Seed Starting Guide: Grow Transplants Like a Pro

Updated March 2026 · By the ZonePlanter Team

Starting seeds indoors gives you a head start on the growing season, access to hundreds of varieties not available as nursery transplants, and significant savings over buying starts. A flat of 72 seedlings costs about $5 in seeds and supplies, compared to $50 or more for the same number of nursery transplants. With the right setup and timing, anyone can grow strong, healthy transplants that hit the ground running when planted outdoors.

When to Start Seeds: Timing Is Everything

Seed starting timing is counted backward from your last frost date. Each crop has a recommended number of weeks to start indoors before the last frost. Tomatoes and peppers start 6 to 8 weeks before last frost. Brassicas like broccoli and cabbage start 4 to 6 weeks before. Squash and melons start only 2 to 3 weeks before because they grow quickly and do not transplant well when root-bound.

Starting too early is a more common mistake than starting too late. Seedlings that spend too long indoors become leggy, root-bound, and stressed. A tomato started 12 weeks before last frost will be a struggling, overgrown plant at transplant time, while one started 6 weeks before will be compact, vigorous, and ready to grow. Check your seed packets for specific timing recommendations and count backward from your local last frost date.

Tomatoes, peppers, eggplant: 6-8 weeks before last frost
Broccoli, cabbage, cauliflower: 4-6 weeks before last frost
Lettuce, kale: 4-5 weeks before last frost
Squash, melons, cucumbers: 2-3 weeks before last frost
Herbs (basil, parsley): 6-8 weeks before last frost

Pro tip: Create a seed starting calendar by listing each crop, its start date, and its transplant date. Post it where you will see it. Missing your window by even 2 weeks can mean the difference between thriving transplants and disappointing results.

Essential Equipment and Setup

You do not need an expensive setup. The essentials are: seed starting trays or cell packs ($2 to $5 for 72 cells), a quality seed starting mix ($5 to $10 for 8 quarts), a light source, and a warm location. Reusable plastic cell trays last many seasons. Clear humidity domes ($2 to $4) help with germination but should be removed once seedlings emerge.

The single most important equipment investment is lighting. A south-facing window provides insufficient light for most seedlings, resulting in leggy, weak plants that stretch toward the glass. A basic T5 fluorescent or LED shop light ($20 to $40) positioned 2 to 4 inches above seedlings provides the intense, even light they need. Dedicated grow lights ($40 to $100) offer the full light spectrum but are not necessary for starting vegetables.

Seed starting trays (72 cells): $2-$5
Seed starting mix (8 qt): $5-$10
Humidity dome: $2-$4
LED shop light: $20-$40 (adequate for most seedlings)
Grow light with stand: $40-$100 (premium option)
Heat mat (for warm-season crops): $15-$30

Pro tip: Do not use regular garden soil or potting mix for starting seeds. Seed starting mix is specifically formulated to be fine-textured, sterile, and well-draining, which prevents damping off disease that kills seedlings. It is worth the few extra dollars.

Sowing and Germination

Fill cells with moistened seed starting mix, pressing lightly to remove large air pockets but not compacting. Plant seeds at the depth recommended on the packet — a general rule is twice the diameter of the seed. Very fine seeds like lettuce and herbs are sown on the surface and pressed in without covering. Water gently with a spray bottle or bottom-water by filling the tray below and letting the mix absorb upward.

Cover trays with humidity domes or plastic wrap to retain moisture during germination. Most vegetable seeds germinate at soil temperatures of 65 to 80 degrees Fahrenheit. Warm-season crops like tomatoes, peppers, and basil germinate faster with bottom heat from a seedling heat mat, which raises soil temperature by 10 to 15 degrees. Remove the dome and provide light as soon as seeds sprout.

Planting depth: 2x seed diameter (fine seeds on surface)
Moisten mix before filling cells, not after sowing
Cover with dome for humidity until germination
Soil temperature for germination: 65-80°F (most vegetables)
Remove dome immediately when first seedlings emerge

Pro tip: Label everything. Use plant markers or tape on the tray edge with the variety name and sowing date. Seedlings of different tomato varieties look identical, and you will not remember which is which in 4 weeks.

Growing Strong Seedlings

Once seedlings emerge, light is the priority. Provide 14 to 16 hours of light daily with the light source 2 to 4 inches above the seedlings. Raise the light as plants grow, maintaining the close distance. Inadequate light is the number one cause of leggy, weak seedlings. If your seedlings are stretching toward the light or have long, thin stems, the light is too far away or too weak.

Water seedlings when the surface of the mix dries out, which is typically every 1 to 2 days. Bottom watering (filling the tray beneath) encourages roots to grow downward and reduces the risk of damping off disease. Begin fertilizing with a diluted liquid fertilizer (quarter strength) once seedlings develop their first true leaves (the second set of leaves that appear after the initial seed leaves).

Light: 14-16 hours daily, 2-4 inches from canopy
Water: when surface dries, bottom-water preferred
Fertilize: quarter-strength liquid after first true leaves
Temperature: 60-70°F after germination (cooler = stockier plants)
Air circulation: a small fan prevents damping off and strengthens stems

Pro tip: Run a small oscillating fan on low near your seedlings for a few hours daily. The gentle breeze strengthens stems (a process called thigmotropism) and reduces fungal disease. Plants grown with air movement are noticeably sturdier at transplant time.

Hardening Off and Transplanting

Seedlings grown indoors are not adapted to outdoor conditions: direct sunlight, wind, temperature swings, and lower humidity. Transplanting them directly outdoors without acclimatization causes transplant shock, sunburn, and sometimes death. The hardening off process gradually exposes seedlings to outdoor conditions over 7 to 10 days.

Start by placing seedlings outdoors in a sheltered, shaded location for 2 to 3 hours on the first day. Each day, increase the time outside and gradually introduce more direct sunlight. By days 5 to 7, seedlings can be in full sun for most of the day. On days 8 to 10, leave them out overnight if nighttime temperatures are above 50 degrees Fahrenheit for warm-season crops. Transplant on a cloudy day or in the late afternoon to reduce shock.

Days 1-2: 2-3 hours in shade, sheltered spot
Days 3-4: 4-6 hours with some direct morning sun
Days 5-7: full day outside, increasing direct sun
Days 8-10: overnight outdoors if warm enough
Transplant on a cloudy day or in late afternoon

Pro tip: Water transplants deeply immediately after planting and again the following morning. Some wilting in the first 2 days is normal as roots establish. Shade cloth or a lightweight row cover for the first 3 to 5 days after transplanting reduces stress in hot weather.

Frequently Asked Questions

Is it worth starting seeds indoors?

Yes, for crops that need a long growing season (tomatoes, peppers, eggplant, broccoli). You save $1-$3 per plant compared to nursery transplants, and you get access to hundreds of varieties unavailable as starts. For fast-growing crops like beans, squash, and direct-sow crops like carrots, starting indoors is unnecessary.

Do I need grow lights for seed starting?

You need some form of supplemental lighting. Even a bright south-facing window is usually insufficient, producing leggy seedlings. A $25 LED shop light positioned 2-4 inches above seedlings for 14-16 hours daily produces excellent results. You do not need expensive full-spectrum grow lights for starting vegetable seedlings.

Why are my seedlings leggy and falling over?

Leggy seedlings are caused by insufficient light. The light is either too far from the seedlings, not on long enough (need 14-16 hours daily), or not bright enough. Move lights to within 2-4 inches of the seedling canopy. A small fan providing gentle air movement also strengthens stems. Leggy seedlings can be planted deeper at transplant time (especially tomatoes).

What is damping off and how do I prevent it?

Damping off is a fungal disease that causes seedlings to collapse at the soil line and die. Prevent it by using sterile seed starting mix (never garden soil), providing good air circulation (a small fan), avoiding overwatering, removing humidity domes promptly after germination, and not overcrowding seedlings.

Can I start seeds in egg cartons?

You can, but they are not ideal. Egg carton cells are too small for most vegetable seedlings, the cardboard wicks moisture away from the soil, and they degrade before transplant time. Inexpensive plastic cell trays ($2-$5 for 72 cells) are reusable for many seasons and provide proper cell depth and drainage.