How to Successfully Start Seed Indoors

Care and How To

For many home gardeners, it's fun to get a head start on the upcoming garden season by starting seedlings indoors. Growing quality seedlings indoors requires high-quality seeds, a germination medium/potting mix, containers, proper temperature and moisture conditions, and adequate light.


When to Sow  |  Soil & Containers  |  Sowing Seed  |  Temperature  |  Watering & Humidity  |  Light  |  Transplanting  |  Hardening & Planting  |  More Information


sowing seeds photo by Adobe StockA seed is a miracle waiting to happen. The embryo comes pre-packaged with a food supply and the vital genetic information needed to become a plant, much like its parents.

Seeds exist in a state of dormancy, absorbing oxygen, giving off carbon dioxide, and slowly using up their stored food reserves. During this process, the seed continually monitors the external environment, waiting for ideal conditions specific to the particular seed. Once the ideal conditions occur, the seed breaks dormancy and germinates. The seedling gathers energy through its leaves through photosynthesis and absorbs nutrients and water from the soil through the roots.

As gardeners, the goal is to provide the optimal environment for germination and seedling growth.

When to Sow Seed

Resist the urge to start seedlings too early in the season. It is easy to get excited for spring, but starting seed too early leads to challenges later.  Seedlings planted too early get large and lanky by the time they can be planted outside, and lanky seedlings do not transplant well.  Additionally, the plants that grow from spindly or lanky seedlings will not be as high-quality as those started from smaller, stockier transplants. 

The crop time (number of weeks from sowing to planting outdoors)  is printed on the seed packet.  You can also get the crop time for several popular flowers and vegetables in this article: Germination Requirements for Annuals and Vegetables.  

Use this number to determine when to sow seeds indoors.  If you intend to plant outdoors on May 15th, the sowing date indoors would be the number of weeks listed in this column before May 15th.  Most annual and vegetable transplants are planted in the garden after the threat of frost has passed for the summer.  For much of Iowa, this is the last week in April or the first week in May.  Find your frost-free date in this article: Frost Dates in Iowa

Some cool-season vegetables and annuals like broccoli, cabbage, or pansy, tolerate light freezing temperatures and can be planted 2 to 4 weeks before the average last frost date.

Soil and Containers

The germination medium should be lightweight, porous, and free of disease pathogens. Excellent germination media are commercially prepared soilless products, such as Jiffy Mix or Redi-Earth. When transplanting seedlings into individual pots or cell packs, use a high-quality, well-drained potting mix.

Various containers can be used to germinate and grow transplants. Gardeners can purchase flats, trays, cell packs, pots, compressed peat pellets, and other commercial products. Cut-off milk cartons or plastic jugs, paper cups, and egg cartons can also be used to start seeds.

Previously used flats, trays, and pots should be cleaned and disinfected before use. Wash the containers in soapy water, then disinfect them in a solution of one part chlorine bleach and nine parts water. Holes should be punched in the bottom of milk cartons, jugs, paper cups, and similar containers to allow for drainage.

Sowing Seed

The size of the seeds and their germination requirements largely determine the type of container and sowing method. Fine seeds, such as begonias and petunias, are typically sown in flats or trays. After germination, the seedlings are transplanted into individual containers. Some seed companies offer coated or pelleted seeds for ease of handling and planting, especially for species with fine or tiny seeds.

Large seeds, such as marigolds and tomatoes, are commonly germinated in flats. However, they can also be sown directly into individual containers, eliminating the need to transplant the seedlings. 

When sowing seeds in flats or trays, fill the container with the germination medium within 1 inch of the top. Firm the medium and water thoroughly, then allow it to drain. Fine seeds and those that require light for germination are sown on the surface of the medium and then lightly pressed into the germination medium. Cover all other seeds with additional media to a thickness of one to two times the seed's diameter.

After sowing the seeds, water the medium by partially submerging the container in water. When the surface becomes wet, remove the container from the water and allow it to drain.

Watering from below prevents the washing of seeds on the surface of the medium. The medium can also be moistened with a rubber bulb sprinkler. The fine mist from the rubber bulb sprinkler will not disturb the seeds or the medium.

When sowing seeds into individual containers, plant two or three seeds per container (peat pots, pellets, soil blocks, etc.). Place the containers in a flat and water.

Temperature

Seeds require a certain temperature to germinate. Each plant has a specific optimum and a range within which germination will occur. The closer the temperature is to the optimum, the quicker germination will occur.

Most seeds germinate when the soil temperature is between 68° and 86°F. Once germination occurs, the optimum growing temperature for the seedling is about 10°F cooler than the optimum germination temperature.

Watering and Humidity

Moisture is critical for germinating seeds. They like a moist but not soggy environment. Seeds require oxygen and, if kept waterlogged, may rot. On the other hand, if the soil dries out, the seed will lose whatever water it has absorbed and die.

Finding the middle ground can be difficult and comes easier with practice. After sowing the seeds, mist the tray with water and cover it with plastic wrap, a plastic bag, glass, or plexiglass to seal in moisture.

As soon as the seed germinates, remove the covering. Ventilation and air circulation are also important to discourage damping off diseases. 

Check the seedlings frequently (as much as daily) for moisture. Thoroughly water the seedlings when the soil surface becomes dry to the touch.

If using a commercial potting mix containing a slow-release fertilizer, fertilization should not be necessary. Applying a dilute fertilizer solution once every two weeks should be sufficient for those potting mixes that don't contain a slow-release fertilizer. 

Light

Some seeds need light to germinate, but many do not. Seed packages will usually indicate what your particular selection requires.

It is important to follow the directions given on the package for planting depth. Seeds planted too deep may not receive the light they need to germinate (if applicable), will not have enough stored energy to reach the soil surface, and may die in the process.

After germination occurs, seedlings require about 12 to 16 hours of light a day. Intense light is necessary to prevent spindly or leggy seedlings. If you are growing under lights, ensure the light source is 4 to 6 inches above the plants. In a sunny window, turn the seedlings regularly to avoid leaning.

Learn more about providing supplemental light for seedlings indoors in this article: Growing Indoor Plants Under Supplemental Lights.

Transplanting

If you are sowing seeds in furrows or flats, transplant individual seedlings into cell packs when the first true leaves appear or when they are large enough to handle.

Seedlings started indoors should be fertilized regularly with a dilute (1/4 strength) water-soluble fertilizer. This will help to produce stockier transplants, provided enough light is available.

Hardening and Planting

Before planting in the garden, harden seedlings by gradually acclimating the transplants to the outdoors. Start by putting them outside on cloudy days or in a shaded location, then after a few days, work them into more light and exposure.

The best time to plant in the garden is overcast skies or late afternoon. Water immediately after transplanting. If plants wilt, provide protection with an open milk carton or row cover for a few days.


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Last Reviewed: 
December, 2023