The Best Backyard Greenhouse Kits

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.

Clear walls so you can see what is happening inside, with a diffusing twin-wall roof over the plants that need protection from hard overhead sun.

The Hybrid name refers to two glazing types in one kit: crystal-clear panels on the walls, twin-wall on the roof. In practice that means high light levels from the sides and diffused light from above, which is a good compromise in a northern or cloudy garden.

It also means you can see in. That sounds trivial until you have walked out in the rain three times a week to check whether anything needs water.

In a hot, south-facing, unshaded spot the clear walls become a liability. If that is your site, choose the fully diffused model or plan on shade cloth from midsummer.

Skip it if: your site bakes in afternoon sun. Clear walls transmit far more light, and heat follows light.

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.

A tunnel, not a building -- but the cheapest way to find out whether you will actually use a greenhouse before spending real money.

This is the honest entry point. A lot of people buy a greenhouse and discover they do not have the habit to use one, and finding that out cheaply is worth something.

It does real work in spring: hardening off seedlings, protecting young plants from a late frost, and giving you somewhere dry to pot up. Those are the jobs a first greenhouse actually does.

Treat the cover as a consumable and stake it properly. Every bad review of a tunnel greenhouse is a wind story, and almost all of them start with skipped guy ropes.

Skip it if: you are in a windy or snowy spot. A covered frame is not a structure, and covers are a consumable.

A greenhouse is measured in shelf area, not floor area, and staging is the cheapest way to double what you have.

The first thing most new greenhouse owners discover is that a 6 x 8 footprint is smaller than it looked in the listing. Staging is the fix: three tiers of trays in the floor space of one.

Look for slatted or mesh tops rather than solid ones. Water has to drain and air has to move, or the trays on the shelf become the place fungus gnats and damping-off start.

Plan for the seasonal switch. Staging is right in February and March when the greenhouse is full of seed trays, and comes out in May when tall crops go in and the shelves would only shade them.

Skip it if: you are growing tall crops to maturity in the ground inside. Staging and mature tomatoes compete for the same light.