Guide to Starting Seeds Indoors

1. Keep everything clean: Wash work surfaces, seeding and watering tools, and any reused trays, domes, flats or pots before you start. Keep supplies and tools off the ground and stored somewhere clean between uses. In our trial greenhouses we also mix in RootShield when filling every tray, as a preventive against damping-off, root rot and Phytophthora. Learn more about damping-off.
2. Use a mix made for seed starting: A good seed-starting medium soaks up water but does not pack down, and it lets air reach the roots. Regular potting soil or garden soil is usually too dense, and garden soil or reused mix can carry diseases that attack seedlings.
   
   Germination and seed-starting mixes usually combine compost or other organic matter with light, porous materials that improve aeration and water holding. Common ingredients are perlite (expanded volcanic rock that looks like tiny white foam beads), vermiculite (a mineral expanded by heating), peat moss and coir (coconut husk fiber).
   • Germination mix is typically very light and works well for small seeds that will be potted up (see below) at least once before going outside.
   • Seed-starting mix generally works well for larger seeds.
3. Choose shallow containers or flats: Be sure containers drain. Skip deep pots, but for crops that resent root disturbance, such as cucurbits, pick a cell or pot big enough to hold the plant until it goes into the field. Instead of plastic trays you can also sow into biodegradable pots or soil blocks. Soil blocks help prevent root-bound plants and lower the chance of transplant shock.
4. Moisten the mix properly before sowing: Add enough water that a squeezed handful keeps its shape without dripping. Too much water can rot seeds before they sprout, while dry spots prevent good contact between seed and mix. How much water you need to add, if any, depends on the product. In our experience, our 512 Mix usually needs no added water, while we moisten Fort Light Potting Mix.
5. Sow seeds at the spacing recommended for the variety: Crowded seedlings are more prone to disease.
6. Firm the seeds into the mix and cover them: Seeds need good contact with the mix to germinate. Firm them in, then cover to the depth given on the seed packet; sowing deeper than recommended can reduce germination. For crops that need light to germinate, we cover seed with vermiculite, which lets light through while helping hold moisture. We also cover very small seeds with vermiculite unless they need darkness. Either way, the vermiculite keeps the surface from drying out and helps keep moisture even.
7. Water in carefully: Don't wash the seeds out of place. Keep them evenly moist, but not soggy, until they sprout. Water gently by hand or with a mister; misting works best for tiny seeds. You can also set each tray inside a tray without holes and water from below. Use plain lukewarm water, around 68–77°F (20–25°C), and hold off on fertilizer until seedlings emerge, because fertilizer salts can make it harder for seeds to absorb water and start growing roots.

Humidity domes in several styles are available for seed starting and grafting.
A well-fitting dome keeps moisture and temperature steady and improves germination. Take it off as soon as seeds sprout.


8. Cover the tray: A humidity dome, a sheet of clear plastic or glass, plastic wrap or an upside-down tray will hold in moisture, warmth and humidity. Vent the cover in warm, sunny weather so it doesn't overheat and hold back germination, and take it off once seeds sprout.
9. Keep the mix at the right temperature for germination: Look up the recommended temperature for each crop and adjust as needed. A heat mat can warm the mix, and a soil probe thermometer helps you keep track. Germination usually calls for warmer conditions than young seedlings need, so lower the temperature once seedlings are up. Some crops also do best with warmer days and cooler nights.
10. Feed lightly, and only when needed: Some mixes hold enough nutrients to carry seedlings for their first few weeks, and fertile mixes such as 512 Mix can carry many crops a good part of the way to transplanting. With low-nutrient mixes, begin feeding once seedlings are up and have their first true leaves, using a diluted fertilizer as the label directs.
11. Use grow lights when starting seeds indoors: A window alone does not give enough light.

Handle the fragile roots gently when potting up.
A widger is a stainless steel tool with a curved blade, handy for lifting small seedlings out of cells, separating them and making planting holes when potting up.


12. Pot up as each crop requires:
    Potting up (also called bumping up or potting on) means moving seedlings into bigger containers before they go into the field. It helps in several ways:
    
    
    1. Roots get more space to grow.
    2. You can switch to a richer mix, since bigger plants need more nutrients and may have used up what was in the starting mix.
    3. You can weed out weak seedlings.
    4. Seedlings that stretched soon after sprouting can be planted deeper. Tomatoes benefit greatly: a leggy stem buried when potting up grows roots along its length, making a stronger, healthier plant.
    When the first true leaves appear, lift seedlings out by holding a leaf, never the stem, and avoid harming the roots. Use a widger to ease seedlings from their cells, separate them and make a hole in the bigger container. Set them a little deeper than before, firm the mix gently around the stem, and water in with lukewarm water.

Germination depends heavily on soil temperature.

Look up the best germination temperature on each product page and adjust conditions to match.

When germination is poor, look at these four things.

1. Soil temperature: Temperature has a major effect on germination, and the best range depends on the crop. Lettuce and spinach like it cool, for example, while peppers and tomatoes like it warm.
   
   A soil thermometer is a must for checking the temperature of your mix; without one you can only guess whether it is right for germination.
   
   You may have to change conditions to reach the right temperature, for instance with a heat mat to warm the mix from below. In hot weather, cool-season crops may need to sit in shade or a cooler spot until they sprout. If you use a heat mat, know its range: some only warm the mix roughly 10–20°F (about 6–11°C) above room temperature, while thermostat-controlled mats can reach much higher temperatures even in a cool room.

Mold caused by too much water.


2. Moisture. Uneven or insufficient watering is a frequent cause of poor germination. Water often enough to keep the mix evenly moist but not saturated. Seeds need steady moisture, yet too much water encourages rot and mold.
3. Days to germination. Some crops are slow and uneven to sprout, so a few seeds come up well ahead of others in the same tray under the same conditions, while other crops sprout quickly and all at once.
   
   Germination times are listed in our Key Growing Information for Herbs and for Flowers. Slow germinators include angelica, asparagus, columbine, and parsley.
4. Light needs for germination. Most seeds germinate without light, but a few, such ascolumbineand both summer savory and winter savoryneed light to sprout. See our vermiculite tips above for these crops.

Natural variation in germination: each row in this tray is a different eggplant variety, all sown the same day.

1. Damping-off refers to the sudden collapse of young seedlings caused by several fungi and fungus-like organisms, most often Rhizoctonia, Fusarium, and Pythium. These organisms invade roots or stems at or near the soil surface, causing rot and a pinched look until the seedling falls over and dies. It is especially troublesome in humid conditions, and since treatment rarely works well, prevention matters most. Several practices covered above help lower the risk:
   • Keep seeding areas and tools clean.
   • Use a mix made for seed starting.
   • Sow at the recommended density; crowded trays are more vulnerable.
   • Take off domes and covers as soon as seeds sprout.
   • Give enough light and nutrients, and keep temperatures suited to the crop.
   Overall, aim for strong, healthy seedlings, since they resist disease better than weak, leggy ones. Other preventive steps include:
   • After germination, let the mix dry slightly between waterings, because constantly soggy mix invites disease.
   • Keep air moving with good ventilation, such as small fans over the seedlings.
   • A biological product like RootShield can be mixed into the germination medium or watered in after sowing. Its beneficial fungi protect seedlings, so it works as prevention, not a cure, and must be applied at sowing time. In our trial greenhouses we add it to every tray when filling, at the label rate.
   Growers often ask why damping-off appeared when it never did before, or why it hit one tray but not the next. There is no single answer, but disease always depends on three factors together, known as the disease triangle: the pathogen must be present, conditions must favor it, and the plant must be susceptible. For more on damping-off, including photos of affected seedlings, see:
   • Hudelson, B. 2004 Damping Off. UW-Madison Plant Pathology.
   • Grabowski, M. 2018. How to Prevent Seedling Damping Off. University of Minnesota Extension.

Healthy (left) and stretched seedlings (right).



2. Stretched, leggy seedlings: After sprouting, seedlings need plenty of light to grow well. Even a sunny window rarely gives enough direct light for sturdy vegetable, herb or flower seedlings. Etiolation is the clearest sign of too little light: seedlings with small, pale leaves (from lack of chlorophyll) and thin, weak stems stretching toward the light.
   
   Unless you grow in a greenhouse, you will probably need grow lights . Pick a full-spectrum fixture made for plants and keep it close, about 3–6" above the seedlings. Most vegetable, herb and flower seedlings need roughly 14–16 hours of light a day.
   
   To learn more, read our Guide to Choosing a Grow Light.


3. Hardening off and transplanting: Before going into the garden, seedlings have to adjust gradually to outdoor conditions, a step called hardening off.
   
   Start by placing plants outside in a sheltered, shady spot for a few hours each day and bringing them in at night. We also water a little less often during this period so plants adapt to field conditions more readily. Over about a week, increase their exposure to direct sun and weather, then transplant on a cool, overcast day once the weather has settled.
   
   Seedlings that skip hardening off can struggle after transplanting and suffer sunscald and wind damage. Sunscald shows as whitish patches on leaves; plants recover from light damage, but it slows them down.
   
   Hardened seedlings are still delicate. Prevent transplant shock by handling them gently and disturbing roots as little as you can. A plug popper pops seedlings out of the flat quickly and gently. Water regularly for the first week after planting to help them settle in.
4. Root-bound seedlings: When seeds are sown too early, roots can outgrow the container and start circling, forming a tight mat. Try to avoid this, but if it happens, gently tease the rootball apart at transplanting so roots grow outward normally afterward.

Healthy (left) and root-bound seedling (right).

These guidelines cover most common crops and should help your seedlings start strong. Paying attention to each crop's and variety's particular needs will improve results further, and the resources below go into more detail on raising plants from seed.

• Key Growing Information for Vegetables, Fruit, Flowers, Herbs and Farm Seed • Index
• Seed-Starting Date Calculator  • Interactive Calculator
• Transplanted Vegetable Crops: Average Plants per Seed Quantity, Average Yield and Feet of Row per Acre • CHARTS
• Seed Starting in Trial Greenhouses: 3 Indoor Seed-Starting Systems • Article
• Soil Blocking: A Better Way to Raise Seedlings• Article
• Seed-Starting Flats, Trays, Domes and Pots  • Comparison Chart (PDF)
• Guide to Choosing a Grow Light • Tech Sheet (PDF)
• Seedling Light Cart  • Comparison Chart (PDF)
• Potting Mixes for Certified Organic Production • ATTRA Sustainable Agriculture Program (PDF)