What To Grow In New Hampshire Greenhouses For Year‑Round Herbs

Introduction

Growing herbs year-round in New Hampshire greenhouses is an achievable and profitable objective if you match plant selection with proper environmental management. Cold winters and short winter days require focused choices about species, heating, lighting, and production scheduling. This guide explains which herbs perform best in New Hampshire greenhouses, how to grow them effectively, and how to set up a year-round production plan that balances quality, energy use, and market demand.

New Hampshire greenhouse realities

New Hampshire climate factors that affect year-round greenhouse herb production include long, cold winters, variable humidity, and short daylight hours in December and January. Even with a well-built greenhouse, you will need supplemental heating and lighting to maintain consistent growth. Insulation, thermal curtains, and efficient heating systems reduce operating cost and smooth temperature swings.
Typical target greenhouse ranges for year-round herbs:

• Day temperature: 65-75 F for many culinary herbs; basil prefers 70-80 F.
• Night temperature: 55-65 F; avoid nights below 50 F for tender herbs.
• Relative humidity: 50-70% depending on species; mint tolerates higher RH, while rosemary and thyme prefer lower RH.
• Light: 12-16 hours of effective light for many herbs in winter; supplemental LED or high-pressure sodium light may be required.

Best herbs to grow year-round in New Hampshire greenhouses

Choose herbs that tolerate greenhouse conditions, have steady market demand, and are manageable in space. Below are herbs grouped by ease, marketability, and cultural needs.

Easiest and most profitable herbs

• Basil (Ocimum basilicum)
• Culture: Warm, high light; 70-78 F day, night above 60 F.
• Notes: Fast growth, high turnover; multiple harvests through pruning.
• Varieties: Genovese, Thai, lemon basil.
• Mint (Mentha spp.)
• Culture: Prefers moisture and moderate temperatures; 60-70 F day.
• Notes: Vigorous and invasive — use containers; steady market for fresh bunches and potted herbs.
• Parsley (Petroselinum crispum)
• Culture: Cooler tolerant; 60-70 F day; long production cycle but steady demand.
• Notes: Flat-leaf and curly types; good for pot sales and bunching.

Mediterranean herbs (drier conditions)

• Rosemary (Rosmarinus officinalis)
• Culture: Likes warm, drier conditions; good drainage; 65-75 F.
• Notes: High-value potted plant; slow-growing but long shelf life.
• Thyme (Thymus vulgaris)
• Culture: Dry, warm; minimal fertilizer; excellent for potted herbs.
• Notes: Low maintenance, good for mixed herb pots.
• Oregano (Origanum vulgare)
• Culture: Similar to thyme; tolerates moderate humidity; 65-75 F.
• Notes: Robust flavor, popular in culinary markets.

Cooler-season and specialty herbs

• Cilantro/Coriander (Coriandrum sativum)
• Culture: Prefers cooler temps; 50-68 F; bolt-prone in heat.
• Notes: Grow in shaded or cooler greenhouse areas; stagger plantings.
• Chives (Allium schoenoprasum)
• Culture: Tolerant of wide temps; 50-70 F; productive in pots.
• Notes: Perennial in pots; good for market bunches.
• Lemon balm (Melissa officinalis)
• Culture: Prefers cool to moderate temps; 60-70 F; moderate humidity.
• Notes: Aromatic, used fresh or dried; spreads easily in bench containers.

Medicinal and specialty herbs

• Lavender (Lavandula spp.)
• Culture: Likes warm, dry conditions; high light; verify varieties suited to greenhouse.
• Notes: Good value for dried bundles and potted plants.
• Stevia (Stevia rebaudiana)
• Culture: Warm, humid tolerant; 70-85 F; good niche crop for sweeteners.
• Notes: Year-round production possible with proper light.

Propagation, containers, and substrates

Propagation methods and container choices affect throughput and labor.

• Propagation: Use seed for cost-effective crops (parsley, cilantro, basil). Use cuttings for true-to-type and faster production (rosemary, mint, basil). Maintain a propagation chamber at slightly warmer temps (72-78 F) with bottom heat for cuttings where appropriate.
• Containers: Use 4-6 inch pots for individual plants; 6-8 inch pots or trade gallon for potted herbs with longer production cycles. Use mezzanine benches for small pots to maximize space.
• Substrates: Use a well-draining, stable mix with 60-80% peat or coir, perlite, and a slow-release fertilizer for potted herbs. Adjust mix for Mediterranean herbs to be grittier and faster draining.

Environmental control: light, heat, and humidity

Maintaining stable environmental parameters is the core of year-round success.

• Light: In December/January, supplement with 12-16 hours of light using LEDs optimized for plant production. Aim for PPFD levels appropriate to the crop (basil and lavender require higher light than parsley or chives). Use timers and dimming to control energy use.
• Heating: Use insulated glazing and thermal curtains to reduce heat loss at night. Consider a dual heating strategy: a primary boiler or unit heater plus localized heat mats or soil warming for propagation. Maintain minimal night setback to prevent chilling-sensitive herbs from bolting or slowing.
• Ventilation and humidity: Use automated ventilation and fans to manage humidity and prevent fungal diseases. Target 50-70% RH; higher humidity improves cuttings but increases disease risk for foliage crops. For Mediterranean herbs, keep RH lower and increase air movement.

Fertilization and irrigation

Herbs are sensitive to overwatering and excess nitrogen.

• Fertilizer: Use balanced fertilizers (N-P-K 20-20-20 or similar) in moderation for fast-growing herb crops. For potted Mediterranean herbs, use lower nitrogen, higher potassium formulations. Slow-release fertilizers in substrate help reduce daily feeding.
• Irrigation: Employ drip or ebb-and-flow systems for high-volume production. Water when the top 1 inch of media is dry for most herbs; rosemary and thyme need less frequent water. Avoid overhead watering in winter to reduce disease.

Integrated pest and disease management

Greenhouses create ideal conditions for certain pests and diseases. A proactive IPM plan is essential.

• Common pests: Whiteflies, aphids, spider mites, fungus gnats. Monitor weekly with yellow sticky traps and visual inspection.
• Common diseases: Botrytis, powdery mildew, root rot. Prevent with sanitation, adequate air movement, and avoiding overwatering.
• Controls: Use biological controls (predatory mites, parasitic wasps) where possible and targeted insecticidal soaps or oils when thresholds are exceeded. Rotate plant sources and sanitize tools to minimize pathogen introduction.

Harvesting, postharvest, and product presentation

Harvest practices influence shelf life and marketability.

• Harvest timing: Harvest in morning after dew dries for best essential oil content and flavor. For basil and mint, pinch above leaf nodes to encourage bushy regrowth.
• Postharvest: Cool harvested herbs quickly to 35-45 F for fresh bunches where appropriate. Keep Mediterranean herbs at slightly warmer temperatures (45-55 F) to avoid chilling damage.
• Presentation: Offer mixed herb pots, single-species pots, bunches, and microgreens to broaden market. Label plants with cultivar and usage notes for retail customers.

Scheduling and batch planning for year-round supply

Stagger plantings and maintain a rolling schedule to provide continuous product.

• Staggered planting: For high-turnover herbs like basil, seed every 2-3 weeks. For slower herbs like rosemary, produce on a 12-16 week cycle with periodic propagation.
• Batch sizes: Determine bench space and pot count per crop based on sales velocity. Keep an active propagation area that feeds 10-25% of your production space.
• Example 12-week rotation for basil:
• Week 0: Seed cells or plug trays.
• Week 2-3: Transplant to 4-inch pots.
• Week 5-7: First harvest/pinching for bushy growth.
• Week 8-12: Final harvests, replace with new plugs.

Energy and cost considerations

Heating and lighting are the largest expenses for year-round greenhouse production in New Hampshire.

• Energy efficiency: Insulate north walls, use double glazing, install thermal curtains, and group crops by environmental needs to avoid over-conditioning large areas.
• Cost-benefit: Prioritize high-value crops for winter months (basil, rosemary, lavender) and move lower-value crops to spring/summer when natural light is abundant.

Practical takeaways

• Choose a mix of fast-turnover herbs (basil, mint, parsley) and slower, high-value Mediterranean herbs (rosemary, thyme, lavender).
• Maintain day temps in the mid 60s to low 70s for most herbs; use higher temps for basil and stevia, lower temps for cilantro and parsley.
• Provide 12-16 hours of supplemental light in winter and target appropriate PPFD for each crop type.
• Propagate by cuttings for consistency and speed; use seed for cost-effective bulk production.
• Stagger plantings on a regular schedule to ensure continuous supply and optimize bench use.
• Implement IPM and maintain airflow to prevent fungal diseases in high-humidity winter periods.
• Use insulation, thermal curtains, and efficient lighting to minimize operating costs in cold months.

Final recommendations

Start by trialing 3-5 herb varieties that match your greenhouse microclimate and local market demand. Track production metrics — time to harvest, yield per pot, sales price — and calculate energy costs per crop to refine your winter lineup. Over time, you can expand into value-added products (dried bundles, potted mixes, culinary gift packs) and optimize greenhouse zones for specific herb groups to maximize year-round profitability in New Hampshire.