What To Plant For Early Spring Success In A Maine Greenhouse

Early spring in Maine presents both opportunity and challenge for greenhouse growers. Cold nights, variable last-frost dates, and limited natural sunlight mean planning, crop choice, and microclimate management determine whether you harvest a steady supply of greens and tender vegetables long before the outdoor garden can be planted. This guide gives practical crop recommendations, sowing schedules, environmental targets, and pest and cultural tactics that will let you maximize a small or medium-sized greenhouse during Maine’s early season.

Understand Maine timing and greenhouse advantage

Maine’s last frost date varies widely by location: coastal southern areas often experience last frost from mid-April to early May; inland and central regions commonly see last frost in late April to mid-May; northern and high-elevation areas can still have serious frosts into late May or early June. Use your local microclimate as your baseline, then add greenhouse protection to move your planting window earlier by several weeks.
A greenhouse gives three main advantages for early spring:

• raises average air and soil temperature compared with outdoors;
• reduces wind stress and desiccation on young seedlings;
• allows for controlled, staggered sowings for continuous harvest.

With modest heating or thermal mass and careful ventilation, you can grow cool-season crops from late winter through spring reliably.

Best crops for an early Maine greenhouse

Choose crops that tolerate or prefer cool soil and air, germinate at lower temperatures, or can produce quickly under moderate light. Prioritize high-value, fast-turnover crops that give repeated harvests.

• Salad greens: lettuce (many cultivars), spinach, arugula, mizuna, tatsoi, mustard greens.
• Cut-and-come-again mixes and baby greens.
• Brassicas for early transplant or for direct greenhouse finishing: kale, collards, broccolini, early cabbage, bok choy.
• Root crops suitable for greenhouse sowing: radishes (very fast), beets, early carrots (select short-root types), turnips.
• Alliums: green onions/scallions, bunching onions, leeks started early for late spring harvest.
• Peas: shelling and sugar snap varieties that tolerate cool soil.
• Herbs: parsley, cilantro, chives, dill (cold-tolerant); basil later when temps permit.
• Microgreens and sprouts for ultra-fast turnover and high value.

Practical sowing schedule and timing

Use the number of weeks before or after your local last frost date to schedule sowings in the greenhouse. These are general guidelines; adjust based on cultivar and greenhouse conditions.

• Lettuce, arugula, mustard, mizuna, baby greens: sow 8 weeks before to 2 weeks after last frost. You can sow continuously every 7-14 days for a steady supply.
• Spinach: sow 8-2 weeks before last frost. Spinach handles cooler soil and will bolt as temperatures rise; plan for succession.
• Kale and collards: start 6-8 weeks before transplanting outdoors, or sow in the greenhouse for winter-to-spring harvest.
• Broccoli, cabbage, cauliflower: start 6-8 weeks before last frost for greenhouse-grown transplants.
• Radish: sow in greenhouse 6-1 weeks before last frost for very fast harvests (often 3-4 weeks).
• Beets and carrots: sow 6-2 weeks before last frost; use deep containers for carrots or select short types.
• Peas: sow as early as 8 weeks before last frost in a greenhouse with cool soil; they prefer 40-60 F germination and will produce early vines.
• Microgreens: sow anytime; harvest 1-3 weeks depending on species.

For Maine, a practical calendar example:

• Coastal southern Maine: begin sowing hardy greens in late February-March; brassicas and onions in March; succession plant through April.
• Central Maine: shift everything 2-4 weeks later than the coast.
• Northern Maine: plan greenhouse sowings in April-May for many crops, with very hardy greens possible earlier if greenhouse nights are above freezing.

Soil mixes, containers, and propagation methods

Healthy seedlings start with a sterile, well-draining medium and clean trays or pots.

• Use a soilless mix with peat or coconut coir, perlite for drainage, and a small amount of compost or slow-release fertilizer. A mix labeled “seed-starting” is appropriate.
• Seed depth: follow packet recommendations, but as a rule: very small seeds (lettuce, celery family) on the surface or lightly covered (1/16″-1/8″); medium seeds (carrots, beets) 1/8″-1/4″; larger seeds (peas) 1/2″-1″.
• Containers: cell packs, soil blocks, and 4″ pots for brassicas work well. For direct greenhouse sowing, sow in wide flats or raised beds for easier harvesting of baby greens.
• Bottom heat (heat mats) speeds germination for many crops and reduces damping-off if paired with good air circulation. Target germination temps: lettuce 60-68 F; spinach 50-68 F; brassicas 60-70 F; peas 40-60 F.
• Label every tray with crop and sow date.

Temperature, light, and humidity targets

Early spring in Maine still has short days and low sun angles; light is often limiting.

• Day temperature targets: 55-70 F for cool-season vegetables. Aim for 10-15 F higher than outdoor daytime temperatures where possible.
• Night temperature targets: 40-55 F for hardy greens and brassicas. Avoid sustained night temperatures below freezing for long periods; aim to keep seedlings above 35-40 F.
• Soil temperature: most cool-season crops prefer soil in the 50-70 F range for consistent germination and root activity.
• Light: provide the brightest possible exposure in south-facing greenhouses. If you need supplemental light, use wide-spectrum LEDs placed close to seedlings for 12-16 hours daily for robust growth. However, many cool-season crops will do well with natural daylight if you limit sowing density and rotate trays for even exposure.
• Humidity: maintain relative humidity around 60-75% to prevent desiccation but ensure adequate ventilation. High humidity and poor airflow encourage damping-off and fungal diseases.

Watering, fertilization, and seedling care

• Water from below when possible to reduce leaf wetness; use a tray or capillary mat for even moisture.
• Avoid waterlogged media. Let the surface dry slightly between waterings for most seedlings.
• Fertilize after the first true leaves appear. Use a half-strength soluble balanced fertilizer (e.g., 10-10-10 equivalent) and increase to full strength as plants grow and root systems develop.
• Thin seedlings to recommended spacings to avoid competition and disease.
• For brassicas, transplant when plants have 4-6 true leaves; for lettuce, 3-4 leaves or when root ball holds together.

Pest and disease management

Greenhouses reduce but do not eliminate pests and diseases. Early spring pests include aphids, fungus gnats, slugs, and mice; cool, damp conditions favor fungal diseases.

• Prevention: clean trays and tools, sterilize old pots, and practice crop rotation if you use multiple seasons per year.
• Ventilation and air circulation (oscillating fans) reduce humidity pockets and prevent damping-off.
• For aphids and whiteflies: inspect regularly, use sticky traps, and spray with insecticidal soap when necessary.
• For fungus gnats: allow surface to dry between waterings, use well-draining media, and consider biological controls (entomopathogenic nematodes) if infestation is severe.
• Row covers inside the greenhouse can protect tender transplants from sudden pest influxes.
• Slugs and snails: use physical barriers, hand-pick, and copper tape around benches or beds.

Succession planting and maximizing harvests

Plan for continual harvests rather than one large sowing. Succession planting keeps your greenhouse productive.

• Sow quick crops like radish and baby greens every 7-14 days.
• For lettuce and mixed greens, sow small batches weekly to create a staggered harvest.
• Space brassica sowings 2-3 weeks apart if you want multiple transplant waves for outdoor planting or continued greenhouse production.
• Use microgreens to fill gaps in labor or to generate quick income in 7-21 days.

Transitioning to outdoors and hardening off

Greenhouse-grown transplants need a careful hardening-off period before going outdoors.

• Start hardening off two weeks before transplanting to the garden: bring plants to a cold frame or open the greenhouse ventilation progressively during the day; reduce fertilizer and water slightly to toughen tissue.
• Gradually increase outdoor exposure over 7-14 days, starting with a few hours in sheltered shade and ending with full sun and overnight exposure if temperatures permit.
• Plant outdoors on a suitable day with mild weather and water transplants in well.

Quick checklist for early spring greenhouse success

• Choose cool-tolerant crops: lettuce, spinach, kale, radish, peas, scallions, microgreens.
• Sow in succession every 7-14 days for steady harvests.
• Use a sterile seed-starting mix, clean trays, and label everything.
• Target daytime temps 55-70 F and nights 40-55 F; manage thermal mass and supplemental heat as needed.
• Provide good light, reduce seedling density, and use fans for airflow.
• Water from below, fertilize after true leaves, and avoid overwatering.
• Inspect for pests weekly and act early with cultural or mechanical controls.
• Harden off transplants for 7-14 days before moving outdoors.

Final practical takeaways

Early spring greenhouse growing in Maine is about matching crop choice to your microclimate, controlling temperature and moisture, and sequencing sowings. Invest a small amount of effort in sanitation, label and schedule, and the right mixes and containers, and your greenhouse will produce fresh, marketable greens and early vegetables long before the outdoor growing season begins. With a consistent weekly routine of sowing, monitoring, and ventilation, you can convert a cold Maine spring into a productive period that sets the tone for the entire season.