Eleven winters ago I put a 6 by 8 foot greenhouse on the coldest corner of the plot, under a north slope. Poor site. Best mistake I have made. A house in a bad position tells you plainly what it can do, because it cannot bluff its way through a hard night.
Here is what mine does.
It shifts the edges of the season by roughly a month at each end — a later spring and an earlier autumn. It keeps salad alive through January. It ripens tomatoes ahead of the outdoor plants in June, then finishes the last of them in October, weeks after the outdoor vines have blackened. It does not make summer in February.
That is the whole job. The rest is working detail.
What a hobby greenhouse actually does
Strip the romance out and a greenhouse is a box. It traps shortwave radiation, blocks wind, and slows heat loss after dark. Each of those jobs is worth something. None of them generates heat.
North Carolina State Extension puts a figure on the return. Warm-season crops such as tomatoes and cucumbers can get as much as a month’s head start in spring, or last a month longer in autumn, than the same plants grown outside (season extenders and greenhouses, NC State Extension). A month at each end is the honest yield of a small unheated house.
What it is not is a tropical room. Holding 18°C (64°F) through a northern winter costs more than most hobby growers will spend. Buy a small greenhouse expecting a later spring and an earlier autumn. Do not buy one expecting a different climate.
Frost protection is not the same as heat
These two get muddled constantly, usually by the people selling greenhouses.
Frost protection means holding the air a degree or two above freezing for the few hours around dawn. Heat means raising the average temperature of the whole night. A greenhouse does the first. It rarely does the second.
Most frosts are shallow. Colorado State Extension separates advective frosts, where a cold front drives temperatures far below the critical level, from radiation frosts on calm clear nights, which at the start and end of a season are typically only a few degrees below the point that damages a crop (frost protection and extending the growing season, Colorado State University Extension). Radiation frosts are the ones a hobby house beats.
The protective band is thin. Floating row covers gave up to 10°F of protection in the trials CSU reports. The RHS puts heavier grades of horticultural fleece at about 2°C (avoiding late frosts, RHS).
I lost a bench of young cosmos and a tray of basil one February when the outside low touched -6°C (21°F). The glass held. The air inside dropped to about -2°C (28°F). Frost protection is not heat. A greenhouse is a blanket, and blankets do not burn.
January and February: holding a crop, not growing one
By the turn of the year, light limits growth rather than temperature. Anything not already grown by mid-October will not grow now. It may germinate, then sit. Lettuce seed will germinate at 35°F (2°C). NC State’s production guide gives that as the floor for the crop (lettuce production, NC State Extension). Germinating is not growing.
My house in January carries winter lettuce, a row of land cress, kale and spring cabbage. All of them tolerate temperatures near 0°C (32°F) without dying. None of them does much before February. Hardy salad crops survive the cold and wait for the light.
The work is maintenance. I strip yellowing leaves. I water sparingly, and only in the morning, so the foliage is dry before the night chill. I open a vent for an hour on any mild day. Still air in a cold house is how grey mould begins.
Want fuchsias, pelargoniums or anything tropical alive in there? Then you are heating. The RHS states that a minimum of 5°C (41°F) keeps fuchsias and pelargoniums from being killed by cold, and that most tropical plants need at least 12°C (54°F), which requires a greenhouse heater (jobs for April, RHS). That is the line between an unheated house and a heated one. It is a fuel bill, not a technique.
March and April: the propagation window
This is when the house earns its keep, and when the soil under your trays matters more than the air above them.
Tomatoes stall below 10°C (50°F). The RHS notes that tomatoes, peppers, basil and cucurbits do better if night temperatures stay above 10°C, and that tender plants should go out only once nights are reliably above at least 7°C (45°F) (when to plant out tender plants, RHS). Florida’s extension service is stricter. It holds greenhouse tomatoes between 62°F and 72°F at night, and warns that temperatures below 50°F cause chilling injury to the fruit (production of greenhouse tomatoes, UF/IFAS).
Lettuce and brassicas sit at the other end of the scale. They germinate and grow in cold soil. I sow them first, in February under cover, and they never sulk about it.
Where the roots sit changes everything. A greenhouse border warms through the day and gives that heat back slowly. A pot on a bench has cold air on every side. On a clear March night my bench runs several degrees colder than the border at the same air temperature, and the difference shows in how quickly seedlings move. Warm the root zone before you warm the room.
Air first: ventilation measured against floor area
Ventilation is the most neglected part of a hobby house. It is also what decides whether summer is manageable.
The RHS gives the proportion plainly. In a larger glasshouse, ridge vent area equal to 20% of the floor area gives one complete air change every two minutes, and smaller houses, which carry a higher ratio of glass to floor, need a larger percentage again. The same page puts plant damage above 27°C (81°F) and advises keeping the house below roughly 25°C (77°F) (ventilation and shading, RHS).
American guidance agrees. UMass Extension reports the ASAE recommendation that combined sidewall vent area should match ridge vent area, each at 15 to 20% of floor area, and that a fan delivering a full air change every minute still leaves an 8 to 10°F rise between the intake and the fan (natural ventilation in greenhouses, UMass Extension).
Most kits sold for gardens give far less than that. Mine has about 1.2 square metres of combined roof vent for a 4.5 square metre floor, which is roughly 26% on paper. On a still April morning it is nowhere near enough, and I open the door at eight rather than waiting to feel the heat. The temperature climbs fastest between nine and eleven. A maximum-minimum thermometer on the bench tells you what actually happened overnight, which is not something you can feel in the afternoon.
Water next: by hand, or on a capillary bench
Hand watering makes you part of the control loop, which is a polite way of saying it is inconsistent. It also wets foliage.
UMass describes capillary mat systems, where pots stand on a mat kept constantly wet and water moves up into the compost by capillary action. The advantages they list are even watering, dry foliage and much less labour. They are blunt that overhead sprinklers and hand watering waste water and wet the leaves. Wet foliage raises the risk of disease and injury (irrigation systems, UMass Extension).
I use both. Seedlings and short-term salad go on a capillary bench, because their roots are shallow and they dry out in an afternoon. Tomatoes, cucumbers and anything in a large pot get watered by hand, at the base, because a mat never flushes the pot and soluble salts build up in compost that is only ever wicked upwards.
The rule that matters more than either method is the wet surface. Water the compost, not the plant. Do it in the morning. If the surface is still dark and damp from yesterday, skip a day. Overwatering kills more greenhouse plants than drought ever will.
Damping-off starts on a wet surface
Damping-off is not a disease you treat. It is a condition you avoid.
Penn State Extension lists what favours it. Excessive soil moisture and excessive overhead misting. Soil temperatures below 20°C (68°F) before germination and above 25°C (77°F) after emergence. Overcrowded flats. Their control advice is to sow into pasteurised or soilless mix and to hold containers at 70°F to 75°F with bottom heat (damping off, Penn State Extension).
The RHS points at the same causes from the gardener’s side. Air that sits too still. Seedlings left too close together. Compost kept too wet, and pots and trays brought back into use without washing. Their non-chemical controls start with thin sowing and fresh trays, then plenty of ventilation, and a covered water butt so debris cannot harbour the pathogens (damping off, RHS).
Once a tray collapses, throw the compost away and wash the tray. Reusing a tray that has already lost seedlings to damping-off is how you guarantee it happens again.
June to August: the crowding bill arrives
The classic beginner error is made in April and paid in July. Every surface gets filled with modules and pots. Nothing is thrown out, because everything might grow. By June the house is a thicket.
Crowding is a ventilation problem wearing a plant costume. I thin mine twice before midsummer now, and I have thrown away healthy seedlings to do it. The plants that remain repay the space. Less leaf on leaf means more air through the house and fewer places for botrytis to take hold.
Heat is the other half of it. UF/IFAS notes that greenhouse tomatoes perform best when day temperatures stay below 85°F (29°C) (UF/IFAS). A closed house passes that line by mid-morning in July. Whatever the vents are doing, the door needs to be open.
I ran forty tomato plants in that 6 by 8 house one summer. It was the worst crop I have had. Blossom end rot, whitefly and a week of grey mould, all of it downstream of one crowded spring. The site’s guide to keeping a greenhouse from overheating works through the fixes in order.
September and October: the sowing that decides winter
Autumn is the second half of the season extension, and it is planned earlier than people expect.
The RHS advises sowing a winter lettuce variety in September or October, either in a greenhouse border or in a container (growing lettuce, RHS). My own cut-off is blunter. If it is not up and growing by the middle of October, it will not feed me in December.
The border is the better place for it. Border soil holds moisture and warmth, and plants in it need far less attention than a pot. Containers work, and the RHS recommends them in the same breath, but they dry quickly and they freeze harder. The sowing itself is rarely the problem. The problem is sowing late and asking weak plants to grow in November light.
October has one more job. Take the shading off, clean the glass, and check the frame before the gales. Clean glazing in a low-sun month is worth more than any heater you can afford to run.
The empty fortnight: cleaning between crops
Here is the habit that separates a greenhouse from an incubator. Empty it.
UMass is direct about timing. Take the chance to clean thoroughly between crop cycles, when the house is totally empty, because infectious microbes accumulate and algae flourish on moist surfaces that harbour fungus gnats and shore flies. Cleaning means physically removing weeds, soil and plant debris first, since organic matter stops disinfectants working, and you work from the top down (cleaning and disinfecting the greenhouse, UMass Extension).
The RHS recommends an autumn clean once the tomato crop has finished and been removed. That clean covers the glazing inside and out, the floor, the pots and the water butt (how to clean your greenhouse, RHS).
I build in a real gap. My tomatoes come out in early July, which is when UF/IFAS expects a greenhouse crop to finish, and the house then stays bare for ten days to a fortnight. Glass washed, floor scrubbed, trays soaked, water butt emptied and rinsed. Then the winter sowings go in.
A house that never empties never gets cleaned. That is the whole of it.
What a small unheated house will not do
It will not give you tomatoes in March. It will not hold a tender plant through an advective freeze. It will not germinate warmth-loving seed without a propagator or a heat mat. It will not save a crop you forget to water, and it will not stay free of pests because it has a roof on it.
What it will do is narrower and worth having. It adds about a month at each end of the season. It keeps hardy salad cropping through the winter. It dries and warms soil early enough for a genuine spring start. It ripens the last tomatoes in a wet October.
That is a fair return on a small structure. It only arrives if you treat the house as a machine with a schedule, and stop asking it to be a warm room.