When To Start Herbs For Year-Round Harvest In A Maine Greenhouse

Growing herbs year-round in Maine requires timing, environmental control, and a plan for succession. This article explains when to start seed and cuttings, how to group herbs by temperature and light needs, a practical monthly schedule for staggered sowings, and concrete cultural details that will let you harvest fresh herbs every week of the year from a heated greenhouse in Maine.

Maine climate and greenhouse basics: why timing matters

Maine has long, dark, cold winters and relatively short, intense summers. Outdoors, most annual culinary herbs will die back with frost, and many Mediterranean perennials struggle with winter cold and low light. A greenhouse changes the equation by letting you control temperature, humidity, and light. But timing still matters because each herb has a predictable rate of growth from seed or cutting to harvest, and because greenhouse energy and light limitations force tradeoffs.
Key greenhouse variables you must manage and understand for timing:

Temperature: day and night targets for each herb group.
Light: sun plus supplemental lighting hours and intensity.
Starting method: seed speed versus cuttings/liners.
Succession cadence: how often to sow to maintain steady harvests.
Space management: bench rotation, pot sizes, and spacing for harvestable plants.

Temperature and light targets by herb group

Understanding which herbs are warm-season, cool-season, or perennial Mediterranean types will determine when to start them and how to maintain them through Maine winters.

Warm-season annuals (need warmth and light)

These herbs grow quickly but suffer below 55 F night or weak light: basil, Thai basil, lemon basil, marjoram, dill (prefers moderate temps but bolts in heat), and savory.

Daytime: 70-85 F.
Nighttime: not lower than 55-60 F to avoid stunting.
Light: 14-16 hours/day of combined sun and supplemental light in winter.

Cool-season annuals (tolerate cool temps)

Parsley, cilantro (prefers cool and bolts in heat), chervil, and some mints. These can be grown at lower temperatures and are useful for late fall and winter harvest if prioritized.

Daytime: 55-70 F.
Nighttime: 40-50 F acceptable for many, but avoid repeated freezes.
Light: 12-14 hours/day ideally.

Mediterranean perennials (require warm light but tolerate drought)

Rosemary, thyme, oregano, sage, lavender. These are slower growers, often better started from liners or cuttings, and can be maintained year-round if you supply moderate heat and good light.

Daytime: 60-75 F.
Nighttime: 50-60 F to avoid cold stress.
Light: high-quality light is critical in winter; 12-16 hours/day for dense growth.

Start times: general rules and concrete windows

The most practical way to think about start times is working backwards from desired harvest plus the herb’s time-to-harvest. Below are common windows for a heated, lit Maine greenhouse. These assume you maintain appropriate temperatures and supplemental lighting during short days.

Basil: 4-8 weeks from seed to first substantial harvest. Start seeds 4-6 weeks before your first desired harvest for baby leaf; 6-8 weeks for larger plants. For continuous winter harvest, sow every 3-4 weeks.
Parsley: 3-4 weeks to germinate, 8-12 weeks to first full harvest. Start 8-12 weeks ahead of desired harvest. Stagger sowing every 6-8 weeks for continuity.
Cilantro: 3-7 weeks to harvest baby leaves; bolts in heat. Start every 2-4 weeks during cool seasons. For winter, grow under cool conditions and shade from greenhouse heat.
Chives: 6-10 weeks from seed to harvest; divide or start from bulbs/crowns for faster production. Start 8-10 weeks ahead.
Thyme, oregano, sage: slow from seed (12-20 weeks). Start from cuttings or liners in late summer to early fall for winter production, or start seeds 3-4 months early if you have space.
Rosemary: difficult from seed; take cuttings and root them 6-10 weeks before you need harvest-quality sprigs. Maintain warm bright conditions.
Mint: vigorous from cuttings; start anytime. Divide established plants in fall or spring for quick production.

Practical month-by-month plan for year-round harvest in Maine (heated greenhouse)

Below is a pragmatic schedule for a greenhouse kept roughly 55-70 F depending on the season and supplemented with lights in winter. Adjust timing earlier or later depending on your actual greenhouse temps and light.

October – November:
Start perennials from cuttings or liners: rosemary, thyme, oregano, sage, lavender. Cuttings root best at warm bench temps (65-72 F) with bottom heat.
Sow cool-season annuals for winter harvest: parsley and chives sown now will produce for late fall into winter.
Plant mint and divide clumps; pot up for year-round harvest.
December – January:
Start basil only under supplemental lights and heated benches; use 4-6 week schedule for microgreen/baby leaf harvest. If you want mature basil plants for winter, start in December with strong lights and maintain 70-75 F.
Continue staggered sowings of parsley every 6-8 weeks if you rely on it heavily.
Root rosemary and thyme cuttings taken in October-November for transplanting to production pots.
February – March:
Sow basil, dill, marjoram, and other warm herbs if you have adequate lighting. Start seeds 6-8 weeks for full-size plants and 4 weeks for baby leaf.
Sow cilantro in mid to late March for spring harvest as temperatures rise.
April – June:
Increase ventilation and reduce night temperature setbacks when outdoor temps permit. Sow succession plantings every 2-4 weeks for fast-turnover herbs such as basil, cilantro, and dill.
Harden off varieties that will be moved outdoors or to unheated hoop houses for summer.
July – August:
Shift focus to harvesting and maintaining perennials. Be ready to shade herbs that bolt in high greenhouse heat (cilantro, parsley).
Take cuttings for fall rooting and winter stock.
September:
Sow a final round of cool-season herbs to carry you into the fall and early winter.
Bring tender perennials inside the heated greenhouse and reduce fertilizer to prepare for winter.

Succession planting and sowing intervals

To have herbs every week, you must stagger sowings. Practical intervals:

Fast, leafy herbs (basil, cilantro when cool, dill for baby leaves): sow every 2-4 weeks.
Moderate growers (parsley, chives, mint): sow or divide every 6-8 weeks.
Slow perennials (rosemary, thyme, sage, oregano): maintain established pots and root new cuttings every 3-6 months to replace aging plants.

Staggering is also spatial: keep production pots on rotation so that as one block is harvested another is maturing.

Starting method specifics: seed, cuttings, or liners

Choose method by herb species and desired speed.

Seed: best for basil, parsley, dill, cilantro. Use a sterile seed mix, small cells for seedlings, and maintain warm, moist conditions. Sow at 1-2 mm depth (most fine herb seeds need light soil contact but not deep covering).
Cuttings: best for rosemary, thyme, oregano, mint, lemon balm, sage. Take 2-4 inch tips, strip lower leaves, insert in a soilless mix with rooting hormone and keep humid until roots form (2-6 weeks).
Liners: buying or producing liners is ideal for perennials. Transplant liners into production pots and acclimate for 1-2 weeks before harvest.

Growing media, containers, and fertilization tips

Use a well-draining, friable mix with peat or coconut coir, perlite, and a small amount of compost or slow-release fertilizer for full-size pots.
For continuous harvest, feed with a balanced water-soluble fertilizer at quarter to half strength every 1-2 weeks, or use a slow-release formula at potting.
Pot sizes: baby leaf production can be done in flats; mature pots should be at least 4-6 inches for basil and 6-8 inches for Mediterranean perennials.

Light, humidity, ventilation, and pest control

Supplemental lighting: use LED fixtures providing full-spectrum light; aim for 12-16 hours daily for warm-season herbs in winter and 12 hours for perennials.
Humidity: maintain 50-70 percent; high humidity reduces transpiration for cuttings but also increases fungal risk. Provide air movement and avoid prolonged leaf wetness.
Ventilation: critical in winter to control disease and in summer to prevent overheating. Use thermostats and automatic vents where possible.
Pests: monitor for aphids, whiteflies, fungus gnats, and spider mites. Use integrated pest management: sticky cards, predatory insects, insecticidal soaps, and periodic sanitation.

Practical takeaways and checklist for Maine greenhouse growers

Know your plant group: start warm-season herbs later and under heat; start cool-season herbs earlier for winter production.
Work backwards from harvest: calculate the weeks-to-harvest for each herb and sow accordingly.
Succession sowing is essential: set a calendar to sow fast herbs every 2-4 weeks and slower ones every 6-12 weeks.
Use cuttings for slow perennials like rosemary and thyme for faster production than seed.
Maintain target temperatures: 70-85 F day/60 F night for warm herbs; 55-70 F for cool herbs; 50-60 F night for Mediterranean perennials.
Provide 12-16 hours of light in winter; reduce water and fertilizer for perennials heading into winter.
Keep a sanitation and pest monitoring routine to avoid losing plants mid-season.

By applying these timing principles and the month-by-month plan, you can structure sowings, cuttings, and maintenance to deliver a continuous supply of fresh herbs from a Maine greenhouse. Start with a modest schedule, track actual growth rates in your specific greenhouse, and adjust sowing intervals and environmental settings. The first season you will learn the real “weeks-to-harvest” for your setup, and after that you can refine a calendar that reliably yields herbs every week of the year.