The Best Greenhouse Kits for Winter

The heavy-glazed model in the range, and the one to buy if the plan is to keep growing through a real winter.

Glazing thickness is the whole argument for this kit. A 10 mm twin-wall panel holds substantially more heat overnight than a 4 mm one, and overnight is when a winter greenhouse either works or does not.

It is also the model to look at if you get real snow. Thin panels and light frames are a three-season specification; if you are in the Cascades or anywhere with a genuine snow load, buy the heavier structure rather than hoping.

None of that makes it heated. A greenhouse without a heat source holds a few degrees over outside, which is the difference between hardy greens surviving and not -- it is not a tropical house in January.

Skip it if: you only want a spring seed-starting space. You are paying a large premium for insulation you will use for three months of the year.

The sensible first greenhouse: an aluminum frame with twin-wall panels, sized to fit a normal backyard rather than a nursery.

Twin-wall polycarbonate is the thing that separates a greenhouse from a plastic tent. Two skins with an air gap between them insulate, and the panel diffuses light rather than throwing hard shadows, which suits seedlings better than clear glass does.

The Mythos sits at the point in the range where you get a proper aluminum frame, a roof vent and a lockable door without paying for the heavy-duty glazing. For starting seeds, hardening off, and stretching the tomato season at both ends, that is the right specification.

Two warnings from the manual that people ignore. Anchor it -- every lightweight greenhouse that ends up in a neighbor's yard was not anchored. And build it with a second person; the panels flex and the frame is not square until the last bolt.

Skip it if: you live somewhere with heavy snow load. 4 mm panels are a three-season specification; the thicker-glazed models exist for a reason.

The cheapest accessory that changes decisions: it tells you the overnight low and the midday high, not the temperature at the moment you happened to look.

Every greenhouse decision -- when to sow, when to vent, whether you need a heater, whether the frost cloth is doing anything -- depends on a number most owners never measure: the overnight low inside the structure.

A min/max thermometer records the high and low since you last reset it, so a morning glance tells you what happened while you slept. That turns guessing into knowing, for the price of a bag of compost.

Hang it at plant height and out of direct sun. A thermometer in a sunbeam reports the sunbeam, not the greenhouse.

Skip it if: nothing. If you own a greenhouse and not one of these, you are guessing about the only variable that matters.

A lid for a corner of a raised bed: no assembly worth the name, and the cheapest few weeks of extra season you can buy.

A cold frame is the least glamorous and most cost-effective piece of season-extending equipment in the garden. It is a box with a transparent lid: sunlight in, wind off, a few degrees of overnight protection.

Used over the corner of a raised bed it will carry spinach, mache, and hardy lettuces through a cold spell, harden off seedlings before they go out, and start spring sowings a couple of weeks early.

The one discipline it demands is venting. A closed frame on a sunny March day can cook its contents by lunchtime. Prop the lid in the morning, close it in the late afternoon.

Skip it if: you need headroom. A cold frame is worked from above, kneeling, with the lid propped open.