How to Start Seeds Indoors: Lights, Trays, and Timing for 20 Popular Vegetables

The Seed-Starting Setup

Lighting

A south-facing windowsill provides 1,000 to 2,000 foot-candles of light in winter, which is 20 to 40 percent of what seedlings need for stocky growth. Seedlings grown on windowsills stretch toward the light, producing thin, weak stems that transplant poorly and snap in wind. Supplemental lighting is not optional for healthy seedlings. It is required.

The most cost-effective setup uses two 48-inch T5 fluorescent tubes (one 6500K cool white, one 3000K warm white) in a shop light fixture ($25 to $35). Hang the fixture on adjustable chains so the tubes sit 2 to 3 inches above the seedling tops. Raise the lights as seedlings grow, maintaining the 2 to 3 inch gap. Run the lights 14 to 16 hours per day on a timer ($8 to $12). The combined output of the two tubes delivers 2,000 to 3,000 foot-candles at the plant canopy, sufficient for stocky growth in all vegetable seedlings.

LED grow lights have become competitive in price. A full-spectrum LED panel (45 watts, 2x2 feet) costs $30 to $50 and delivers 3,000 to 5,000 foot-candles at 12 inches. LEDs produce less heat than fluorescents, which reduces the need to raise the lights frequently. The electricity cost for either system is $3 to $5 per month for a 4-foot shelf running 16 hours per day.

Trays and Cells

Use 1020 standard nursery trays (10x20 inches) with cell inserts. Cell sizes determine how long seedlings can stay indoors before needing transplant. 72-cell trays (each cell is 1.5x1.5x2 inches) suit small-seeded crops like lettuce, basil, and peppers that transplant in 6 to 8 weeks. 50-cell trays (each cell is 2x2x3 inches) suit tomatoes, eggplant, and broccoli that need 8 to 10 weeks indoors. 36-cell trays (each cell is 2.5x2.5x3.5 inches) suit squash, cucumbers, and melons that grow rapidly and transplant in 3 to 4 weeks. Trays with drainage holes are mandatory. Set the drained trays inside solid trays (no holes) to catch runoff.

Cost: 1020 trays are $1.50 to $2.50 each. Cell inserts are $5 to $8 per pack. A reusable system of 10 trays with cell inserts costs $50 to $80 and lasts 5 to 8 years with proper cleaning. Wash trays in a 10 percent bleach solution (1 part bleach to 9 parts water) between uses to prevent damping-off and other seedling diseases.

Seed-Starting Mix

Use a soilless seed-starting mix, not garden soil or potting soil. Seed-starting mixes are formulated from sphagnum peat moss or coconut coir, perlite, and vermiculite. They are sterile (no weed seeds, no pathogens), lightweight, and hold moisture while draining well. Commercial mixes like Pro-Mix BX, Burpee Organic Seed Starting Mix, or Espoma Organic Seed Starter cost $8 to $12 per 8-quart bag. One 8-quart bag fills roughly three 72-cell trays. Moisten the mix before filling cells. Dry mix repels water and creates air pockets around seeds. The mix should feel like a wrung-out sponge: damp but not dripping.

Heat Mats

Seed germination is controlled by soil temperature, not air temperature. Most vegetable seeds germinate fastest at 70 to 85 degrees Fahrenheit. A typical indoor room temperature of 65 to 68 degrees is 5 to 20 degrees below optimal for many crops. A seedling heat mat ($20 to $30 for a 10x20 inch size) raises the soil temperature 10 to 20 degrees above ambient. Use a heat mat thermostat ($15 to $20) to prevent overheating. Peppers, eggplant, and melons germinate in 5 to 7 days at 80 to 85 degrees but take 14 to 21 days at 65 degrees. Tomatoes germinate in 5 to 7 days at 75 to 80 degrees but take 10 to 14 days at 65 degrees. The heat mat pays for itself in improved germination rates and faster emergence.

Labels and Spray Bottle

Label every tray with the variety name and planting date using waterproof labels ($6 to $10 for 100) or wooden craft sticks ($3 for 100). Seedlings look identical across varieties for the first 2 to 4 weeks. Mislabeling creates confusion at transplant time that persists through the entire season. A spray bottle ($2 to $4) is essential for watering newly planted seeds. A stream of water from a watering can displaces seeds and washes them out of shallow cells. Mist the surface gently until seeds germinate, then switch to bottom watering.

Sowing Technique: The Steps That Determine Germination Rate

Fill cells to within 0.25 inches of the top with pre-moistened seed-starting mix. Press the mix gently to firm it, but do not compact it. Sow seeds at the depth specified on the packet. A general rule: sow seeds at a depth of 2 to 3 times their diameter. Lettuce seeds (1 millimeter) get barely covered. Squash seeds (10 millimeters) go 1 inch deep. Fine seeds like lettuce, basil, and oregano can be surface-sown and pressed into the mix without covering.

Mist the surface after sowing. Cover the tray with a clear humidity dome ($3 to $5) to maintain moisture during germination. Remove the dome as soon as 50 to 60 percent of the seeds have germinated. Leaving the dome on too long promotes damping-off, a fungal disease that kills seedlings at the soil line. Damping-off thrives in warm, humid conditions with poor air circulation. If you see fuzzy white growth on the soil surface or seedlings falling over at the base, increase ventilation immediately by removing the dome and running a small fan nearby.

After germination, switch from misting to bottom watering. Pour 0.5 to 1 inch of water into the solid bottom tray and let the cells wick moisture up through the drainage holes over 30 to 60 minutes. Empty any remaining water from the bottom tray after 1 hour to prevent root rot. Bottom watering encourages roots to grow downward toward the moisture source, producing stronger root systems than top watering.

Planting Dates and Details for 20 Vegetables

All dates below are calculated from the average last frost date in your area. In zone 5, the average last frost is May 10 to 15. In zone 6, it is April 15 to 25. In zone 4, it is May 20 to 30. In zone 7, it is April 1 to 10. Adjust the weeks-before-frost numbers accordingly.

1. Tomatoes

Start 6 to 8 weeks before the last frost. Sow 0.25 inch deep. Germination: 5 to 7 days at 75 to 80 degrees Fahrenheit. Grow at 65 to 70 degrees after emergence. Transplant when seedlings have 2 to 3 sets of true leaves and are 6 to 8 inches tall. Harden off for 7 to 10 days before transplanting. Varieties: Celebrity (determinate, 70 days to maturity, 10 to 15 fruits per plant), Better Boy (indeterminate, 72 days, 30 to 50 fruits per plant), Sungold (indeterminate cherry, 57 days, hundreds of fruits per plant), Roma (determinate paste, 76 days, 20 to 30 fruits per plant).

2. Peppers (Bell)

Start 8 to 10 weeks before the last frost. Sow 0.25 inch deep. Germination: 7 to 14 days at 80 to 85 degrees. Peppers are slower to germinate than tomatoes and benefit from consistent heat. Grow at 65 to 70 degrees after emergence. Transplant when 4 to 6 inches tall with 4 to 6 true leaves. Varieties: California Wonder (75 days, 8 to 12 fruits per plant), King of the North (68 days, reliable in cool climates, 6 to 10 fruits), Big Red (75 days, large 4-inch fruits, 6 to 8 per plant).

3. Peppers (Hot)

Start 8 to 10 weeks before the last frost. Same sowing depth and temperature as bell peppers. Jalapeno (65 to 75 days, 30 to 50 fruits per plant), Serrano (75 to 80 days, 40 to 60 fruits), Habanero (90 to 100 days, 20 to 30 fruits, needs sustained heat above 80 degrees). Hot peppers require more heat for germination than bells. Use a heat mat set to 85 degrees for best results.

4. Eggplant

Start 8 to 10 weeks before the last frost. Sow 0.25 inch deep. Germination: 7 to 14 days at 80 to 85 degrees. Eggplant seedlings grow slowly for the first 3 weeks, then accelerate. Grow at 70 to 75 degrees. Transplant when 6 to 8 inches tall. Varieties: Black Beauty (74 days, 4 to 6 fruits per plant), Ichiban (64 days, Japanese type, 8 to 12 slender fruits), Hansel (55 days, miniature, 15 to 20 fruits per plant).

5. Broccoli

Start 6 to 8 weeks before the last frost for a spring crop. Sow 0.25 inch deep. Germination: 5 to 10 days at 70 to 75 degrees. Broccoli tolerates cooler growing temperatures (60 to 65 degrees) better than warm-season crops. Transplant when 4 to 5 inches tall with 3 to 4 true leaves. Varieties: Green Magic (57 days, 6 to 8 inch heads), Belstar (66 days, heat-tolerant, side shoots after main harvest), Packman (52 days, early, reliable).

6. Cabbage

Start 6 to 8 weeks before the last frost. Sow 0.25 inch deep. Germination: 4 to 10 days at 70 to 75 degrees. Grow at 60 to 65 degrees. Cabbage seedlings that are kept too warm (above 70 degrees) become leggy and fail to form tight heads. Transplant when 4 to 6 inches tall. Varieties: Early Jersey Wakefield (63 days, 2 to 3 pound conical heads), Stonehead (68 days, round heads, 4 to 6 pounds), Red Acre (75 days, 3 to 4 pound red heads).

7. Cauliflower

Start 6 to 8 weeks before the last frost. Sow 0.25 inch deep. Germination: 5 to 10 days at 70 to 75 degrees. Cauliflower is the most temperature-sensitive brassica. Heads button (form small, premature heads) if exposed to temperatures below 50 degrees for more than a week during early growth. Grow at 65 to 70 degrees. Varieties: Snow Crown (55 days, early, 6 to 8 inch heads), Cheddar (68 days, orange curds, high beta-carotene), Graffiti (80 days, purple heads, 8 to 9 inches).

8. Lettuce

Start 4 to 6 weeks before the last frost. Sow on the surface or barely cover with 0.125 inches of mix. Germination: 7 to 14 days at 60 to 68 degrees. Lettuce germinates poorly above 75 degrees. If your indoor space is warm, place seeded trays in a cool location (60 to 65 degrees) for germination. Grow at 60 to 65 degrees. Transplant at 3 to 4 inches tall. Varieties: Buttercrunch (55 days, butterhead, bolt-resistant), Romaine (Cos, 70 days, 10 to 12 inch heads), Saladbowl (45 days, oakleaf, cut-and-come-again).

9. Onions (from Seed)

Start 10 to 12 weeks before the last frost. Sow 0.25 inch deep. Germination: 7 to 14 days at 65 to 75 degrees. Onion seeds lose viability quickly. Use fresh seed (dated for the current year). Grow at 60 to 65 degrees. Transplant when pencil-thick (0.125 to 0.25 inches in diameter). Varieties: Copra (104 days, storage onion, hard, 3 to 4 inch bulbs), Walla Walla (95 days, sweet, 4 to 6 inch bulbs, short storage life), Red Wing (110 days, red storage onion, 3 to 4 inch bulbs).

10. Leeks

Start 8 to 10 weeks before the last frost. Sow 0.25 inch deep. Germination: 10 to 14 days at 65 to 70 degrees. Leek seeds are slow and uneven germinators. Sow 2 to 3 seeds per cell and thin to the strongest seedling. Grow at 60 to 65 degrees. Transplant when 6 to 8 inches tall by planting in 6-inch-deep trenches and filling as the leeks grow. Varieties: King Richard (75 days, long slender stalks, 10 to 15 inches), American Flag (105 days, larger stalks, winter-hardy).

11. Kale

Start 4 to 6 weeks before the last frost. Sow 0.25 inch deep. Germination: 5 to 7 days at 65 to 70 degrees. Grow at 60 to 65 degrees. Transplant when 3 to 4 inches tall. Varieties: Winterbor (60 days, frilly blue-green leaves, cold-hardy to 10 degrees Fahrenheit), Lacinato (Dinosaur kale, 65 days, dark blistered leaves, Italian heirloom), Red Russian (50 days, red-veined, tender leaves, cold-hardy).

12. Swiss Chard

Start 4 to 6 weeks before the last frost. Sow 0.5 inch deep. Germination: 7 to 14 days at 65 to 75 degrees. Soak seeds in water for 24 hours before planting to speed germination. Each seed is actually a cluster of 2 to 4 seeds, so thin to one seedling per cell after emergence. Transplant when 3 to 4 inches tall. Varieties: Bright Lights (50 days, multicolored stems, ornamental and edible), Fordhook Giant (60 days, large white stems, prolific), Ruby Red (55 days, red stems, 20 to 30 leaves per plant).

13. Basil

Start 6 to 8 weeks before the last frost. Sow on the surface or barely cover. Germination: 5 to 10 days at 70 to 75 degrees. Basil is extremely sensitive to cold. Do not transplant until nighttime temperatures stay above 50 degrees. Grow at 65 to 70 degrees. Transplant when 3 to 4 inches tall. Varieties: Genovese (70 days, large sweet leaves, standard for pesto), Sweet Thai (70 days, anise flavor, purple stems), Spicy Globe (60 days, compact, 6 to 8 inch mounds, good for containers).

14. Cilantro

Start 4 to 6 weeks before the last frost. Sow 0.25 inch deep. Germination: 7 to 14 days at 65 to 70 degrees. Cilantro has a long taproot that does not transplant well. Use deep cells (3 inches minimum) or start in small pots. Transplant carefully to avoid root disturbance. Cilantro bolts in temperatures above 75 degrees, so start early and plan for succession planting. Varieties: Santo (50 to 55 days, slow-bolting, standard), Leisure (45 days, compact, bred for leaf production).

15. Parsley

Start 10 to 12 weeks before the last frost. Sow 0.25 inch deep. Germination: 14 to 28 days at 65 to 70 degrees. Parsley is notoriously slow to germinate. Soak seeds in warm water for 24 hours, then drain and sow. Keep the mix consistently moist but not saturated. Grow at 60 to 65 degrees. Transplant when 3 to 4 inches tall. Varieties: Gigante d'Italia (75 days, flat-leaf, large leaves, strong flavor), Moss Curled (70 days, curly, decorative garnish).

16. Cucumbers

Start 3 to 4 weeks before the last frost. Sow 1 inch deep. Germination: 3 to 7 days at 80 to 85 degrees. Cucumbers grow fast and resent root disturbance. Start in 3-inch peat pots or large cells that can be planted directly into the garden, pot and all. Transplant when the first true leaves appear. Do not let seedlings become root-bound. Varieties: Marketmore 76 (58 days, slicing, 8 to 9 inch fruits, disease-resistant), Spacemaster (60 days, bush type, 7 to 8 inch fruits, for containers), Lemon (65 days, round yellow fruits, mild flavor).

17. Summer Squash

Start 3 to 4 weeks before the last frost. Sow 1 inch deep. Germination: 3 to 7 days at 80 to 85 degrees. Like cucumbers, squash grows rapidly and dislikes root disturbance. Start in 3-inch peat pots. Transplant when the first true leaves appear. Varieties: Black Beauty Zucchini (50 days, 6 to 8 inch fruits, 10 to 15 per plant), Gold Rush (50 days, yellow zucchini, 8 to 12 fruits per plant), Pattypan Scallop (50 days, disc-shaped, 10 to 15 per plant).

18. Melons

Start 3 to 4 weeks before the last frost. Sow 0.5 to 0.75 inches deep. Germination: 4 to 10 days at 80 to 85 degrees. Melons need warm soil and long growing seasons. Use a heat mat set to 85 degrees. Start in 3 to 4 inch peat pots. Transplant when seedlings have 2 to 3 true leaves. Varieties: Hale's Best Cantaloupe (80 days, 3 to 4 pound fruits, sweet orange flesh), Sugar Baby Watermelon (80 days, 8 to 10 pound icebox type, 2 to 3 fruits per plant), Minnesota Midget Cantaloupe (60 days, compact, 1 to 2 pound fruits, for short seasons).

19. Watermelon

Start 3 to 4 weeks before the last frost. Sow 1 inch deep. Germination: 4 to 10 days at 85 degrees. Watermelons need a long, hot growing season. In zones 4 and 5, starting indoors is essential for reliable maturity. Use a heat mat. Start in 4-inch peat pots. Transplant when seedlings have 2 true leaves. Varieties: Sugar Baby (80 days, 8 to 10 pounds, icebox type), Charleston Gray (85 days, 25 to 35 pounds, classic oblong), Yellow Doll (68 days, 5 to 7 pounds, yellow flesh, early).

20. Pumpkins

Start 3 to 4 weeks before the last frost. Sow 1 inch deep. Germination: 4 to 7 days at 80 to 85 degrees. Pumpkins have sensitive roots and do not tolerate root disturbance. Start in 3 to 4 inch peat pots. Transplant the entire pot into the garden. Varieties: Jack O'Lantern (100 days, 10 to 18 pound carving pumpkins, 3 to 5 per plant), Sugar Pie (100 days, 5 to 7 pound pie pumpkins, sweet flesh), Big Max (120 days, 50 to 100 pound giant, 1 to 2 per plant).

Hardening Off: The 7-Day Schedule

Seedlings grown indoors under stable conditions have thin cuticles (the waxy outer layer of leaves) and are not prepared for direct sun, wind, or temperature fluctuations. Hardening off gradually exposes seedlings to outdoor conditions over 7 to 10 days, thickening the cuticle and building stress tolerance. Skipping this step kills 30 to 50 percent of transplants within the first week in the garden.

Day 1: Set seedlings outside in a shaded, sheltered location for 1 to 2 hours. Bring them back indoors. Day 2: 2 to 3 hours in shade. Day 3: 3 to 4 hours, with 1 hour of dappled sunlight. Day 4: 4 to 5 hours, with 2 hours of direct morning sun. Day 5: 5 to 6 hours, with 3 hours of direct sun. Day 6: 6 to 8 hours, leave outside overnight if temperatures stay above 50 degrees. Day 7: Full day outside, transplant into the garden. If temperatures drop below 40 degrees during the hardening-off period, bring seedlings indoors or cover with a row cover. Reduce watering during hardening off to slow growth and toughen stems.

Troubleshooting Seedling Problems

Leggy Seedlings (Stretched, Thin Stems)

Leggy seedlings are caused by insufficient light or excessive warmth. The stem stretches to reach the light source, resulting in a weak plant that cannot support itself outdoors. Solutions: move lights closer (2 inches above the tops), increase light duration to 16 hours, and reduce room temperature to 60 to 65 degrees. You can bury leggy tomato stems up to the first set of true leaves at transplant time. Tomatoes will grow roots from the buried stem. Peppers and eggplant cannot be buried deeper and should be discarded if severely leggy.

Damping-Off (Seedlings Collapse at Soil Line)

Damping-off is caused by Pythium, Rhizoctonia, and Fusarium fungi that thrive in wet, warm conditions. Infected seedlings develop a pinched, water-soaked area at the soil line and fall over within 24 to 48 hours. Prevention: use sterile seed-starting mix, clean trays with bleach, maintain good air circulation with a fan, avoid overwatering, and remove humidity domes as soon as seeds germinate. If damping-off appears, remove affected seedlings immediately, increase air circulation, and water with chamomile tea (1 tea bag steeped in 2 cups of water, cooled) which has mild antifungal properties.

Yellow Leaves on Seedlings

Yellowing cotyledons (the first two seed leaves) on seedlings with 2 or more true leaves is normal. Cotyledons feed the seedling for the first 2 to 3 weeks and then naturally yellow and drop. Yellowing true leaves indicate nitrogen deficiency or overwatering. Fertilize with half-strength liquid fish emulsion (1 tablespoon per gallon of water) once seedlings have 2 to 3 sets of true leaves. Apply every 10 to 14 days. Overwatered seedlings have yellow leaves with a soft, limp texture. Underwatered seedlings have yellow leaves with crisp, dry edges.

Fuzzy White Mold on Soil Surface

White mold on the soil surface is saprophytic fungus feeding on organic matter in the seed-starting mix. It does not infect seedlings but indicates the mix is too wet. Scrape off the mold, reduce watering frequency, and increase air circulation. Bottom watering instead of top watering keeps the soil surface drier and reduces mold growth.

Cost Breakdown: Seed Starting vs. Buying Transplants

By year two, the equipment is paid for and your only costs are seeds and soil mix. Over 5 years, seed starting saves $800 to $1,300 compared to buying transplants, while giving you access to 5 to 10 times more varieties than any garden center carries.