Most beginners treat the greenhouse like an outdoor garden with a roof. It's not. The season extends in both directions — you can start months earlier in spring and keep harvesting months later into winter. But you have to manage the environment deliberately every month. Here's what I actually do, month by month.
❄️ Winter
January
What to Plant
Onions, leeks, celery, celeriac from seed (they need the longest season). Microgreens and sprouts for immediate harvests.
Maintenance
Minimum watering — cold temps + wet soil = root rot. Inspect glazing after storms. Check heating overnight.
Cold: heat required
Temperate: cold-hardy greens only
Hot: cool-season crops
January is for seed orders, not sowing. The only things I start are onions — everything else can wait three more weeks.
February
What to Plant
Tomatoes, peppers, eggplant from seed under grow lights. They need 6–8 weeks to reach transplant size. Chillies too — they're slow.
Maintenance
Check glazing for winter cracks. Clean algae off glass (blocks early-season light). Test thermometer calibration — accuracy matters now.
Cold: 60°F / 15°C minimum at night
Temperate: unheated is fine for greens
I start tomatoes February 15th every year. Earlier and they outgrow their pots before it's warm enough to transplant.
🌿 Spring
March
What to Plant
Cucumbers, melons, squash, basil, aubergine. Direct-sow radishes, peas, and spinach in beds — they germinate fast in cool soil.
Maintenance
Open vents on sunny days — March sun heats fast. First aphid check of the year. Introduce sticky yellow traps before populations build.
Cold: still below freezing at night
Temperate: greenhouse warm enough for tender seedlings
Hot: full summer crops going in
The biggest March mistake is overwatering seedlings. They're growing but the soil dries slowly — feel it before you water, don't water on schedule.
April
What to Plant
Marigolds and nasturtiums as companion plants — they deter pests and attract pollinators. French beans direct-sow in pots. Herbs: dill, cilantro, parsley.
Maintenance
Harden off transplants going to outdoor beds — 7–10 days of increasing exposure. Aphid colonies will appear on new growth. Act early: blast with water or introduce ladybirds.
Cold: last frost risk, protect overnight
Temperate: tomatoes can go into final pots
April is when impatience kills. People transplant tomatoes outside too early. Wait for consistent 50°F nights — no exceptions.
May
What to Plant
Everything is in by now. Late herbs: Thai basil, lemongrass. Second succession of beans and radishes. Outdoor beds are now filling up from your greenhouse seedlings.
Maintenance
Ventilation is critical — May can hit 100°F inside by midday. Full open on both roof and side vents. Start training tomatoes up string or wire.
Temperate: prime growing month
Hot: shade cloth needed if over 90°F
I pinch out tomato laterals every week in May. Miss two weeks and you're pulling out a jungle instead of training a plant.
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☀️ Summer
June
What to Plant
Succession cucumbers and beans. Sweet corn in deep pots. Basil goes in everywhere — it loves the heat.
Maintenance
Add 30–40% shade cloth on south-facing glass. Deep water in the morning. Watch for early blight on tomatoes — remove any yellowing lower leaves immediately.
All zones: manage heat first
The greenhouse in June feels like a sauna. That's fine for peppers. Not fine for you working in there at noon — schedule greenhouse tasks for early morning.
July
What to Plant
Fast crops for succession: radishes (25 days), rocket (30 days), baby leaf lettuce. Remove bolted lettuce immediately — it signals the rest to bolt.
Maintenance
Whitefly and spider mite pressure peaks in July heat. Inspect leaf undersides weekly. Introduce biological controls (Encarsia for whitefly, Phytoseiulus for spider mite) before infestations take hold.
All zones: pest pressure highest
I wet the paths morning and evening in July. The evaporation cools the greenhouse by 5°F and keeps spider mites down — they hate humidity.
August
What to Plant
Start fall crops now: kale, chard, spinach, pak choi, autumn lettuce varieties. They need 6–8 weeks to establish before temps drop.
Maintenance
Begin removing exhausted summer plants to make space. Stop feeding tomatoes nitrogen — switch to high potassium to ripen remaining fruit.
Cold: start fall crops early August
Temperate: mid-August start
Hot: late August / September
August is the transition month. You're winding down summer and starting fall at the same time. The temptation is to let summer plants linger too long.
🍁 Fall
September
What to Plant
Fall brassicas, winter lettuce varieties (Arctic King, Winter Density), mâche, lamb's lettuce. Overwintering onions and garlic sets.
Maintenance
Nighttime temps drop fast in September. Watch for cold damage on any remaining tropical crops. Remove shade cloth — you need every photon of light now.
Cold: first frost possible
Temperate: great growing month
Hot: best growing weather of year
September harvests are underrated. Cool nights + warm days = the best flavour all year. Tomatoes in September beat July tomatoes every time.
October
What to Plant
Overwintering herbs: chives, parsley, thyme in pots to bring inside or shelter under row cover. Winter spinach and mâche direct sow.
Maintenance
Close up for winter — seal drafts, check door seals, bubble-wrap the north wall if it's single-skin. Deep clean the structure before pathogens overwinter in debris.
Cold: heating needed below 40°F nights
Temperate: unheated fine for hardy greens
I spend a full day in October deep cleaning the greenhouse. It's the most boring task and pays off the whole rest of the year in lower disease pressure.
November
What to Plant
Garlic cloves in deep pots for early spring shoots. Microgreens under lights. Nothing else — the light levels are too low for most crops to establish.
Maintenance
Minimal watering — cold wet soil is the number one cause of winter losses. Check insulation. Slug traps: slugs move into warm greenhouses in autumn.
Cold: survival mode for plants
Temperate: cold-hardy crops only
Hot: full cool-season growing
November in a cold climate is honestly a rest month. Keep what's alive alive, and don't force growth. The energy bill will make you regret it.
December
What to Plant
Nothing that needs to grow. Microgreens if you have grow lights. Paperwhite bulbs for indoor decoration. This is a planning month.
Maintenance
Inspect structure for winter damage. Order seeds — the best varieties sell out by February. Sketch your planting layout for next year. Service any heating equipment.
Cold: structure check priority
Hot: active cool-season growing
December is when I plan the year. I review what worked, what failed, what I want to try. An hour of planning in December saves six hours of improvising in March.
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