For commercial growers, greenhouse seed starting is the foundation of a profitable season. Get it right and you'll have strong, uniform plugs ready to sell or transplant on schedule. Get it wrong and you're looking at uneven germination, damping off, and lost revenue. This guide covers everything you need to know — from tray selection and germination temperatures to the best bulk varieties for plug production — so you can run a tighter, more profitable greenhouse operation.
Whether you're growing for a retail garden center, a farmers market, a landscaping operation, or your own cut flower farm, the principles are the same: start with quality bulk seed, control your environment, and time your successions carefully.
Why Greenhouse Seed Starting Beats Buying Plugs
Buying finished plugs from a wholesale greenhouse is convenient, but the economics rarely work in your favor at scale. Here's why growing from bulk seed wins:
- Cost per plant: A 128-cell plug tray from a wholesale supplier can cost $40–$80. Starting from bulk seed, your cost per plant drops to pennies.
- Variety control: Plug suppliers offer a limited selection. Growing from seed gives you access to hundreds of varieties — including specialty cultivars that command premium prices.
- Timing flexibility: You control when your plugs are ready, not a supplier's schedule.
- Succession planting: You can stagger multiple rounds of the same variety for continuous production — impossible when buying plugs in fixed quantities.
The upfront investment in a good seed starting setup pays for itself quickly when you're buying quality bulk seed and managing your germination environment well.
Step 1: Choose the Right Trays and Growing Media
Tray selection depends on the crop and your end use. Here's a quick guide:
- 512-cell trays: Best for tiny seeds like petunias and begonias. Produces small plugs ideal for transplanting into 4-packs or 6-packs.
- 288-cell trays: The workhorse of commercial plug production. Good for most annual flowers and vegetables.
- 128-cell trays: Better for larger-seeded crops like zinnias, snapdragons, and tomatoes. Produces a bigger plug with more root mass.
- 72-cell trays: For crops that need more space to develop before transplanting, like lisianthus and large-flowered begonias.
Growing media: Use a professional-grade, fine-textured germination mix with good moisture retention and low fertilizer charge. Avoid heavy potting mixes — they compact in small cells and restrict root development. Many commercial growers use a peat-perlite blend or a dedicated plug mix.
Step 2: Understand Germination Temperature Requirements
Temperature is the single most important factor in germination success. Most growers make the mistake of using air temperature as their guide — but what matters is media temperature (the temperature inside the tray). Here are the key ranges for common greenhouse crops:
- Petunias: 72–76°F media temp. Light required for germination — do not cover seeds.
- Lisianthus: 72–75°F. One of the most temperature-sensitive crops. Use a heat mat and monitor closely.
- Snapdragons: 65–72°F. Cool-season crop — avoid high temps which cause dormancy.
- Zinnias: 75–85°F. Warm-season crop that germinates fast in heat.
- Begonias: 75–78°F. Tiny seeds — surface sow, do not cover, keep humidity high.
- Sunflowers: 70–85°F. Fast and reliable — one of the easiest crops to germinate.
Invest in a good infrared thermometer to check media temps regularly. Bottom heat mats or heated germination chambers are essential for maintaining consistent temperatures, especially in early spring when greenhouse air temps fluctuate.
Step 3: Seeding Techniques for Small and Pelleted Seeds
Many of the most profitable greenhouse crops — petunias, lisianthus, begonias — have extremely small seeds that require careful handling. Here's how to get consistent results:
Raw Seeds
- Use a vacuum seeder or a fine-tipped dibble for precision placement
- Sow 1–2 seeds per cell for most crops
- For tiny seeds like begonias and petunias, mix with fine sand to improve distribution
- Do not cover light-requiring seeds (petunias, begonias) — just press gently into the media surface
Pelleted Seeds
Pelleted seeds are coated in a clay or inert material to make them larger and easier to handle. They're ideal for precision seeding and are especially popular for petunias and lisianthus. Our Petunia FotoFinish Mix Pelleted Seeds are a top choice for greenhouse growers — the pellet coating makes single-seed placement easy even in 512-cell trays.
For lisianthus, pelleted seed is almost always the professional choice. Our Lisianthus Croma Champagne Pelleted Seeds and Lisianthus Arena Purple Pelleted Seeds are bred for commercial plug production with excellent germination rates and uniform growth.
Important: Pelleted seeds require consistent moisture to dissolve the coating and allow germination. Never let trays dry out during the germination phase — a dry period can cause the pellet to harden and block emergence.
Step 4: Build a Succession Planting Schedule
The most profitable greenhouse operations run on a tight succession schedule — new trays going in every 1–3 weeks so there's always product at the right stage for sale or transplanting. Here's how to build yours:
- Work backwards from your target sale date. If you want finished 4-packs ready for May 1, count back the weeks needed for germination + plug development + finishing in the final container.
- Know your crop timing. Petunias: 10–12 weeks seed to finished 4-pack. Lisianthus: 20–24 weeks seed to bloom. Snapdragons: 12–16 weeks. Zinnias: 8–10 weeks.
- Stagger by 2–3 weeks. Start a new tray of your anchor crops every 2–3 weeks to maintain continuous production.
- Track everything. Label every tray with the variety, seeding date, and target transplant date. A simple spreadsheet or whiteboard system works fine.
Step 5: Top Bulk Varieties for Greenhouse Plug Production
Not all varieties perform equally in a plug production system. Here are the top performers that commercial growers rely on, all available in bulk quantities:
Petunias — The Greenhouse Staple
Petunias are the backbone of most spring greenhouse operations. They germinate reliably, grow fast, and sell in huge volumes. Our Petunia FotoFinish Mix Pelleted Seeds are a professional-grade choice with excellent uniformity and a full color range. For smaller trial quantities before committing to bulk, visit www.trailingpetunia.com.
Lisianthus — The Premium Cut Flower
Lisianthus is one of the highest-value crops a greenhouse grower can produce, but it requires patience and precision. It's slow — 20–24 weeks from seed to bloom — but the returns are exceptional. Our Lisianthus Corelli White Pelleted Seeds are a reliable commercial variety with strong stems and excellent vase life.
Snapdragons — Cool-Season Workhorses
Snapdragons are a must for spring and fall greenhouse production. They thrive in cool conditions and are one of the best-selling cut flowers at market. Our Snapdragon Legend Pink Cut Flower Seeds and Snapdragon Twinny Mix are both excellent for plug production and finished container sales.
Zinnias — Fast Turnaround, High Volume
Zinnias are the fastest crop in the greenhouse — 8–10 weeks from seed to finished plant. They're heat-loving, easy to grow, and sell out fast at market. Our Zinnia Magellan Mix Bulk Flower Seeds produce large, sturdy blooms in a full color range that customers love.
Sunflowers — Easy and Profitable
Sunflowers are one of the simplest crops to start in the greenhouse and one of the easiest to sell. Our Sunfinity Hybrid Sunflower Seeds are a branching, continuous-blooming variety that gives growers multiple harvests from a single plant — exceptional value for cut flower production.
Step 6: Managing Humidity, Airflow, and Disease
The biggest enemy of greenhouse seed starting is damping off — a fungal disease that kills seedlings at the soil line. Here's how to prevent it:
- Don't overwater. Keep media moist but never waterlogged. Allow the surface to dry slightly between waterings once seeds have germinated.
- Maximize airflow. Run horizontal airflow fans continuously. Good air movement dramatically reduces fungal pressure.
- Reduce humidity after germination. High humidity is fine during germination, but once seedlings emerge, drop relative humidity to 50–60% to harden them off.
- Sanitize everything. Clean and disinfect trays, benches, and tools between crops. Fungal spores persist on surfaces.
- Use clean water. If your water source has high pathogen pressure, consider a UV sterilizer or hydrogen peroxide treatment.
Step 7: Fertilizing Seedlings in the Plug Stage
Most professional germination mixes have little to no fertilizer charge, so you'll need to start feeding early. Here's a simple fertilization approach for plug production:
- Start feeding at cotyledon stage (when the first seed leaves appear) with a dilute, balanced fertilizer at 50–75 ppm nitrogen.
- Increase to 100–150 ppm once true leaves appear.
- Use a calcium-magnesium supplement every 2–3 weeks to prevent deficiencies, especially in peat-based mixes.
- Watch EC levels. High salt buildup in small cells can burn roots. Flush trays with clear water periodically.
Frequently Asked Questions
What temperature should a greenhouse be for seed starting?
Media temperature (not air temperature) is what matters most. Most annual flowers germinate best at 72–78°F media temp. Use bottom heat mats and an infrared thermometer to monitor accurately. Cool-season crops like snapdragons prefer 65–72°F, while warm-season crops like zinnias and sunflowers prefer 75–85°F.
How many seeds should I sow per cell?
For most crops, sow 1–2 seeds per cell and thin to one seedling after germination. For pelleted seeds, sow one pellet per cell. For very small seeds like begonias and petunias, precision seeding equipment helps achieve single-seed placement.
Why are my greenhouse seedlings leggy?
Legginess is almost always caused by insufficient light. Seedlings need 16–18 hours of bright light per day. If natural light is inadequate, supplement with LED grow lights positioned 4–6 inches above the tray tops. Also check that temperatures aren't too high — excessive heat combined with low light accelerates stretching.
When should I start seeds for spring sales?
Work backwards from your target sale date. For a May 1 sale date, petunias should be seeded around mid-February (10–12 weeks). Lisianthus needs to be started in November or December (20–24 weeks). Zinnias can be started as late as mid-March for a May sale (8–10 weeks).
What is the difference between pelleted and raw seeds?
Pelleted seeds are coated in a clay or inert material to make them larger and easier to handle for precision seeding. They're ideal for tiny seeds like petunias and lisianthus. Raw seeds are uncoated and typically less expensive per unit, but harder to place precisely in small cells. For commercial plug production, pelleted seeds often pay for themselves in labor savings and improved uniformity.
Where can I buy bulk seeds for greenhouse production?
For professional-grade bulk flower seeds in commercial quantities, Trailing Petunia Bulk Seeds carries a wide selection of top varieties for greenhouse growers. If you want to trial a variety in smaller quantities first, smaller packs are available at www.trailingpetunia.com.
Ready to build a more profitable greenhouse operation? Browse our full catalog of professional-grade bulk seeds at trailingpetuniabulkseeds.com and find the varieties that will make your next season your best one yet.