What to Plant First in a Pennsylvania Greenhouse

Understanding Pennsylvania climate and greenhouse types

Pennsylvania covers several hardiness zones and a variety of microclimates. In practical terms, winters are cold and long in the north and mountains, milder in the southeast, and variable in valleys and urban areas. That variation determines what you can start and when in a greenhouse. The single most important local datum to determine timing is your average last frost date. Regardless of county, work backward from that date in weeks for seed starting times rather than relying on a calendar date alone.
Greenhouses fall into three functional categories that change what you plant first:

unheated greenhouses that offer frost protection but not supplemental heat,
minimally heated greenhouses where small heaters prevent freezing but do not provide steady, high temperatures,
fully heated greenhouses that allow year-round warm-season production.

Your first choices for planting will depend on which type you have and the resources you are willing to invest in heat, light, and ventilation.

The seasonal strategy: cool-season first, warm-season later

A greenhouse in Pennsylvania is most powerful as an extension of the outdoor season. That means using the protected environment to start cool-season crops earlier and to give warm-season crops a head start before transplanting outside. The general strategy is:

Use your greenhouse in late winter and early spring to start hardy vegetables, cuttings, and flats of seedlings that can tolerate cool temperatures.
As temperatures and day length increase, progressively shift greenhouse territory toward more tender seedlings that need higher heat and light.
Use staging and succession planting to maintain continuous production and avoid crowding.

What to plant first: priority list with timing

Below is a practical priority list organized by crop type and relative timing using “weeks before last frost” as the guide. Adjust weeks depending on your greenhouse heating and local microclimate.

12 to 16 weeks before last frost:
Onions (from seed), leeks, shallot seeds. These need a long growing period and do best started early.
Perennials and woody ornamental cuttings that need early rooting and growth before outplanting.
8 to 12 weeks before last frost:
Tomatoes, peppers, eggplants (tomatoes 6-8 weeks is common but in colder parts of PA 8-10 weeks helps).
Basil and other warm-season herbs that require steady warmth to germinate.
6 to 8 weeks before last frost:
Cucumbers, squash, melons if you are transplanting; many growers direct-seed these outdoors, but greenhouse transplants can speed harvest.
10 to 2 weeks before last frost (cool-season crops can be sown anytime early):
Lettuce, spinach, arugula, mizuna, tatsoi, and other salad greens. These germinate in cool soils and can be seeded very early.
Brassicas: broccoli, cauliflower, cabbage, kale, collards. These prefer cool growing conditions and tolerate cool greenhouse temperatures.
As early as late winter in an unheated greenhouse:
Microgreens and cut-and-come-again herbs (parsley, chives). These are high-value and tolerate lower temperatures under protection.
Hardy annuals and bedding plants that prefer cool nights.

Use these ranges to stagger sowings for succession: for example, sow lettuce every 2-3 weeks to create a steady supply.

Practical greenhouse setup for getting seeds off to a strong start

Proper setup is often more important than the exact planting date. For Pennsylvania conditions, pay attention to the following practical details:

Soil and mixes: Use a sterile seed-starting mix for the first flats. These mixes are lighter, free of pathogens, and drain well. For later potting up, transition to a well-draining potting mix with added compost or a balanced slow-release fertilizer.
Containers: Use cell trays for small seeds and 3- to 4-inch pots for early pot-ups. For tomatoes and peppers, plan to move from small cells to 3-4 inch pots and then to 1-gallon pots prior to final transplant.
Heat: Bottom heat (seedling heat mats) is one of the most cost-effective investments for consistent germination of warm-season crops like tomatoes, peppers, and basil. Aim for 70-85 F soil temperatures for those seeds.
Light: Pennsylvania spring light is often bright but low-angle. Supplement with full-spectrum grow lights if seedlings become leggy. Provide 12-16 hours of light for most vegetable seedlings.
Ventilation and air circulation: Condensation and high humidity in cool months breed disease. Install thermostatically controlled vents or use fans to move air and reduce leaf wetness.
Watering: Water from the bottom when possible to minimize damping-off. Use tepid water and keep surfaces dry between watering events.

Preventing common early-season problems

Seedlings in Pennsylvania greenhouses face a few predictable problems. Address these proactively.

Damping-off: Causes include cool, wet conditions and contaminated mixes. Use sterile mixes, avoid overwatering, space trays for air circulation, and do not reuse flat-bottom containers without sanitizing.
Leggy seedlings: Often caused by insufficient light or temperature too warm relative to light. Lower night temperatures if possible, increase light intensity/duration, and pinch back or repot deeper to bury weak stems.
Mold and algae on mix surfaces: Improve air movement, reduce humidity, and avoid constant surface moisture. Scrape affected surfaces and replace top layer of mix if persistent.
Nutrient deficiencies: Seed mixes contain little nutrition. Start a weak liquid feed once true leaves appear. For transplants moving to larger pots, use a balanced fertilizer mixed at half strength.

Crop-specific notes: what to watch for

Tomatoes: Start 6-8 weeks before last frost in warmer parts of PA, 8-10 in colder zones. Keep soil warm for germination (75 F) and provide strong light and steady fertilizer later. Harden off gradually before transplanting outdoors.
Peppers and eggplants: Give extra lead time and keep them warmer (night temps above 60 F). They are slower than tomatoes and will benefit most from heat mats.
Brassicas: Germinate well at cool temperatures (45-65 F) and can be started very early. They can be grown in minimally heated greenhouses through early spring.
Lettuce and salad greens: Sow thinly in flats and harvest as microgreens or leaves. They thrive in cool greenhouse conditions and are ideal early-season crops.
Herbs: Parsley and cilantro can be sown early; basil should wait for warmer conditions unless you can supply heat.

Scheduling and space planning

A greenhouse is finite space; detailed scheduling will pay dividends.

Map your greenhouse space in zones: near the door (warmer/drier), center (most stable), and bench edges (cooler). Use these zones strategically.
Start long-lead crops in zones with supplemental heat or under lights early. Place the hardier crops in cooler bench edges.
Stagger sowings by weeks rather than calendar months. Record each sowing and expected transplant date relative to your last frost.
Use succession planting: after harvesting early salad greens from a bench, replace them with warm-season seedlings that need room before moving them outside.

Hardening off and transplant timing

Hardening off is critical in Pennsylvania where spring days can be warm and nights still freezing. Even greenhouse-grown plants can be shocked if moved outdoors too soon.

Begin hardening off 7-14 days before transplanting outdoors. Start with a few hours in a sheltered spot and gradually increase exposure to sun and wind.
Avoid transplanting outdoors until night temperatures consistently stay above the crop’s tolerance threshold (for tomatoes, ideally above 50 F).
If cold snaps are forecast, be ready to cover transplants or move them back into the greenhouse for protection.

Practical takeaways

Use the last frost date as your timing anchor. Convert desired germination and growth time into weeks before last frost rather than a fixed calendar date.
Prioritize long-lead crops (onions, peppers, tomatoes) early under heat; use unheated or minimally heated space for brassicas, salad greens, and microgreens early in the season.
Invest in simple tools: a seed-heat mat, a small fan for air movement, and a good-quality sterile seed mix deliver outsized returns in germination and disease prevention.
Schedule and zone your greenhouse: allocate space by temperature and light, stagger sowings, and rotate crops to maintain steady production.
Hardening off is not optional: gradual acclimation protects investment and improves field performance.

By planning with the local frost date, using the greenhouse microclimates intelligently, and prioritizing crops by their heat and time needs, you can maximize early yields and create a productive, year-round greenhouse practice in Pennsylvania.