What an unheated greenhouse can and cannot do for you in winter
A greenhouse buys you a controlled space rather than a warm one, and the budget models cut corners on the two parts that decide whether it lasts. A cold frame does more than people expect.
The picture in most people's heads when they price a greenhouse involves picking something in January. The reality of an unheated structure is closer to a few extra weeks at each end of the season and a reliable crop of tomatoes, which is genuinely worth having and is a different purchase from the one being imagined.
Working out which structure fits is mostly a matter of being clear about what you want from it, because a cold frame costs a tenth of what a greenhouse does and covers a surprising amount of the same ground.
What a cold frame is good for
A cold frame is a box with a sloping transparent lid, and it does four jobs well. It hardens off seedlings without anyone carrying trays in and out twice a day. It gets salad, spinach and hardy greens through winter in usable condition. It brings sowings forward by two or three weeks in spring and holds them going two or three weeks later in fall. And it keeps rain and wind off plants that would otherwise be killed by winter wet rather than by winter cold, which is the real cause of death for a lot of borderline-hardy things.
The catch is that it needs opening. A closed frame in March sunshine will pass 100 degrees inside while the air outside is barely above freezing, and a tray of seedlings does not survive that. Prop the lid on sunny mornings and close it in the late afternoon. If that sounds like too much attention, the same automatic openers sold for greenhouses fit cold frames and solve it for the price of a couple of plants.
What a greenhouse adds
The honest case for a greenhouse is not winter. It is standing room, a bench at a workable height, and a place where heat-loving crops that ripen unreliably outdoors in a cool summer, tomatoes and peppers and cucumbers in particular, come good every year rather than most years. If you propagate a lot, the second case is having somewhere to put several hundred pots and work at them dry and out of the wind in March.
What it does not give you is warmth. On a still, clear night an unheated greenhouse holds perhaps a degree or two over the outside air, because glass radiates heat away almost as fast as the space gained it during the day. There is also a limit that no amount of heating gets around: once daylight drops below roughly ten hours, growth in most plants slows to nearly nothing regardless of temperature. A winter greenhouse is somewhere for crops to sit in good condition until you pick them, not somewhere they grow.
Ventilation is where the cheap ones fail
Overheating is a far more common cause of losses than cold, and it starts earlier in the year than people expect. A sealed greenhouse on a bright March day can climb from 40 degrees to well past 90 inside while it is still cold and windy outdoors. The usual design guideline is roof vent area equal to something like a fifth of the floor area, plus low-level louvre vents that let cool air in underneath as the hot air leaves through the top. A large number of budget greenhouses ship with a single small roof vent and nothing else, which is nowhere close.
Count the vents before you compare prices, and add the cost of extra ones to the cheaper model before deciding it is cheaper. Automatic openers are worth fitting to every vent you have. They work off a wax cylinder that expands as it warms and pushes the vent open, need no power or wiring, and mean the greenhouse looks after itself on the weekday in April when nobody is home to notice the sun came out.
Glazing, and what each one costs you
| Horticultural glass | Best light transmission, never clouds, panes cheap to replace. Breaks, and breaks sharply |
|---|---|
| Toughened safety glass | The same light with far less risk if something hits it. Meaningfully more money |
| Twin-wall polycarbonate | Insulates better, diffuses light evenly, light and safe. Yellows over years and blows out of weak frames |
| Polythene film on hoops | Most growing area per dollar by anything. Replace the cover every few years, and ventilation is harder |
Diffused light is not the drawback it sounds like. Light scattered by twin-wall glazing reaches leaves lower down the plant instead of leaving them in the shade of the ones above, and it reduces scorch on seedlings in high summer. As for the frame, aluminum needs no maintenance, is light, and has thin enough bars to cast very little shadow, though it conducts heat out efficiently. Wood insulates slightly better, casts more shade with its thicker sections, holds screws so you can fix shelves and hooks wherever you want, and requires attention every few years.
Where it goes and what it stands on
The base is the item most often left off the quote and the one that decides whether the structure survives its first serious gale. A greenhouse is a large light box, and wind getting underneath it will lift it. It needs a proper base frame bolted down to a level foundation of concrete or paving slabs, or ground anchors driven in if the manufacturer supplies them. Skipping this to save a couple of hundred dollars is how greenhouses end up in a neighbor's yard in pieces.
- Out from under trees, both for the light and because a falling branch and a glass roof are a bad combination.
- Off the low point of a sloping yard, where cold air pools on frosty nights.
- On genuinely level ground. A frame twisted out of square cracks panes and stops doors closing.
- Within reach of a hose. Carrying watering cans in July gets old within one season.
- Somewhere you walk past. A greenhouse at the far end of the yard is checked far less often in February than one by the back door.
Getting it through winter
Heating the whole volume of a greenhouse through winter costs more than the produce is worth in almost every case. Heat the small part that needs it instead: a propagator on a bench, or a partitioned end of the house closed off with bubble insulation. Line the north-facing glazing with bubble film, which loses you some light and keeps meaningful warmth, and throw horticultural fleece over tender plants on the coldest nights.
The winter enemy is damp rather than cold. Still, humid air and dead leaves lying on the bench are what gray mold needs, and it will take out a bench of overwintering plants faster than a frost would. Open a vent on mild days even in December, water in the morning so surfaces dry before evening, water much less than you would in summer, and pick off yellowing leaves as they appear. If you do only one of those things, make it the vent.