What to Plant in a Maryland Greenhouse Each Season

Greenhouses transform Maryland’s variable climate into a controlled environment that extends growing seasons, improves yields, and allows cultivation of crops that might otherwise struggle outdoors. This guide gives practical, location-aware recommendations for what to plant in a Maryland greenhouse each season, plus temperature targets, propagation timing, variety suggestions, and management strategies that produce consistent results.

Greenhouse essentials for Maryland growers

A successful greenhouse harvest begins with the basics: temperature control, ventilation, light management, water and growing media, and sanitation. Getting these right creates a flexible environment for both cool-season and warm-season crops.

Temperature and humidity targets

Maintaining appropriate temperatures and humidity is the most important daily task in a greenhouse.

Seedlings and most transplants: daytime 65-75degF, nighttime 55-65degF.
Warm-season vegetables (tomatoes, peppers, eggplant, basil): daytime 70-85degF, nighttime 60-70degF.
Cucumbers and squash: daytime 70-80degF, nighttime 60-68degF.
Cool-season greens (lettuce, spinach, kale, arugula): daytime 50-65degF, nighttime 40-50degF.
Overwintering hardy plants: maintain 45-55degF minimum to prevent frost and injury.
Relative humidity: aim for 50-70%; high humidity (>75%) increases fungal disease risk, while low humidity stresses some tropical plants.

Use thermostatically controlled heaters for winter, automatic vent openers and exhaust fans for hot days, and shade cloth (30-50%) for peak summer sun. Consider thermal mass (water barrels, concrete) and bubble-wrap insulation to stabilize night temperatures in winter.

Light and supplemental lighting

Maryland winters have short days and lower sun angles. For winter greens and seedlings, provide supplemental fluorescent or LED grow lights for 12-16 hours per day depending on crop. Warm-season crops need full sun in spring/summer; add 30-50% shade when temps exceed recommended daytime targets.

Growing media, pots and watering

Use sterile, well-draining potting mixes for seedlings and containers. For beds or ground soil in a greenhouse, amend with compost and ensure good drainage. Water early in the day to allow foliage to dry and reduce fungal problems. Bottom-watering or wick systems reduce leaf wetness. Fertilize with balanced soluble feed (e.g., 10-10-10 or 20-20-20) or slow-release granular fertilizer according to crop needs.

Sanitation and airflow

Sanitize benches and tools between crops, remove old plant debris, and space plants to allow air movement. Good horizontal and vertical airflow reduces disease pressure and improves crop vigor.

Seasonal planting guide: what to grow and when

Maryland spans USDA zones roughly 5b to 8a, with coastal southern Maryland earlier by a few weeks and higher elevation western counties later. Adjust “weeks before/after last frost” accordingly. As a rule, know your local last frost date and plan relative to it.

Spring (late winter through May)

Spring is about starting warm-season crops early, producing early cool-season harvests, and planting quick-turn crops.

Start seeds indoors or on greenhouse benches early:
Tomatoes: start 6-8 weeks before intended transplant date. For Maryland, start in mid-February to early March for spring transplants if your last frost is mid-April to early May.
Peppers and eggplants: 8-10 weeks before transplant due to slower growth.
Basil and tender herbs: 6-8 weeks before transplant.
Cool-season crops grown for early harvest:
Lettuce, spinach, arugula, mizuna, and other salad greens — sow in late winter/early spring for harvest as soon as light and temperatures allow.
Peas and radishes — quick, cold-tolerant crops that can go in early spring for a fast turnover.
Suggested spring varieties and tips:
Tomatoes: ‘Early Girl’, ‘Sungold’, ‘Stupice’ for earlier fruiting.
Lettuce: butterhead and looseleaf varieties such as ‘Buttercrunch’ and ‘Rouge d’Hiver’ for greenhouse success.
Peas: sugar snap or snow peas trained on trellises.
Practical takeaways:
Harden off transplants for several days before moving outdoors if they will go outside — reduce humidity and gradually expose to cooler night temps.
Use row covers inside the greenhouse for an extra frost buffer in late spring cold snaps.

Summer (June through August)

Summer is for full production of warm-season crops, continuous harvesting, and managing heat and humidity.

Best summer greenhouse crops:
Tomatoes — indeterminate varieties trained vertically to maximize space.
Cucumbers — pick compact or parthenocarpic varieties bred for greenhouse use to avoid pollination issues.
Peppers and eggplants — thrive with steady warmth.
Basil, cilantro (morning harvest to reduce bolting), and other culinary herbs.
Bush beans and yardlong beans for trellis systems.
Heat management:
Use shade cloth on hot days and keep vents/fans running. Temperature spikes above 90degF stress tomatoes and peppers, reducing fruit set.
Increase ventilation and use evaporative cooling if needed.
Water and fertility:
Consistent moisture and regular feeding (weekly soluble fertilizer or drip fertigation) prevent blossom end rot in tomatoes and tip burn in greens.
Pest watch:
Summer is peak pest season — scout weekly and use sticky traps, beneficial insects (ladybugs, predatory mites), or targeted organic controls.

Fall (September through November)

Fall is ideal for a second harvest and for extending cool-season production into colder months.

Autumn greenhouse crops:
Fast-maturing lettuce and salad mixes sown in late summer to early fall for fall harvest.
Spinach, Swiss chard, kale, and beets for cold-tolerant greens and roots.
Asian greens (bok choy, tatsoi) and radishes for quick fall crops.
Late tomatoes and peppers: continue harvesting indeterminate tomatoes and ripen green fruit by maintaining warm greenhouse conditions into early fall.
Overwintering and cover crops:
Sow cover crops (hairy vetch, clover) in benches or beds to restore nitrogen and organic matter if you plan to clean out and rest soil.
Bring in tender potted perennials (roses, figs, citrus) before first hard frost and maintain minimal heat for overwintering.
Practical tips:
Start insulating early — apply bubble-wrap to north walls and check seals for drafts.
Reduce nutrient strength slightly to slow overvigorous growth and focus on hardening plants for cooler weather.

Winter (December through February)

Winter greenhouse gardening in Maryland is about careful crop selection and energy-efficient climate control.

Best winter greenhouse crops:
Cold-tolerant greens: mache (corn salad), winter-hardy lettuce, spinach, and perpetual spinach.
Microgreens and sprouts: fast turnarounds and high value; can be grown on trays under lights.
Herbs: parsley, chives, thyme, and cold-tolerant rosemary if temperatures are kept above 45-55degF.
Citrus and tender exotics: with supplemental heat and light, you can maintain lemons, limes, kumquats, figs, and even dwarf olives, but expect increased energy costs.
Energy and light strategies:
Use thermal mass and insulation to reduce heating needs. Night temperatures of 45-55degF are adequate for hardy greens and significantly reduce fuel use.
Supplement light to reach 12-14 hours per day for leafy production and to keep seedlings from becoming leggy.
Practical winter takeaways:
Focus on high-value, low-energy crops like microgreens and leafy herbs.
Plan succession plantings and use flats of seedlings under lights so you can transplant quickly when a warm spell appears.

Propagation, timing and practical schedules

Propagation timing in a greenhouse is flexible, but success depends on predictable schedules.

Determine your local last frost date and work backward for seed starting: tomatoes 6-8 weeks, peppers 8-10 weeks, eggplant 8-10 weeks, basil 4-6 weeks, lettuce and spinach 3-4 weeks for transplant or direct-sow early.
Seed depth and pot size: sow most seeds at 2-3 times their diameter. Use 2- to 3-inch plug cells for lettuce and herbs, 4-inch pots for larger seedlings, and 1- to 2-gallon pots for slow-developing peppers before transplanting.
Harden off and transplant: reduce humidity, lower daytime temps for a week, and introduce outdoor light gradually if moving plants outside.
Successive sowing: plan rotations every 2-3 weeks for lettuce and radishes in spring and fall for continuous harvest.

Pests, diseases and greenhouse maintenance

Good management keeps production high and reduces losses.

Common greenhouse pests in Maryland and responses:
Aphids: spray with insecticidal soap, release lady beetles, or use reflective mulches early.
Whiteflies: sticky traps, parasitic wasps (Encarsia), and strong air movement to reduce populations.
Spider mites: keep humidity up to discourage outbreaks; use predatory mites or miticides if needed.
Fungus gnats: avoid overwatering, use sticky traps, and apply biological control (Bacillus thuringiensis israelensis or predatory nematodes).
Disease prevention:
Practice strict sanitation: remove diseased plants, rotate crops, use clean potting media, and disinfect tools and benches between plantings.
Reduce leaf wetness: water at the base and provide good airflow to prevent botrytis and powdery mildew.
Use resistant varieties where possible and avoid overcrowding.

Year-round practical takeaways

Keep detailed records: planting dates, germination rates, varieties, pest outbreaks, and harvest yields. Records will refine timing for your microclimate.
Prioritize energy-efficient strategies in winter: thermal mass, bubble-wrap insulation, and choosing crops that tolerate lower temps reduce heating costs.
Plan succession crops and staggered planting for continuous production rather than single large plantings that all mature at once.
Start with reliable, compact varieties adapted for greenhouse culture before experimenting with more exotic plants.
Use integrated pest management: combine cultural practices, biological controls, and targeted organic products rather than relying solely on pesticides.

Growing in a Maryland greenhouse gives you the power to harvest earlier, extend the season, and diversify the crops you produce. By matching plant choices to seasonal conditions, maintaining good environmental control, and following the practical propagation and pest-management steps outlined here, you can achieve steady, productive harvests year after year.