How to Start Seeds Indoors: The Complete Guide

Guidance on Watering, Lighting, and Other Growing Factors

seedlings receiving sunlight

The Spruce / K. Dave

Project Overview
  • Working Time: 30 mins - 1 hr
  • Total Time: 30 mins - 1 hr
  • Skill Level: Intermediate
  • Estimated Cost: $10 to $30

Gardening can be an expensive hobby if you purchase all your plants as potted nursery specimens. Fortunately, most vegetables and ornamental plants can be started from seeds, which offers a much less expensive way to populate your garden. Each type of plant has its own particular needs for starting seeds indoors. Seed depth, type of growing medium, and water and light exposure needs will all vary depending on the species. But the general process is the same for growing seedlings you can transplant into the outdoor garden.

There are a few basic steps to starting most types of seeds indoors before they turn into seedlings that are ready to be planted outdoors:

  1. Choose the right container to start off the seeds.
  2. Select the right spot with enough light for seeds to germinate and augment the space with grow lights and a heat mat if necessary.
  3. Keep seeds moist, but not too wet.
  4. Gently prepare seedlings for the outdoors by hardening them off if you plan to move them into your garden.

Now that you know the road ahead, here's exactly how to start seeds indoors, with pro tips every step of the way to ensure success. You'll be eating fruits and vegetables and enjoying flowers in no time!

Small containers with plant sprouts next to glove on marbled surface

The Spruce / Heidi Kolsky

When to Start Seeds Indoors

A package of seeds will usually announce if the plant should be started indoors, with instructions that include phrases such as "start indoors eight weeks before last expected frost date in your area." A simple internet search will tell you the date of the expected last frost in your area. Count backwards to, for example, eight weeks before that, and that's the date you should start your seeds.

2:46

Everything You Need to Know About Starting an Edible Seed Garden

Reading a Seed Packet

The printed instructions on the back of a seed package will give you a lot of information on how (and if) you should start the seeds indoors. These elements are among the most important information to look for:

  • Planting time: Most seed packets will tell you quite clearly if the seeds can or should be started indoors. For some species (tomatoes, for example), it is virtually mandatory to start seeds indoors in cold-weather climates. For other species it may be optional, and for other fast-growing species, there may be no indoor starting information at all—these plants are best planted directly in the outdoor garden.
  • Days to maturity: This will tell you how long the plants take to produce edible fruit or ornamental flowers. Some tomato plants take as much as 100 days to reach fruit-producing maturity. If you want tomatoes in July, this means the seeds need to be started in early April.
  • Light and water needs: The seed package will tell you if the seeds need lots of light. If so, starting them indoors may require a fluorescent grow light—or you may need to reserve your sunniest window for seed-starting.
  • Soil needs: Some seeds can be started in ordinary potting soil, while others require a porous, fine-grained seed-starting mix. The package may also suggest an optimal soil temperature for seeds to germinate. Seeds that require 70-degree soil to germinate will clearly need to be started indoors in cold-weather climates since the soil does not get adequately warm until late into May.

The seed package will also give a wealth of other information, such as days to germination, fertilizing needs, planting depth, and transplanting techniques.

Before Getting Started

There are many good commercial potting mixes available that are suitable for starting seeds. Although they may be called "potting soil," they actually contain no garden soil at all. Instead, they are a soilless mix containing materials such as peat moss, perlite, vermiculite, compost, and more. This ordinary potting mix is fine for starting many seeds.

Some seeds—especially those that are small—may do better in what is known as a seed-starting mix. Seed-starting mix is a special form of soilless potting mix that is especially porous and fine-grained. It omits the organic materials found in standard potting soil. This is because seeds do not require the nutrients provided by organic material to germinate and sprout.

For many plants, a seed-starting mix is the best choice, because the organic material in a standard potting mix can lead to fungal problems. Avoid starting your seeds in outdoor garden soil, which can become compacted. And outdoor soil often contains weed seeds and disease pathogens that interfere with seeds germinating and sprouting.

What You'll Need

Equipment / Tools

  • Marker
  • Grow light

Materials

  • Planting trays and small containers
  • Seeds
  • Seed-starting mix or potting mix
  • Labels
  • Plastic bags or tray covers

Instructions

Materials and tools to start seed indoors on marbled surface

The Spruce / Heidi Kolsky

How to Start Seeds Indoors

  1. Prepare the Growing Medium

    Loosen and dampen the potting mix before you put it into seed-starting trays or individual 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.

    Growing medium for seeds held by hand over large container

    The Spruce / Heidi Kolsky

  2. Fill the Containers

    Use the pre-dampened potting mix to fill your chosen seed-starting trays or containers about two-thirds full. Tap the container on the tabletop to help the potting mix settle.

    Gently firm the top of the mix with your hand or a small board. Don't pack the potting mix tightly into the container—you want it to remain fluffy and aerated. 

    Seed starting mix added to small plant pots

    The Spruce / Heidi Kolsky

    Tip

    A seed starting tray is the easiest way to start seeds indoors for beginners and expert gardeners. These cells are ideal since they are usually made of a porous material that allows for water absorption and roots, when big enough, to grow right through the walls. If you need a quick alternative, use an egg carton and poke a small drainage hole at the bottom of each cell in the carton, or you can use empty toilet paper rolls.

  3. Plant the Seeds

    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. Some seeds need complete darkness to germinate and others require light to germinate.

    Proper planting depth is usually provided on the seed packet. If there is no information on the packet, the rule of thumb is to plant seeds two to three times as deep as they are wide. Determining depth can be a challenge, but if you are not sure, err on the shallow side.

    Seeds added in middle of small pots for planting

    The Spruce / Heidi Kolsky

  4. Finish Planting

    Cover the seeds with 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 seeds, the less you need to cover them. There are a few seeds, such as lettuce, which require light to germinate and should barely be covered with potting mix.

    Seed starting mix added to tops of pots covering seeds

    The Spruce / Heidi Kolsky

    Tip

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

  5. Label the Seeds

    To be able to identify seedlings as they grow and to know when they will be ready for transplanting, you should label the seed containers as you are sowing. For every type of seed sown, use popsicle sticks or plastic plant markers and permanent ink pens to record the plant name and date sown. Insert the plant labels into the soil near the edge of the container or tray.

  6. Keep Seeds Warm and Humid

    The hardest part of starting seeds indoors is providing the optimal temperature, light, and humidity levels for them to germinate and sprout into seedlings.

    Start by covering the trays or containers with clear plastic. This can be provided by rigid plastic domes or covers, as is included with commercial seed-starting trays, or with clear plastic bags or cling wrap if you are using repurposed containers for starting your seeds. The plastic covering serves to hold in heat and moisture. (If your seed labels are too tall for the cover, remove the label and set it on top of the cover directly above its corresponding cell.)

    Move the container to a warm, draft-free spot where you can check it daily. Most seeds germinate best when the temperature is between 65 and 70 degrees Fahrenheit, but check the information on the seed packet for specifics. 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.

    Seed containers covered with plastic to control environment

    The Spruce / Heidi Kolsky

  7. Monitor Seedling Growth

    Remove the plastic as soon as you see a seedling emerging and move the containers into indirect light. In general, seeds will not need light until they emerge.

    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 becomes capable of photosynthesis. This is the point at which you should move your seedlings under a light source.

    Small sprout growing from seed starting tray

    The Spruce / Heidi Kolsky

  8. Add Light

    Seedlings need a lot of light to grow into sturdy, healthy plants. Chances are that you do not have enough natural light in your home to grow robust seedlings. Even a south-facing window usually will not do the job. You can, however, use artificial light (i.e., grow lights) to achieve the right amount of light required by seedlings.

    Keep the lights as close to the seedlings as possible without touching them (2 to 3 inches). When seedlings first appear, keep the lights turned on for 12 to 16 hours per day. To reduce your hands-on time, use a timer to turn the lights on and off automatically.

  9. Keep Seedlings Moist

    You must keep the sterile seed-starting medium damp but not wet. Check soil moisture at least once a day to make sure the soil hasn't dried out. Improper conditions can lead to damping off disease, a fungal disease that quickly kills seedlings. You can minimize the chances for damping-off disease by watering the containers from below, and by providing good air circulation once the seedlings have sprouted.

    Watering from the bottom enables the seedlings to soak up water through the container's drainage holes or porous walls. Set the seed starting trays on a dish and add a thin layer of water to the bottom of the dish multiple times for 10 to 30 minutes. Use your finger to touch the top of the soil to ensure that moisture has reached the top of the container, then move the seed trays out of the water-filled dish and back under the light.

    Alternatively, you could buy a self-watering, seed-starting system.

    Fertilizer added to sprouts in seeding trays for growth

    The Spruce / Heidi Kolsky

  10. Begin Feeding

    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. Excessive fertilizer will overwhelm the seedlings, so use a water-soluble fertilizer diluted to one-half the normal strength. The seedlings should be lightly fed every two weeks.

  11. Pot Up If Necessary

    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. Pots of 3 to 4 inches are good sizes to pot up to, allowing plenty of room for root growth. When you do this, you can use potting soil in the larger pot, instead of a sterile seed-starting mix.

    If more than one seedling is growing in the same cell, cut off all but the strongest seedling. (This is called "culling.") Don't try to pull out the extra seedlings, since this may damage the roots of the remaining seedling.

    making sure to label seedlngs

    The Spruce / K. Dave

  12. Harden Off the Seedlings

    By the time the temperature warms outside, you should have stocky, healthy young plants. When your seedlings are large enough to plant outdoors, you need to prepare them for the transition by hardening off. Hardening off gradually prepares them for outdoor conditions like wind, rain, and sun.

    Move the plants to a shady, sheltered outdoor spot for increasing lengths of time each day, over a period of seven to 14 days. Gradually increase the amount of outdoor time, and introduce direct sunlight as they grow accustomed to outdoor conditions. At the start of this period, you will bring your seedlings indoors or cover them at night if the temperature looks like it will dip overnight. By the end of the hardening off period, you can leave them outdoors all night, uncovered, so long as the overnight temperature doesn't dip below about 50 degrees Fahrenheit.

    Once they can comfortably thrive outdoors through the night, your seedlings are ready to transplant into the garden or into permanent outdoor containers. Water your seedlings well before and after transplanting. Try not to transplant during the hottest, sunniest part of the day.

    Seed trays with small sprouts hardened outside before transplanting

    The Spruce / Heidi Kolsky

too many seeds in a planting tray

The Spruce / K. Dave

FAQ
  • How many seeds should I sow?

    Sow 3 to 5 seeds per cell in the seed starting container. If the seeds are large, just sow 2 to 3. Ultimately you will be culling the weaker seedlings and will have maximum one per cell. Beginners should be careful not to get too ambitious. If you sow more seeds than you can reasonably maintain, it will become challenging to nurture the seedlings into adulthood.

  • How do you plant seeds that need light to germinate?

    For seeds that need light to germinate, make sure the seeds are in contact with the seed starting medium but are not covered. To do this, gently press the soil medium to create a firm surface. Then, place the seed on top of the medium and gently press down, making sure the seed is still exposed.

  • When should you start seeds indoors?

    Start seeds six to eight weeks before the last frost in your area. Most plants are ready to go outside four to six weeks after you start the seeds.

  • Does seed starter need fertilizer in it?

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

watering seedlings

The Spruce / K. Dave

2:04

Watch Now: Mistakes to Avoid When Growing Seeds Indoors

Article Sources
The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Seedling Care. University of Maryland Extension.

  2. Indoor seed starting 101. Seed Savers Exchange.