't forget to place a drip tray underneath your container to catch the excess moisture and dirt.
  3. Add seed starting mix to your container. Seed starting mix is different from potting soil and doesn't actually contain any soil at all. This mix is made of peat moss or coco coir, perlite, vermiculite, and sometimes compost. It provides excellent drainage, makes it easy for sprouts to surface, and is sterile, so you don't have to worry about fungal diseases. Before filling the containers, you must first moisten your starting mix with water. To know if you have the ideal water to mix ratio, tightly squeeze a handful of mix. If water streams out, it's too wet. If no water comes out, it's too dry. If a few drops of water fall out, it's just right. Once your starting mix is properly moistened, fill your containers within a quarter-inch of the top and compress it so it's firmly packed and flat on top.
  4. Plant your seeds. Reference your seed packet for instructions on planting depth and spacing. If you've misplaced your seed packet, a rule of thumb is to bury a seed twice as deep as it is long. Once buried, tamp the soil firmly with the palm of your hand. Many types of tiny seeds—including snapdragon, petunia, and lettuce—require light to germinate, so you should leave these on the mix surface instead of burying.
  5. Cover your container. Enclose your seeds with a layer of plastic wrap or your seed starter tray's plastic dome cover to lock in the moisture and heat necessary for your seeds to germinate. It’s usually best to store your container in a warm location that receives indirect sunlight, but always check your seed packet for specific guidelines, as some seeds require total darkness to germinate. To speed up the germination process, try using a heat mat to warm the starting mix from the bottom.
  6. Water your seeds. Every day or so, check to see if the starting mix is still moist. If it appears dry, do not use a watering can because it might wash away the delicate seeds. Instead either use a spray bottle to spray a layer of mist over the mix surface or place your container in a larger tray of water so the mix absorbs water from below.
  7. Uncover your container once seeds germinate. The germination process usually takes around two weeks, but it can be shorter or longer depending on the type of seed. As soon as your seeds sprout, remove the cover from your container.
  8. Move seedlings into the sunlight. Most seedlings need at least six to eight hours of direct sunlight a day. Place your seedlings in a bright light location, such as a south-facing window sill. If you allowed your seeds to germinate in darkness, make sure to gradually move them into brighter light levels in stages so they're not shocked by the sudden change of environment. For homes without much natural light, or when sowing seeds in the darker winter months, place an artificial grow light six inches above the seedlings. Whether using natural or artificial light, rotate the seedlings regularly so they're evenly illuminated.
  9. Fertilize as needed. Typically, begin fertilizing four to six weeks after sowing. A seedling is ready for fertilizer once its second set of leaves are formed. These leaves are known as its true leaves and are an indicator that your seedling is preparing to mature. Fertilizing is unnecessary when using a seed starting mix made with compost, as compost already provides the necessary nutrients. Avoid fertilizing the week before you plan to start hardening off the seedlings outside.
  10. Thin overcrowded seedlings. Seedlings that grow too large to share the same container space need to move to their own container to avoid overcrowding. Before transplanting, make sure your seedling appears healthy enough to handle the move. To begin, fill the new container with moist, room-temperature potting mix. Then use a popsicle stick or a teaspoon to dig a hole in the potting soil large enough for the seedling's roots. Delicately use the same tool to tease the seedling out of the starting mix, doing your best to avoid its roots. Pick the seedling up by its uppermost leaves and drop it into the hole you dug into the potting soil. Gently press the potting soil back around the seedling's roots and lightly compress the soil.
  11. Harden off your seedlings. Hardening off refers to the process where indoor seedlings are gradually exposed to outdoor conditions like colder temperatures, wind, and direct sunlight so that they don't go into shock due to the jarring change in environment. Begin this process about 10 to 14 days before your transplant date by placing your seedlings in an outdoor spot protected from wind and sun for an hour a day. Every day, extend your seedlings' time spent outside by another hour and gradually expose them to more and more sunlight.
  12. Transplant your seedlings outdoors. Once the weather is ideal—generally right after the last frost of the season—and your seedlings have adjusted to the outdoors, it's time to transplant them to an outdoor garden bed or pot. Try to transplant when the sky is overcast, if possible. Reference your seed packet or a seed catalog to determine how much space in the garden each seedling needs to grow. When placing a seedling in its new home, carefully spread out its delicate roots without damaging them. Lastly, water the seedling to allow its roots to properly break into its new soil.
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Grow your own food with Ron Finley, the self-described "Gangster Gardener." Get the MasterClass All-Access Pass and learn how to cultivate fresh herbs and vegetables, keep your house plants alive, and use compost to make your community - and the world - a better place.

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