How to Successfully Start Seed Indoors

  • 01 of 10

    Gather Your Supplies

    A little girl lifting mulch with a spade
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    Here are the basics of what you'll need to get your garden seeds started growing indoors:

    • Containers: Either purchased pots or flats or containers you've saved, like egg cartons and yogurt cups. Used pots should be cleaned and disinfected by soaking in 1 part bleach to 10 parts water.
    • Potting mix: Seeds do best in a soilless mix where there are fewer inherent problems than with garden soil.
    • Seeds: These can be up to you. They can be for plants, vegetables, fruits, etc. 
    • Labels and markers: Do this when you plant your seeds, because you won't remember what's what.
    • Plastic bags or covers: These will trap warmth and humidity when the seeds require damp soil.
    • Water
    • Light source: If you don't have a bright window, you will need some kind of florescent or high-density plant light.
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  • 02 of 10

    Preparing the Potting Mix

    Two hands grinding soil between their fingers
    The Spruce / Marie Iannotti

    There are many good potting mixes available. Using a soilless potting mix rather than outdoor soil is preferable because potting mixes don't readily compact, don't contain weed seeds and don't have disease spores and other possible problems.

    The following steps will help get your potting mix ready for planting: 

    • Loosen and dampen the potting mix before you put it into your seed-starting containers. This process helps to achieve a uniform level of moisture.
    • Dampen the mix to the consistency of a wrung-out sponge. It should be wet, but not dripping, with no dry lumps.

    Since new seedlings don't require fertilizer until they sprout their first true leaves, you don't really need a mix that has additional fertilizer.

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  • 03 of 10

    Filling the Containers

    Hands filling seedling pots with potting mix
    The Spruce / Marie Iannotti

    Use the pre-dampened potting mix to fill your seed starting containers:

    • Fill about two thirds full and tap the container on the tabletop, to help the potting mix settle.
    • Gently firm the top with your hand or small board.
    • Don't pack the potting mix tightly into the container—you want it to remain fluffy and aerated. 
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  • 04 of 10

    Planting the Seeds

    Hands putting seed into seedling pots
    The Spruce / Marie Iannotti

    Once you have your containers prepared, you can begin planting the seeds:

    • Make sure you read the seed package for special instructions. Some seeds may require a period of pre-chilling or soaking, and some seeds need exposure to light, to germinate.
    • Small seeds can be sprinkled on top of the potting mix. Larger seeds can be counted out and planted individually.
    • Use at least 3 seeds per container, since not all seeds will germinate and not all that do germinate will survive. You can thin extras later.
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  • 05 of 10

    Finish Planting

    Two hands putting seeds in seedling pots
    The Spruce / Marie Iannotti

    Here are some final touches to your planting:

    • Cover the seeds with some more dampened potting mix and then gently firm again.
    • Re-check your seed packet for information on how much potting mix should go on top of the seeds. Generally, the smaller the seed, the less you need to cover them.
    • There are a few seeds, like lettuce, which require light to germinate and should barely be covered with potting mix.
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  • 06 of 10

    Watering Newly Planted Seeds

    Seeds and seed mix in seed pots being watered
    The Spruce / Marie Iannotti

    Although the potting mix was pre-dampened, it is still a good idea to sprinkle some additional water on top of the newly planted seed. This insures that the top layer of mix won't dry out and it also helps to firm the potting mix and insure good contact between the seed the mix.

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  • 07 of 10

    Creating the Right Atmosphere for Your Seeds

    Seedpot covered with a plastic bag to simulate a greenhouse
    The Spruce / Marie Iannotti

    Here's how you can create the perfect atmosphere for your seeds:

    • G​reenhouse fffect: Your seeds are now ready to be covered loosely with some type of plastic. This will help hold in both heat and moisture. You can place the whole container into a plastic bag or simply lay a sheet of plastic over the container. If you have special seed starting trays with plastic covers, use those.
    • Heat: Move your container to a warm, draft-free spot and check it daily. Most seeds germinate best when the temperature is between 65 and 70 degrees F. The top of a refrigerator is an ideal spot, or you could consider purchasing heating mats specially made for germinating the seeds. Heating mats go under the potting containers and heat the soil from below. You will usually need to water more frequently when using heating mats. Caution: Only use heating mats certified for seed starting use.
    • Light and air: In general, seeds will not need light until they emerge. They will need air circulation under the plastic, or you will be encouraging mold.
    • Signs of life: Remove the plastic as soon as you see a seedling emerging and move the plant into indirect light. Be sure the potting mix stays moist, but not wet.
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  • 08 of 10

    How to Handle Emerging Seedlings

    Seedlings emerging from their seedpots
    The Spruce / Marie Iannotti

    Once your seedlings begin poking through the soil, they will start to straighten up and unfurl. What looks like two leaves will appear. These are leaf-like structures, called cotyledons, that are part of the seed and serve as food sources until true leaves are formed, and the plant is capable of photosynthesis. At this point, you should move your seedlings under a light source.

    Your seedlings will need between 12 to 18 hours of light each day. This may seem extreme, but artificial light and even the low rays of the winter sun are not as intense as the full summer sun. The best way to ensure regular, long doses of light is to attach your fluorescent or high-intensity plant lights to an automatic timer.

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  • 09 of 10

    What to Do When True Leaves Appear

    The first true leaves on a tomato seedling
    The Spruce / Marie Iannotti

    As the seedling grows, the cotyledons will wither and the first "true" leaves will form. This is when your seedling begins actively photosynthesizing. Since it is growing in a soilless mix, you will need to give it some supplemental feeding at this point. Use a balanced fertilizer or one high in nitrogen and potassium, to encourage good roots and healthy growth.

    Seedlings can remain in their original containers until you are ready to plant them in their permanent spots. However, it's common to move the seedlings into a larger pot once several sets of leaves have formed and the seedling is a couple of inches tall. This is called "potting up," and it allows the roots more room to develop. Three to four-inch pots are good sizes to pot up to, allowing plenty of room for root growth.

    If more than one seedling is growing in the same pot, either separate the seedlings into individual pots or cut off all but the strongest seedling. Don't try to pull out the extra seedlings, since this might hurt the roots of the seedling you are keeping.

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  • 10 of 10

    Hardening Off

    Potted plants arranged in front of a garden
    The Spruce / Marie Iannotti

    By the time the temperature warms outside, you should have stocky, healthy young plants. Before moving them out into the garden, take a week or two to gradually introduce them to their new growing conditions. This is called hardening off. It gives the plants a chance to acclimate to sunlight, drying winds and climate changes.

    • Move the plants to a shady spot for increasing amounts of time, several days in a row.
    • Bring them in or cover them if the temperature looks like it will dip.
    • Gradually increase the amount of time they spend outside and the amount of sunlight they receive until you see that they are growing strong and appear ready to go out on their own.
    • Water your seedlings well before and after transplanting. Try not to transplant during the hottest, sunniest part of the day.