Growing tomato plants from seed is relatively easy and growers both big and small have been doing so for centuries. However to produce top quality seedlings a well orchestrated plan for germination and cultural practices based on the latest research, is most advantageous. The following plan/guide lays out step by step how to produce top notch tomato seedlings.
Potting media: Use a commercially prepared peat-lite mix (a soilless mix), or blend your own.
Basic recipe for homemade peat-lite mix:
- 1 part moistened Sphagnum peat moss – (Cornell University advises using imported sphagnum peat moss over domestic peats. “Domestic peats frequently contain large quantities of nutrients or other materials in unknown amounts and are usually too decomposed to provide the desired structural and water drainage characteristics”(Boodley J. W. & Sheldrake R. Jr., n.d.).
- 1 part either horticultural grade vermiculite, perlite or sand (or even a mix of 2 or 3 of these).
- Ground dolomitic limestone (to supply calcium and to bring the pH into the proper range for tomatoes).
- Regular super phosphate (for flowering and fruiting).
- Calcium or potassium nitrate (to reduce blossom end rot).
Plastic cell packs: new or sanitized
Tomato seeds: Tomatoes come in two different types; determinate and indeterminate. Determinates grow to a determined size and then stop. They are compact and bushy. They are a good choice for small gardens and containers. They also tend to fruit earlier. The fruit tends to be ready for harvest all around the same time making it a good choice for canning and sauces. Indeterminates are more viney than determinates; their vines continue to grow throughout the season and they will need support. Indeterminates produce fruit until later than determinates, allowing you to enjoy fresh tomatoes in your salads and sandwiches until first frost.
Choose fungicide treated seeds to help prevent damping off. Sow at least 20% more seed than required to allow for losses. Purchase only quality seed from a reputable seed supplier.
Plastic bags or covers
Start date: Begin seedlings indoors or in your heated greenhouse approximately 6 weeks before your average last frost date. (i.e.: begin seedlings at the end of March to be ready for May long weekend.
Method: Moisten the soilless media until it is evenly damp, (like a wrung-out sponge). Then fill the cell packs and gently firm the soil. Place one seed then cover with ¼” of peat moss or soilless media. Note: the peat moss has anti-fungal properties that are helpful in keeping seeds and seedlings disease-free. Gently firm the surface media to ensure contact with the seed and mist well to moisten the seed and media mix. Place a plastic cover over the seed trays and place in a warm location out of direct sunlight. Ideal temperature range for seed germination (according to Cornell University) is 21°C to 35°C (60° F to 95°F) with the optimal temperature being 85°F (30°C). Use heat mats if required to maintain this temperature. Inspect daily for germination. Note: If not all seeds germinate at the same time, cut strips of plastic or cloth, and keep un-germinated cells covered until seedlings appear. Seeds are quickly killed if allowed to dry during germination
Moisture levels: Keep evenly moist (not wet); misting the soil would be ideal as they should only require very light watering (if any), as the plastic covers serve to hold the moisture in. Note: do not use a forceful stream of water it may dislodge the fragile seedlings.
Relative Humidity: 75 to 80%
Fertilizing: No additional fertilizers should be required until after the second set of true leaves appears.
Seed germination time: Typically 5 to 10 days
Cultural Practices After Germination
Light levels: After the seeds have germinated move the new seedlings to an area of full sun and remove the plastic covers. The tomato seedlings will need 14 to 16 hours per day of natural or fluorescent light to keep from becoming leggy.
Moisture levels: Rather than keeping the soil constantly moist begin watering the seedling trays thoroughly, then wait until they are almost dry before watering again (this will help to prevent damping off). Preferably water the containers from below, (this will help to keep the leaves dry) or use a wand with a small nozzle and carefully water the soil around the plant.
Relative Humidity: Try to maintain a humidity level around 70 to 75%
Temperature: To encourage slow steady growth that will produce sturdy, stocky plants maintain a daytime temperature of about 18°C and a night time temperature of 12°C Note: these temperatures can be safely (gradually) increased by as much as 12°C if seedling growth is to slow.
Thinning: If more than one tomato plant has germinated in a cell pack clip the weakest, leaving only one.
Physical Stimulation: For stronger stems run your finger tips over the plants periodically or place a fan on low speed to blow over them (this helps to make them stronger by simulate their natural environment,). You will sometimes hear this technique called tickling your tomatoes.
Fertilizer: Once the second set of true leaves appear water with a half-strength solution of a water soluble complete fertilizer (like a 10-10-10 or better yet a 5-10-10) and gradually increase the strength of the solution over time. If crystallized salts appear on soil surface reduce fertilizing. (Note: the first set of leaves are not the true leaves. They typically are a different size and shape. The second set of leaves are the real leaves and will look like tomato leaves.)
Disease Controls: the most common diseases to guard your young tomato seedlings against are fungal diseases like damping off. To guard against fungal diseases purchase fungicide treated seed, use sterile potting media, allow the soil to dry slightly between watering, allow for good air circulation around the plants, keep the leaves dry (especially at night) and ensure good sanitation.
Pest Insect Control: The most common insect pests are white flies, aphids and spider mites. To help control these pest populations inspect the plants daily, If you are growing in a green house or growing several you may want to set up sticky cards to monitor pest populations If white flies become a problem in a green house setting or are anticipated Encarsia formosa (a parasitic wasp) may be released once a week at a rate of two to five parasites per plant. A hand held vacuum can also be useful for gathering white flies. Pests may be removed by hand, washed off or sprayed with an approved of insecticide. To protect the seedlings from tobacco mosaic virus ensure that no smokers are allowed to handle the plants (especially with unwashed hands).
Harden Off Seedlings: Once the weather warms up begin setting the tomato seedling outdoors for a few hours a day to gradually get used to the wind and weather. Gradually increase the amount of time spent out doors over a period of about 10 days.
Plant in the Ground or In large Containers: Tomatoes can be planted outdoors once the nighttime temperatures remain above 10 °C. (Note: tomatoes hate the cold and planting them out to early will set the plants back.) Make sure that your soil has been properly prepared. Preferably choose an overcast day to plant. Plant the tomatoes deeper than soil level they are currently growing at. Tomatoes form roots all along any portion of buried stem. The deeper you plant them the sturdier they will be and the more nutrients and water they will be able to take up. (Note: if you are having a cooler spring than normal delay hardening off and planting out, rather repot your seedlings into slightly larger pots to buy you a little extra time.)
Summary Use a soilless media mix in sterile containers. Cover the containers with plastic, keep evenly moist and maintain a temperature around 30°C and a relative humidity of 75 to 80%. Once the seeds have germinated remove the plastic covers and provide the seedlings with 14 to 16 hours per day of natural or fluorescent light. Water the seedlings from below (preferably) and allow them to almost dry out between watering. Monitor the seedling daily for signs of insect pests or diseases and begin a graduated fertilizer program (using a complete soluble fertilizer) once the second set of true leaves appear. Maintain a relative humidity of 70-75% with air temperature between 12°C to 25°C.
Provide some physical stimulation for the seedlings. Harden off the seedlings in a sheltered space outdoors for 10 days prior to planting in the garden.
Boodley J. W. & Sheldrake R. Jr., n.d.; Information Bulletin 43, Cornell Peat-Lite Mixes for Commercial Plant Growing, Retrieved from: http://www.greenhouse.cornell.edu/crops/factsheets/peatlite.pdf
Cornell University, Vegetable Growing Guides, ‘Tomatoes’
Information Bulletin 43. Cornell Peat-lite Mixes for Commercial Plant Growing.
UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Marjorie Peronto, Bulletin #2751, Starting Seeds at Home
The University of Georgia, Georgia Homegrown Tomatoes, Bulletin 1271
The University of Georgia, Commercial Tomato Production Handbook http://extension.uga.edu/publications/detail.html?number=B1312&title=Commercial%20Tomato%20Production%20Handbook
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For photos: Kimberley Pacholko Growing Tomato Plants From Seed – Horticulture For Home Gardeners
For content: Kimberley Pacholko 2021, Ornamental Garden Specialist, Growing Tomato Plants From Seed – Horticulture For Home Gardeners