Step-by-Step Guide to Starting Seeds Indoors

By Melinda Myers - horticulturist and gardening expert
February 11, 2023

Extend the gardening season, save money, and increase your plant selection by starting your plants from seeds indoors. Don’t be intimidated by the thought of trying something new or fear of failure. Starting plants from seeds is a great way for new and experienced gardeners to keep gardening even when it’s too early to garden outdoors. 

What Seeds to Start Indoors

Start by perusing the seed racks at your favorite garden center or check out the many seed catalogs, most of which are now online. Consider purchasing seeds of your favorite plants, especially those you typically have to purchase as transplants.  These are often the plants that require a longer growing season than yours to begin flowering or producing fruit.

You’ll also want to check for newer varieties, heirlooms, or other plants that aren’t readily available at the garden center. Starting these from seeds may be the only option for adding them to your garden.

Boost your confidence by growing a few seeds that sprout quickly and tend to be easy to grow like zinnias and marigolds. Tomatoes are a favorite vegetable to grow in gardens and containers and are fairly easy to start from seeds indoors.

Small seeds like begonias or slow-growing plants like geraniums require more indoor growing time and patience. Others require special treatment for the seeds to complete or break dormancy and sprout. These are great plants for those who want to stretch their skills.  If you are new to seed starting you may want to save the more challenging seeds for when you gain more confidence and experience.

When to Start Seeds Indoors

Your state’s University Cooperative Extension Service likely provides information on the best plants to grow in your area. They often provide charts with the best time to start seeds indoors or directly in the garden.

You can also find recommended indoor and outdoor planting dates for your region on the back of most seed packets or in the catalog descriptions. Many of these sources also provide any special planting directions you need to know.

The planting dates are guidelines most often based on the average last spring frost and the number of days needed for the plant to reach full size. Planting earlier than recommended means the plants will spend more time growing indoors. This often results in seedlings with tall, leggy, and weak stems. Starting too late can result in the plants not having enough time to flower or fruit before the end of your growing season.

Calculate the number of frost-free dates in your area to help you decide when it is too late to start seeds indoors. Find the average dates of the last spring frost and first fall frost in your area. The number of days between these two dates is the average length of the frost-free growing season in your area.

Compare this number to the number of days from planting to harvest or flowering found on the seed packet or in the catalog descriptions.  If the plant needs more than the available growing days you need to grow a variety that requires fewer days to reach maturity or use cold frames or row covers to extend the harvest season. Otherwise, wait until next year to start these seeds indoors at the proper time.

How to Start Seeds Indoors 

• Clear a table, shelves or other space for starting and growing your seedlings. A warm spot will help speed seed sprouting while a bright sunny window or better yet artificial lights are necessary as soon as the seeds sprout.
• Gather or purchase supplies needed for starting seeds indoors. Invest in a sterile seed starting or quality potting mix. These should be lightweight and contain peat moss, coir, or compost to hold moisture and perlite, vermiculite or rice hulls for drainage.
• Purchase, clean, and reuse pots, or recycled yogurt cups, fast food containers or other items into seed starting containers. You may opt for biodegradable peat or cow pots or compressed peat pellets for starting seeds. The seedlings can be left in these biodegradable containers when planting them in the garden. You can also make pots from newsprint or other paper.  Just roll and secure the sides and fold one end to create the bottom.

• Clean used containers to avoid disease problems and increase your growing success. Disinfect used flats and pots by soaking them in a one-part bleach and nine-part water solution for ten minutes. Rinse thoroughly with clear water before planting.
• Fill the containers with a sterile seed starter or potting mix. Follow planting directions on the seed packet for proper planting depth. Plant two seeds per individual container. This way if one seed fails to sprout or grows poorly you have one good plant left in each pot.
• You will plant more seeds when sprouting them in flats or other large containers. Flats are long shallow containers used for starting seeds or holding smaller seed starting containers. Plant seeds in rows or scatter them throughout the larger container.  The seedlings will eventually be lifted, divided and each moved into their own pots.

• Once planted, gently water to moisten the potting mix without washing away the seeds. This step also removes air pockets and insures seed-to-soil contact.  You can cover the seeded containers with plastic to help keep the soil moist and extend the time between watering.
• Starting seeds in compressed peat pellets is a bit different. Soak these in warm water and watch them expand. It’s actually pretty cool. Once the pellets are full-size, you can plant your seeds.
• Place the planted containers in a warm location to speed sprouting (germination). Most seeds need warmth and moisture not light to sprout. Water often enough to keep the potting mix evenly moist.
• Be patient as it can take anywhere from one to three weeks or more for seeds to germinate. Times vary based on soil temperature and the specific plant you are growing. Check the seed packet for the average time between planting and sprouting.

• Move your containers, if needed, to a sunny location or under artificial lights as soon as any green appears. Regularly turn pots growing in the window to encourage even growth. Since most of us do not have sufficient light to grow healthy stout seedlings adding artificial lights can help. Keep grow light 4 to 6 inches above the top of the seedlings. This means you will be raising the lights, or lowering the plants as the seedlings grow.
• Thin seedlings planted in containers or move seedlings to individual containers if started in flats. Wait for the seedlings to develop two sets of true leaves. These are the leaves that look like those of the plant you are growing. Once the plant has two sets of true leaves prune out the weaker of the two seedlings that were started in individual pots.
• Move strong, and healthy seedlings started in flats or other larger containers to their own pot. Gently lift the seedling out with a fork or similar utensil to minimize root disturbance and avoid damaging the stem.
• Continue watering often enough to keep the soil evenly moist but not soggy. Once the thinned and transplanted seedlings start growing, you can start fertilizing. Use a dilute solution of a flowering houseplant fertilizer every two weeks.
• Check the seed packet or your local Extension service publications for the best time to move your plants into the garden. Timing varies with weather and the plant's tolerance to cool air and soil temperatures.

• Prepare your plants for their outdoor home. Stop fertilizing and allow the soil to go slightly drier before watering again. Set the plants in the shade or other sheltered location. Move them into the sun for an hour the first day, two hours the next and so on until they have adjusted to the sunlight. A wagon or snow sled makes this job easier. Cover or move your plants indoors if frost is in the forecast.
• In the meantime, prepare your soil for planting. Incorporate Milorganite low nitrogen, slow-release fertilizer into the soil before planting. It won’t harm young seedlings or any of your plants when the weather turns hot and dry. Plus, the 85% organic matter feeds the soil as well as the plants.

Once the soil is prepared, your plants are conditioned, and the weather is right start moving your plants to their outdoor location. Water plants thoroughly at planting and often enough to keep the soil surrounding the roots moist but not soggy wet.  Gradually extend the time between watering to encourage deep more robust roots that are drought tolerant. Then enjoy the next phase of the growth of the plants you started from seed.

Make this the year you start a few or many more plants from seeds indoors. Just like growing houseplants and tending gardens outdoors, you will find it satisfying and fun.