Your First Year With a Greenhouse: A UK Month-by-Month Beginner's Diary (From a 16-Year Installer)
Your first year with a greenhouse in the UK is mostly about learning what your own garden actually does month by month. After 16 years installing greenhouses across the country, I have watched hundreds of first-time owners discover that February is busier than June, that ventilation matters more than heat, and that the biggest first-year killer is not frost, it is water. This is a real diary, not a brochure: what to do, when to do it, and the rookie mistakes that quietly cost you a season.
This guide is based on the homes we install for at greenhousestores.co.uk from Cornwall up to the Outer Hebrides, plus the follow-up calls we get six, twelve, and twenty-four months later. The pattern is remarkably consistent. If you have just bought a greenhouse, or you are about to, the year below is what your first twelve months actually look like.
Key Takeaways
- Year one is split into three real phases: set-up (Jan-Feb), peak growing (Mar-Aug), and shut-down learning (Sep-Dec). Each one has different jobs and different mistakes.
- Late January is the start of the year, not March. Chilli seeds go in under a heat mat by 25th January or your fruit will not ripen before October.
- Ventilation beats heat every time. The number-one cause of dead seedlings in a first-year greenhouse is fungal damping-off from still, damp air, not cold.
- Realistic year-one yield from a 6x8 greenhouse: 12-18kg of tomatoes, 30-50 cucumbers, 2-3kg chillies, plus enough seedlings to fill the rest of the garden. That number doubles in year two as your routine settles.
- You will lose plants. All of us do. Year one is when you find out what your microclimate is. The diary below tells you what to watch for at each stage so the losses are recoverable.
- Cleaning matters more than you think. An end-of-year clean in October-November cuts year-two pest pressure by an order of magnitude. Skip it and you start the next year fighting whitefly.
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Installer's Note
The first-year owners who succeed are not the ones with the biggest greenhouse or the best gardening books. They are the ones who walk into the greenhouse every day for the first three months, even just for two minutes, and look around. You spot the seedling that has wilted, the vent that did not open, the bag of compost that has gone musty. By June it becomes automatic. The customers who only visit on Saturdays are the ones who ring us in May to say "everything has died" — and almost always it is the same trio of problems: no airflow, no daily watering check, no pest scan. The greenhouse rewards little-and-often. Build the habit in January and your first year will be your best teacher.
January: site, plan, sow the slow stuff
January is the month most first-year owners waste, and the one most experienced growers protect. Your greenhouse is either freshly installed, or it has sat through one winter and you are walking into it for the first season. Either way, the work in January is quiet, low-effort, and disproportionately important.
Start with the base and the orientation if you have not already. An east-west ridge gives the most even winter light in the UK; a south-facing door makes year-round access painless. If your greenhouse is still in the box, get the foundation right before March — retro-fitting a proper base after the frame is up is the single most expensive mistake in greenhouse ownership.
Sowing in January is selective. Chillies, sweet peppers and aubergines need an indoor heat mat from 25th January at the latest, or fruit will not ripen before the first October frost. Onion seed, sweet peas, and broad beans can go in a cold greenhouse from mid-month with no heat at all.
The other January job is to think honestly about what you actually eat. The single biggest year-one regret I hear is "I grew six varieties of tomato I could not give away". Grow one tray of standards, one tray of cherry, and use the rest of the bench for things that are genuinely better when you grow them yourself: chillies, cucumbers, basil.
February: heat mats, propagators, and the first big sowing
February is when the greenhouse year really starts. A heat mat or heated propagator under tomato seed by the second week of February gives you transplant-ready plants by mid-April, fruit by July, and a long harvest into October. Skip the propagator and you are three weeks behind for the rest of the year.
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Soil temperature is what matters, not air temperature. Most first-year owners watch the air thermometer and panic at 2C nights. Your seeds do not care — they care about the compost under them. A heated propagator holds compost at 18-22C through any UK winter and is, for the money, the most useful piece of kit you can own in year one.
Other February jobs: clean any over-wintered staging with a mild horticultural disinfectant before it gets crowded; sow early lettuce, rocket and spring onions in modules; chit your seed potatoes on a bright windowsill ready for outdoor planting in March. If you are growing strawberries under glass for early fruit, force-pot them by the first week of February for picking from mid-May.
For the full month-by-month checklist beyond the greenhouse, our February gardening jobs guide covers the wider garden. Pair it with the seed germination temperature reference for exact target temperatures by crop.
March: pricking out, ventilation, and the first warm day trap
March is the most dangerous month for a first-year greenhouse. The first 20C afternoon in mid-March will cook unventilated seedlings in two hours. Greenhouses lag the outside air by 15-20C in spring sunshine, and the door must come open by 10am on any sunny day, even if the morning frost is still on the lawn.
This is the month you start using the auto-vent openers. If your greenhouse came without them, fit a pair now — they are £40-£60 a vent, take fifteen minutes, and they are the difference between a holiday next month and a tray of dead seedlings. The full installation steps are in our auto vent opener guide.
Pricking out is the other big job. February-sown tomatoes will have two true leaves and need moving from seed trays into 9cm pots from the second week of March. Hold them by the seed leaves, never the stem; bury them deep so the seed leaves sit just above the compost; water once with a fine rose and leave them alone for four days. The temptation to keep watering is what kills more March seedlings than anything else.
The wider garden is also waking up. The March gardening jobs checklist covers what to do outdoors while your greenhouse seedlings are establishing.
April: hardening off, the longest job
April is the month where the greenhouse goes from quiet to chaotic. Your tomato seedlings are 25-30cm tall; your peppers and chillies are stocky and starting to set first flowers; the bench is full and the floor is filling up with trays of brassica and salad seedlings destined for outdoors. The job that defines April is hardening off — moving outdoor crops from the greenhouse to their final home over 7-14 days.
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Hardening off is dull and easy to skip and absolutely non-negotiable. Day one: outside for two hours, sheltered. Day three: four hours, more sun. Day five: half a day. Day seven: all day, back in at night. Day ten: out overnight if frost is past. Cold-shock a tomato seedling at this stage and you have set it back three weeks. Patience pays compound interest in April.
April is also the month to plant your greenhouse tomatoes into their final position — borders, large pots, or grow bags — from the third week. Allow 45cm between plants, support each one with a 1.8m cane or string, and pinch out side shoots from week one of growth. The April gardening jobs guide covers the wider garden.
May: peak sowing, first feeding, first pest scan
May is the busiest month of the greenhouse year for a beginner. Tomatoes are flowering; cucumbers go in by mid-month; aubergines and chillies are setting first fruit; bench space is at a premium. This is the month you start a proper weekly routine: water in the morning, scan for pests at lunchtime, vent and damp-down in the afternoon.
The first feed goes on when the first tomato truss has set. Use a high-potash tomato feed at half strength once a week to begin with, then full strength once fruit is the size of a 5p coin. Over-feeding before fruit set produces leafy plants and no harvest — the single most common May error in a first-year greenhouse.
Pests start in May. Whitefly arrive on warm afternoons; aphids find the new soft tips of tomato plants; red spider mite shows up on the underside of cucumber leaves if humidity drops. Walk the greenhouse once a week with the back of your hand on the underside of three random leaves — if anything moves, identify it and act the same day. Our pest-repelling companion planting guide covers what to plant alongside your tomatoes to keep pressure down naturally.
Damping down — wetting the floor and staging on hot afternoons — is the technique that quietly separates good greenhouse growers from average ones. Raising humidity to 70-80% in the afternoon discourages red spider mite, helps tomato flowers set, and cools the air by 4-5C. Five minutes with a watering can on a 25C afternoon, every day, all summer.
June: the greenhouse hits full speed
June is the month the greenhouse finally feels like the project you signed up for. Tomatoes are setting fruit on the second and third trusses; cucumbers are climbing; basil, coriander and chillies are productive on the bench; the first early lettuce is ready to cut. The job switches from sowing to maintaining.
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The new June discipline is heat management. UK greenhouses regularly hit 35-40C in June afternoons if vents and doors are not both open. Shade paint or shade netting goes on by the second week of June and stays on until the second week of September. Without it, tomato flowers abort, cucumbers split, and your basil bolts in a fortnight.
Watering doubles. By late June a vigorous tomato plant is using one to two litres a day. A drip-irrigation system on a timer, run through a £20 garden hose connector, is the single biggest quality-of-life upgrade in a first-year greenhouse. You can be away for the weekend without worry.
Pinching out side shoots becomes a daily walk-through, not a weekly one. Tomato laterals grow 5-7cm a day in June; miss a week and the plant becomes a hedge. The June gardening jobs guide covers what is happening in the rest of the garden at the same time.
July: first harvest, biggest mistakes
July is harvest month for most first-year greenhouses. First ripe tomatoes appear from the second week; cucumbers start cropping mid-month; chillies turn from green to colour from late July onwards. The pleasure of the first home-grown tomato is genuinely the moment first-year owners decide they are doing this forever.
July is also when the classic first-year mistakes show up. Blossom end rot — the black sunken patch on the bottom of tomatoes — is almost always caused by irregular watering, not calcium deficiency. Keep the compost evenly moist, never bone dry then flooded. Splitting cucumbers point to the same root cause. The fix is a watering timer.
Pick everything as soon as it is ready. Cucumbers left two days too long go to seed and the plant stops producing; courgettes hidden under leaves become marrows; chillies left on the plant slow the whole crop down. A daily 30-second walk-through with a pair of snips is the difference between a productive plant and a sulky one.
Whitefly explode in July. Yellow sticky traps catch flying adults but they only ever catch 10% of the population. Hang traps as monitors, not as a control. The real control is encarsia formosa, a parasitic wasp you can buy by post from any UK biocontrol supplier for £15-£25, and which will clear a 6x8 greenhouse in three to four weeks.
August: holidays, heat, and the second-half plan
August tests every first-year setup. You go on holiday for ten days, come home, and discover whether your watering system, your ventilation, and your neighbour's willingness to pop in every other day are actually up to the job. Two-thirds of the catastrophic first-year losses we hear about happen in the last week of an August holiday.
The August fix is investment in automation. A timer-driven drip-irrigation system, automatic vent openers on every vent, and a thermometer-triggered shade system if you have the budget. The total spend is £100-£200 and it pays back the first time you can leave for a week without a plant-sitter.
August is also when you start the second-half plan. Sow autumn lettuce, pak choi, mizuna, and winter rocket from the second week of August for cropping into November. Take basil cuttings to root in water on a windowsill for an indoor crop through autumn. Sow spring cabbage and over-wintering onion seed. The greenhouse is not done in August — it is restocking for the dark half of the year.
Last sowings of tomato side shoots, rooted in water, give you late-season plants that will fruit into October. This is a free crop and a useful insurance against a poor main-season harvest.
September: insulation prep, last harvests
September is the month the greenhouse year quietly turns. The first 5C night arrives at the end of the month in most of the UK; the first proper frost is six weeks away. The work shifts from production to protection.
Pick all the cucumbers and pinch out the tops of tomato plants to push the last fruit into ripening. Cut the chilli plants back by a third — the second flush is sweeter and ripens faster. Sow next year's autumn-planted broad beans, garlic, and onion sets outside while the soil is still warm.
Bubble wrap insulation goes up by the last week of September if you intend to grow through winter. Use UV-stabilised horticultural bubble wrap — proper kit lasts three seasons. The 50% heat-loss saving makes winter growing financially worthwhile; without it, you are paying twice for the same amount of heat.
Clean the glass inside and out before the bubble wrap goes on. Algae and grime block up to 40% of winter light, and once the bubble wrap is up you cannot reach the glass. An hour with warm water, washing-up liquid, and a squeegee is the single highest-value job of late September.
October: the deep clean
October is the most important month for the year that follows. Whatever pests, fungal spores and bacterial residues remain in the greenhouse on the 31st of October will be waiting for your seedlings in February. The end-of-season clean is the most under-rated job in a first-year greenhouse owner's calendar.
The drill is the same every year. Empty the greenhouse completely. Compost spent crops; bag and bin any leaves showing fungal lesions. Scrub the glass, the frame, the staging and the floor with a mild horticultural disinfectant. Wash pots and trays in hot soapy water and dry them outside in the sun.
Inspect the structure while it is empty. Any cracked panes, loose glazing clips, or missing rubber W-clips get fixed now. A loose pane in October is a smashed pane in the first January storm. Our seasonal maintenance checklist covers the full inspection in detail.
If you ran a soil border this year, refresh the top 15cm with fresh compost and a sprinkle of pelleted chicken manure. Greenhouse borders run out of nitrogen quickly under continuous cropping; a feed in October sets up next year's growth before the soil cools.
November: heater fit, winter sowings, dormancy
November is the quiet month, and the right month to set up your winter heating. If you are heating through winter — and most first-year owners do not need to — a 2kW thermostatic electric tubular heater is the cheapest reliable option. Paraffin heaters add useful CO2 but introduce damp; gas is best avoided for first-year owners. Our greenhouse heating costs guide shows the real running-cost numbers.
Most first-year greenhouses are best run as a frost-free cold house through winter rather than a heated one. Hardy lettuce, winter spinach, claytonia, mizuna and pak choi crop happily down to -2C; tender herbs come indoors. The bubble wrap alone is usually enough to keep a UK greenhouse 3-5C above outside temperature, which is enough for hardy winter crops.
Sow sweet pea seed in deep root-trainers by mid-November for spring planting; sow onion seed (not sets) under cover for next year's exhibition-grade bulbs. Pot up garlic cloves into 9cm pots for an early indoor crop. The greenhouse is not closed for winter — it is just running slower.
December: ventilation, monitoring, planning
December is about ventilation and prevention. A closed-up greenhouse with bubble wrap insulation and any kind of heat is a humidity trap, and humidity at low temperatures is what causes the grey botrytis mould that wipes out winter lettuce. Open a vent or the door for 30-60 minutes on every dry day, even when it is below freezing outside.
Check minimum-maximum thermometers daily for the first week of any cold snap. If the overnight low is dropping below 0C with winter crops in, double up the bubble wrap on the north wall, fleece the staging at night, and consider a thermostatic frost heater set to come on at 2C.
December is the planning month. Pull out a seed catalogue, review what worked and what did not, write down three changes for next year, and order seed by Christmas to avoid the January rush. The single most useful first-year habit is a paper notebook in the greenhouse with one line per week: sowed, fed, problem, harvest. Year two becomes much easier because you have data, not memory.
Year-one greenhouse calendar at a glance
| Month | Main greenhouse jobs | Year-one beginner watch-out |
|---|---|---|
| January | Sow chillies, sweet peppers, aubergines on heat mat; plan crops; pour base if new | Sowing tomatoes too early — they get leggy |
| February | Sow tomatoes (2nd week), early lettuce, broad beans; clean staging | Skipping the propagator — sets you back 3 weeks |
| March | Prick out tomatoes; fit auto-vent openers; first warm-day ventilation | Forgetting to open vents on first 20C day |
| April | Plant tomatoes into final position; harden off outdoor crops over 7-14 days | Rushing the hardening-off; cold-shock sets plants back |
| May | Plant cucumbers; first tomato feed at truss set; weekly pest scan; damp-down | Over-feeding before fruit sets — all leaf, no fruit |
| June | Shade paint up; daily side-shoot pinch; full ventilation; first drip irrigation | Letting greenhouse hit 40C — flowers abort |
| July | First harvests; biocontrol against whitefly; daily picking walk-through | Irregular watering — blossom end rot, split cucumbers |
| August | Holiday automation; sow autumn salads; spring cabbage; basil cuttings | Going on holiday without a watering plan |
| September | Clean glass; fit bubble wrap; pinch out tomato tops; final cucumbers | Putting up bubble wrap before cleaning the glass |
| October | Full deep clean; refresh borders; inspect structure; bag diseased material | Skipping the clean — pests over-winter for year two |
| November | Fit heater if used; sow sweet peas, onion seed; hardy winter salads | Heating a leaky greenhouse — wasted money |
| December | Ventilate on dry days; monitor min/max thermometer; plan year two | Sealed-up humid greenhouse — botrytis on winter lettuce |
Matt's Tip: keep a one-line-a-week paper diary
The single most useful thing I ever did in my first greenhouse year was hang an A5 notebook from a piece of string by the door. One line per week: date, what I sowed or planted, what I fed, what pest I spotted, what I harvested. Year two I had a real reference, not a vague memory. By year four I knew, from my own data in my own garden, that my first tomatoes always ripen the third week of July and my last cucumber comes off the second week of October. Soil thermometers and indoor-outdoor min-max thermometers are useful, but a paper notebook and a biro outperform every gardening app I have ever tried. Hardware does not have to be expensive to be transformative.
Matt's Pick for First-Year Owners
Best For: A first greenhouse you will never need to replace — heavy-gauge frame, toughened glass, room to grow into
Why I Recommend It: Most first-year owners outgrow a 4x6 in 12 months. The Elite Titan 6x8 is heavy-gauge box-section aluminium that holds up on exposed sites, has enough bench space for tomatoes and cucumbers side by side, and is the greenhouse I see customers still using ten and fifteen years later. The toughened glass means you can run grandchildren and footballs near it without holding your breath.
Price: £1,669
Five mistakes that ruin a first greenhouse year
Almost every first-year disaster I get called out to involves one of these five mistakes. None of them are obvious if you have never grown under glass. All of them are easy to avoid once you know.
1. Over-watering in cold weather. A cold, wet seedling damps off in 48 hours. From February to mid-April, water sparingly in the morning only, never in the evening, and let compost surfaces dry between waterings. Most "frost killed my seedlings" calls turn out to be over-watering, not cold.
2. Closing up too tight, too long. The instinct is to keep the greenhouse warm. The reality is that still, damp air is the enemy — botrytis, mildew, whitefly all love a sealed greenhouse. Vent every dry day, even in winter, even when cold.
3. Skipping hardening off. Plants moved straight from greenhouse to garden lose 30-50% of their growth potential to cold shock. A 7-14 day staged transition is non-negotiable.
4. Feeding before fruit set. High-nitrogen feed before the first truss has set means leafy tomato plants and no harvest. Wait for the first flowers, then feed.
5. No end-of-year clean. Pests and spores over-winter in pots, on staging, and in soil borders. Skip October's clean and you start February's seedlings into a contaminated environment. By May you are fighting issues that should not exist.
Frequently asked questions
What should I do in my first month with a greenhouse?
Get the base right, work out the orientation, and start the slow sowings — chillies, sweet peppers, aubergines — on a heat mat by 25th January. The first month is about set-up and the small number of seeds that need the longest growing window. Resist the urge to fill the bench in January; February is when the real sowing begins. Take measurements of where the sun falls in your garden over a few weekends so you know which end of the greenhouse stays warmest.
How long does it take a new greenhouse to start paying for itself?
Most first-year UK greenhouses pay back the running costs but not the capital cost in year one. By year two — once the routine is set and yields rise — a 6x8 greenhouse typically returns £200-£400 of produce a year against £30-£80 of running costs. A full capital payback in produce alone usually takes 5-7 years; the real return is in quality, variety, and the things you simply cannot buy at supermarket level. See our greenhouse ROI guide for the full numbers.
Do I need a heater in my first year?
Most UK first-year owners do not, unless you want to over-winter tender plants or grow into December. A bubble-wrapped unheated greenhouse stays 3-5C above outside temperature, which is enough for hardy winter salads, garlic, broad beans and over-wintered onions. Heating only makes sense if you are committed to year-round production or you are over-wintering pelargoniums, fuchsias, or other tender perennials.
What is the easiest thing to grow in a new greenhouse?
Cherry tomatoes, basil and cut-and-come-again lettuce are the three most forgiving year-one crops in a UK greenhouse. Cherry tomatoes ripen weeks before standard varieties and tolerate watering mistakes; basil thrives on the same conditions; loose-leaf lettuce will give you four cuts from one sowing between April and October. Add a cucumber plant in May and you have a respectable first-year harvest with very little risk.
How often should I water my greenhouse plants?
Water in the morning, once a day from May to September, and roughly every 3-5 days from October to April. The exact frequency depends on outside temperature, plant size, and pot type — clay pots dry faster than plastic, and a 25C July day can mean two waterings for established tomatoes. The rule is: water when the top inch of compost is dry, never on a schedule. A simple moisture meter is £6 from any garden centre and removes the guesswork.
What size greenhouse should a beginner buy?
A 6x8ft is the sweet spot for a UK first-year owner — big enough to grow the four classic crops, small enough to manage in 20 minutes a day. 4x6ft sells well as a starter and gets outgrown within twelve months; 8x10ft is comfortable but a real commitment. A 6x8 gives you space for tomatoes down one side, cucumbers up the other, a propagation bench at the back, and a clear walking aisle. Our first-time setup checklist walks through the full initial set-up.
When can I start using my greenhouse after installation?
Immediately for hardy crops; from late January under heat for tender ones. Glass needs no curing time; the base needs to have set properly before heavy staging goes in. If your greenhouse went up in November or December, by January you can be sowing chillies on a heat mat. The greenhouse year does not wait for spring — late January is the start, not a soft launch.
Related articles
- How to Set Up a New Greenhouse: First-Time Owner's Checklist
- Greenhouse Foundation Types: Complete Base Comparison Guide UK
- Greenhouse Aftercare: Seasonal UK Maintenance Checklist
- Seed Germination Temperatures: The Complete UK Reference Table
- How Much Does It Cost to Heat a Greenhouse? UK Running Costs 2026
For the full range of starter greenhouses and accessories, visit Greenhouse Stores — we install across the UK and the team are on the phone six days a week if you want a hand picking your first model.
