What To Plant First In A Delaware Greenhouse

Growing in a greenhouse in Delaware gives you a valuable head start on the season, the ability to extend harvests into shoulder months, and a protected environment for tender crops. Deciding what to plant first depends on your greenhouse type (heated or unheated), the time of year, and whether you want early harvests or continuous production. This guide walks through practical, location-specific choices and a clear planting schedule, with concrete steps for soil, temperature, watering, pest control, and hardening off.

Delaware climate context and greenhouse types

Delaware lies roughly in USDA hardiness zones 6b to 7b, with coastal influence moderating winters and inland areas a little colder. Outdoor last frost dates typically occur from mid-April to early May depending on location. A greenhouse lets you start seeds weeks to months earlier than outdoor planting and protect crops from late cold snaps.
Consider which greenhouse you have:

• If heated, you can maintain steady warmth and start warm-season crops very early (late winter to early spring).
• If unheated or minimally heated, prioritize cool-season crops early and only start warm-season crops once outside temperatures rise or you can supplement heat and light.
• High tunnels or hoop houses are intermediate – they trap solar heat and extend seasons but still rely on sun and ambient temperatures.

Principles for choosing what to plant first

Start with crops that match the greenhouse environment you can reliably provide.

• If you can keep night temps above 60-65 F, you can start tomatoes, peppers, basil, and eggplant 6-8 weeks before the last frost.
• If your greenhouse is unheated and nights drop near freezing, begin with cool-season crops that tolerate 35-45 F nights: leafy greens, brassicas, peas, and root crops.
• Consider turnover and space: fast crops like lettuce and radishes give quick results and allow multiple successive plantings.
• Start slow-season transplants (tomatoes, peppers) as seedlings and not as immediate production. Use bench space effectively to raise many seedlings before transplanting.

What to plant first – concrete recommendations

Here are specific crops to plant first in a Delaware greenhouse by category and why they are good early choices.

Cool-season starters (best for unheated or minimally heated greenhouses)

• Lettuce and mixed salad greens – Germinate at cool temps, quick harvest (3-6 weeks for baby leaves), tolerate light frosts.
• Spinach and chard – Cold-hardy and produce early; can be direct-seeded or transplanted.
• Peas – Tolerate cool soil, early sowing allows early sugar snap or shelling pea harvest.
• Radishes and baby carrots – Fast maturing; good for early turnover and to use spotty bench space.
• Brassicas (broccoli, cabbage, kale, cauliflower) – Start 6-10 weeks before transplanting out for an early main-crop transplant.
• Onions and leeks – Start from seed in late winter to early spring or set out onion sets for early bulbs.

Warm-season starters (use heated greenhouse or start indoors early)

• Tomatoes – Start seeds 6-8 weeks before the last frost; require steady warmth and good light.
• Peppers and eggplant – Require warmer soil and air; start 8-10 weeks before last frost for strong transplants.
• Basil and other warm herbs – Start 4-6 weeks before last frost; bring in warmth and keep away from cold drafts.
• Cucumbers and melons – Often better started later to avoid damping off from cold, but can be started early in a warm greenhouse if vigorous.

Specialty early crops

• Microgreens – Extremely fast, harvest in 7-21 days, ideal for taking immediate advantage of greenhouse space.
• Cut-and-come-again salad mixes – Plant successive trays for continuous harvest.
• Early flowers (marigolds, calendula, cosmos) – Useful for companion planting and early pollinator support when moved outside.

A practical month-by-month greenhouse schedule for Delaware

Below is a pragmatic sequence you can adapt to your specific last frost date. Assume last frost in your area is around mid-to-late April; adjust a few weeks earlier or later accordingly.

• January – February:
• Start microgreens for quick harvest and to build your transplant routine.
• In a heated greenhouse, start tomatoes, peppers, eggplant, and basil seeds.
• Start onion seed indoors for transplants or grow onion sets.
• February – March:
• Start brassicas (broccoli, cabbage, cauliflower) for early transplanting out or in greenhouse beds.
• Sow lettuce and spinach in flats for staggered harvests.
• Start peas in pots or directly sow in cold frames; transplant later into greenhouse benches if needed.
• March – April:
• Continue succession sowings of salad greens and radishes every 2-3 weeks.
• Begin hardening off early transplants on sunny, protected days as outside temps rise.
• Transplant cool-season crops into greenhouse or outside protected beds once established.
• April – May:
• Transplant tomatoes and peppers into the greenhouse after risk of hard frost passes or when nights are reliably warm.
• Direct sow cucumbers, squash, and melons in the greenhouse if soil and air temperatures are warm.
• June – July:
• Plant heat-loving crops for summer production; monitor ventilation to avoid heat stress.
• Start second wave of brassicas in July for fall harvest.
• August – September:
• Transplant fall brassicas and succession-sow cold-tolerant salads for autumn and winter harvest.
• Prepare the greenhouse for fall by servicing heating and sealing drafts if you plan to overwinter crops.
• October – December:
• Grow winter greens and cold-hardy crops in unheated greenhouses, use row covers or cloches for extra protection.
• Start planning seed orders and bench rotation for the next year.

Soil, containers, and starting mix – practical details

Healthy seedlings start with a sterile, well-draining mix. Use a good commercial seed-starting mix or make your own: 1 part peat or coco coir, 1 part vermiculite or perlite, 1 part compost or well-aged potting mix.

• Pot sizes: use cells or 2-3 inch pots for most transplants; move to 4-6 inch pots for tomatoes and peppers before final planting.
• Fertility: seed mixes contain little fertilizer. Begin light feeding with a balanced soluble fertilizer once true leaves appear (dilute to half strength).
• Soil beds inside greenhouse: top-dress with compost and use sterile raised beds if possible to reduce disease pressure.

Temperature, light, and humidity control

• Temperature:
• Cool-season crops: optimal germination 40-70 F; growth 45-60 F nights, 55-70 F days.
• Warm-season crops: germinate best 75-90 F; grow at 65-80 F nights, 70-85 F days.
• Light:
• Provide strong, consistent light. In late winter/early spring, supplemental LED or fluorescent lighting for 12-16 hours may be necessary for stocky, healthy seedlings.
• Humidity and ventilation:
• Avoid stagnant humid conditions that encourage fungal diseases. Vent early and often on sunny days.
• Use circulation fans and automatic vents if possible to reduce humidity and strengthen seedlings.

Watering, disease prevention, and pests

• Watering:
• Water from the bottom for seedlings or use a fine mist; avoid waterlogging.
• Let mixes dry slightly between waterings to reduce damping off and root rot.
• Disease prevention:
• Use sterile trays and clean tools. Remove and destroy diseased seedlings immediately.
• Avoid watering overhead when leaves are cool and wet for long periods.
• Pests:
• Monitor for aphids, whiteflies, fungus gnats, and slugs. Introduce biological controls like predatory insects when appropriate.
• Sticky traps and neem or insecticidal soaps can help control outbreaks without harming seedlings.

Hardening off and transplant timing

• Hardening off:
• Gradually acclimate greenhouse-grown seedlings to outdoor conditions over 7-14 days before transplanting outside.
• Reduce water and slightly lower temperatures during hardening to toughen stems.
• Transplant timing:
• Wait to plant warm-season crops outdoors until night temps consistently stay above 50-55 F, or keep them inside the heated greenhouse.
• For greenhouse-grown production, transplant into greenhouse beds when seedlings have 3-6 true leaves and roots fill their pots.

Space management and succession planting

Efficient greenhouse production uses succession planting: sow a new tray of salad greens every 2-3 weeks, rotate crops to reduce disease buildup, and use quick crops like radishes to fill gaps between long-season transplants.

• Plan bench space by crop maturity: fast-turn crops occupy space briefly, while tomatoes and peppers need benches for months.
• Sell or consume early microgreens to free space for larger transplants if you run small operations.

Practical takeaways – quick checklist

• Start cool-season crops first in an unheated greenhouse: lettuce, spinach, peas, radishes, and brassicas.
• Use a heated greenhouse to begin tomatoes, peppers, basil, and eggplant 6-10 weeks before outdoor transplanting.
• Use sterile seed-start mix, provide adequate light, and maintain consistent temperatures for vigorous seedlings.
• Succession-sow fast crops every 2-3 weeks to maximize harvests from limited bench space.
• Harden off seedlings gradually before outdoor planting and monitor ventilation to prevent disease in warm months.

Final thoughts

The best first plantings in a Delaware greenhouse depend on your infrastructure and your goals: early market produce, a steady home supply of salad greens, or a season of tomatoes and peppers. Start with cool-season crops in unheated structures to see fast results, and use heated space for warm-season crops to take full advantage of early starts. With careful scheduling, sanitation, and environmental control, a Delaware greenhouse will let you plant earlier, harvest longer, and experiment across seasons with both edible and ornamental crops.