GreenhouseGuide

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Starting Seeds in a Greenhouse

Starting your own seeds in a greenhouse gives you access to far more varieties than garden centers stock, produces stronger transplants adapted to your conditions, and saves significant money over buying nursery starts. A heated greenhouse bench with grow lights and seedling heat mats is a seed-starting powerhouse.

Timing Your Seed Starts

Work backward from your last frost date. Most warm-season crops need 6–8 weeks of indoor growing time before transplanting outdoors. Cool-season crops can be started 4–6 weeks ahead. Here’s a general timeline:

Weeks Before Last Frost	Start These Seeds
10–12 weeks	Onions, leeks, celery, artichoke
8–10 weeks	Peppers, eggplant
6–8 weeks	Tomatoes, basil, annual flowers
4–6 weeks	Brassicas (broccoli, cabbage, kale), lettuce
2–4 weeks	Cucumbers, squash, melons (fast-growing, don’t transplant well if root-bound)

Temperature for Germination

Soil temperature is more important than air temperature for germination. Most vegetable seeds germinate best between 65–80°F in the soil. A seedling heat mat raises soil temperature by 10–20°F above ambient, which is often the difference between reliable germination and sporadic, slow emergence.

Pair the heat mat with a thermostat probe inserted into the soil to maintain a consistent target temperature. Without a thermostat, heat mats can overshoot and cook seeds. See our heater guide for heat mat recommendations.

Lighting for Seedlings

Seedlings need 12–16 hours of strong light daily to grow stocky and strong. Natural greenhouse light may be sufficient in March–April, but for January–February starts, supplemental lighting is essential. T5 LED grow light strips mounted 4–6 inches above seedling trays provide excellent, even illumination at minimal cost. See our Grow Lights guide for specific recommendations.

Without adequate light, seedlings “reach” for the nearest light source, becoming tall, thin, and weak (“leggy”). Leggy seedlings are fragile, transplant poorly, and produce less than stocky, compact ones.

The Seed-Starting Setup

A productive seed-starting station includes:

• Cell trays or soil blocks: 72-cell trays for small seeds, 50-cell or 36-cell for larger seeds. Soil blocks (made with a block maker) eliminate transplant shock entirely because there’s no container to disturb roots.
• Seed-starting mix: Use sterile, lightweight seed-starting mix (not garden soil). It drains well, retains consistent moisture, and is free of weed seeds and pathogens.
• Heat mat + thermostat: Under the tray, set to your target soil temperature.
• Grow lights: Above the tray, on a timer for 14–16 hours daily.
• Humidity dome: A clear plastic dome over the tray retains moisture during germination. Remove once seeds sprout to prevent damping-off.
• Labels: Always label your trays. You will not remember which tray is which in two weeks.

Watering Seedlings

Bottom-watering is the gold standard for seedlings. Set trays in a shallow tray of water and let the soil wick moisture upward. This keeps the soil surface drier (reducing damping-off risk) and encourages roots to grow downward toward the water source. Remove trays from water once the top surface glistens with moisture.

Hardening Off

Seedlings grown in the sheltered greenhouse environment need gradual exposure to outdoor conditions before transplanting. The hardening-off process takes 7–10 days:

• Days 1–3: Move seedlings outside for 2–3 hours in a sheltered, shaded spot. Bring back to the greenhouse before evening.
• Days 4–6: Increase outdoor time to 4–6 hours with some direct morning sun. Reduce watering slightly.
• Days 7–10: Leave seedlings outside all day in progressively more sun. Bring in only if frost threatens.
• Transplant day: Plant on a cloudy day or in late afternoon to reduce transplant shock.

For gardening tools and seed-starting supplies, visit our partner site Garden Gear.