When To Start Seeds In New Hampshire Greenhouses

Understanding Last Frost Dates in New Hampshire

New Hampshire spans a range of climates from coastal, relatively mild zones to cold, high-elevation zones. That variation means there is no single “right” date to start seeds across the state. The most important anchor for any seed-start calendar is your local average last frost date. Use that date as your target transplant date and count backward the number of weeks each crop needs in your greenhouse before it can safely go to the garden.

Regional last frost ranges (practical guidance)

Southern New Hampshire (coastal and southern counties): average last frost typically falls in mid to late May, but can range from early May to late May depending on microclimate and year.
Central New Hampshire (Manchester, Concord area): average last frost generally falls in late May, often between May 15 and May 31.
Northern New Hampshire and higher elevations: last frost commonly arrives in late May into mid June. Mountain sites can see frosts well into June.
Always verify your microclimate. A sheltered town in the southerly coastal strip may reliably be a week or more milder than an exposed inland valley. Greenhouse growers should maintain a record of local frosts and successful transplant dates to refine timing year to year.

Why a greenhouse changes the math

A greenhouse lets you control temperature, moisture, and often light. Those controls allow you to begin seed germination earlier and grow stronger transplants than direct indoor windowsill starts. However, a greenhouse is not a substitute for understanding cold hardiness and timing.
Key greenhouse advantages:

• You can heat soil and air to ideal germination temperatures so seeds sprout faster and more uniformly.
• You can keep seedlings drier and provide ventilation, reducing damping-off and fungal problems.
• You can harden seedlings in the greenhouse bench or an attached cold frame before moving them outside.

Limitations to remember:

• Overheating, especially during sunny spring days, can cause leggy growth and stress. Ventilation and shade cloth matter.
• A heated greenhouse encourages earlier pests and diseases if sanitation is lax.
• Transplant shock remains a risk if you expose greenhouse-grown plants prematurely to cold nights, wind, or bright unfiltered sun.

Seed-start timing: general rules by crop

The simplest way to plan is: determine your last frost date, then count backwards the number of weeks recommended for each crop. The weeks listed below are common ranges for greenhouse-started transplants. They assume starting from seed and growing plants to a robust transplantable size.

• Tomatoes: 6-8 weeks before last frost. Provide night temperatures around 55-65 F and day temps 70-75 F once seedlings emerge.
• Peppers and eggplants: 8-10 weeks. These require warmer germination temps (80-85 F soil) and more time to reach good size.
• Brassicas (broccoli, cabbage, cauliflower): 6-8 weeks. These are cool-season and tolerate earlier outdoor planting or hardening.
• Onions from seed (long-day varieties): 10-12 weeks to get sturdy sets or transplants.
• Lettuce and leafy greens: 4-6 weeks. Can be succession-sown every 2-3 weeks early in the season.
• Cucumbers, summer squash, melons: 2-4 weeks for greenhouse transplants. These set deeper roots and resent being started too early because they prefer warm soil outdoors.
• Peas: generally direct sow outdoors as soon as soil can be worked; start in greenhouse only if you need an early crop and can harden well.
• Beans: direct sow after last frost; do not start too early indoors–beans dislike root disturbance.

Practical seed-start calendar examples for New Hampshire growers

The following examples use three hypothetical last frost dates that represent common New Hampshire situations. To use them, replace the example last frost date with your local average and count back the weeks shown.
Example A – Southern NH, last frost May 15:

• Start peppers and eggplants: sow Feb 27 to Mar 5 (approximately 10-12 weeks before).
• Start tomatoes: sow Mar 20 to Apr 3 (6-8 weeks before).
• Start brassicas: sow Mar 20 to Apr 3 (6-8 weeks before), or earlier for large transplants.
• Start lettuce and greens: sow Apr 3 to Apr 17 (4-6 weeks before).
• Start cucurbits and squash: sow Apr 24 to May 1 (2-3 weeks before), or sow directly outdoors after last frost.

Example B – Central NH, last frost May 25:

• Peppers and eggplants: sow Mar 11 to Mar 25.
• Tomatoes: sow Apr 1 to Apr 11.
• Brassicas: sow Apr 1 to Apr 11.
• Lettuce: sow Apr 11 to Apr 25.
• Cucurbits: sow May 1 to May 9.

Example C – Northern NH/high elevation, last frost June 5:

• Peppers and eggplants: sow Mar 25 to Apr 8.
• Tomatoes: sow Apr 11 to Apr 25.
• Brassicas: sow Apr 11 to Apr 25.
• Lettuce: sow Apr 25 to May 9.
• Cucurbits: sow May 15 to May 22.

These are starting guidelines. If you run a heated greenhouse and can raise night temps and soil temps reliably, you can shift schedules earlier by one to three weeks, but only if you can harden off and protect transplants from late cold snaps.

Soil temperature, light, and humidity details

Germination happens best when soil temperatures match species-specific needs. Use a soil thermometer and heat mats where needed.

• Tomatoes: germinate best at 75-85 F; grow at 65-75 F.
• Peppers and eggplants: germinate best at 80-85 F; grow at 70-75 F.
• Brassicas: germinate well at 60-70 F; grow at cooler day temps, 55-65 F.
• Lettuce: germinates at 50-70 F; grow at 55-65 F to avoid bolting.
• Cucurbits: germinate at 75-90 F; prefer warm soil and air.

Light: Provide strong light as seedlings develop. In cloudy New England springs, natural light in a greenhouse may be sufficient but still monitor for legginess. Supplemental LED or fluorescent grow lights can help maintain compact growth and are particularly useful for peppers and eggplants that need longer warm periods to develop well.
Humidity and air movement: Keep humidity moderate and provide steady air movement to prevent damping-off and fungal disease. Ventilation, oscillating fans, and staged dehumidification are good practices.

Staging, potting up, and hardening off

Seedlings should be potted up once they reach the size where roots fill their cells or if they become root-bound. Harden seedlings gradually before transplanting outdoors: reduce greenhouse night temperatures, increase air flow, and expose them to outdoor conditions over 7-14 days. Hardening is essential in New Hampshire because late cold snaps and wind are common.
Hardening protocol example:

• Day 1-3: set plants in a sheltered, shaded spot outdoors for 2-3 hours, then bring back inside.
• Day 4-7: increase outdoor exposure by 2-3 hours each day and add morning sun.
• Day 8-10: leave plants out overnight only if temperatures are above 40 F; otherwise protect overnight.
• Day 11-14: plants should tolerate a full day in the garden if temperatures are stable.

Common problems and fixes in greenhouse seed starting

Damping-off: caused by overly wet conditions and poor air flow. Use fresh sterile mix, avoid overwatering, increase air circulation, and remove infected seedlings promptly.
Leggy seedlings: usually from insufficient light or too-high temperatures during germination. Lower night temperatures, provide more light, and use a cooler germination temperature where possible.
Root-bound transplants: pot up earlier and use deeper containers for long-running seedlings like peppers and onions.
Cold shock at transplant: always harden off and avoid setting out plants during wind, frost threats, or cold rain. Use row covers, cloches, or low tunnels when marginal nights are expected.

Practical checklist for New Hampshire greenhouse growers

• Know your local last frost average and maintain a log of the season.
• Create a master calendar: list each crop and the sowing date calculated by counting back the recommended weeks.
• Control soil temperature for germination with heat mats for warm-loving crops.
• Monitor greenhouse temps daily and ventilate on sunny spring days to avoid overheating.
• Use sterile, fast-draining seed mix and clean trays to reduce disease risk.
• Harden off transplants for 7-14 days before planting outdoors.
• Stagger sowings to avoid a single glut of transplants and to provide continuous harvests.

Final practical takeaways

1. Use your local last frost date as the starting point; adjust for microclimate and yearly variability.
2. Tomatoes 6-8 weeks, peppers and eggplants 8-10 weeks, brassicas 6-8 weeks, onions 10-12 weeks, and cucurbits 2-4 weeks are reliable rules of thumb in New Hampshire.
3. Heated greenhouses let you start earlier, but only if you manage temperature, light, and hardening to prevent transplant shock.
4. Keep records each season to refine your timing for your specific site; that accumulated local knowledge is the most valuable tool a grower has.

With careful planning and attention to greenhouse environment, New Hampshire growers can successfully start seeds that lead to healthy transplants and robust harvests despite the variability of spring weather.