The Best Small 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.

A shelved cover for seed trays rather than a greenhouse you walk into, and the right answer where there is no room for anything larger.

Calling this a greenhouse oversells it, which is why it disappoints people who expected one. What it actually is: a shelf unit with a cover, sized for standard seed trays, that fits on a balcony or against a wall.

For that job it is good. Seedlings started indoors need somewhere sheltered to harden off, and this gives them a few protected degrees and shelter from wind and heavy rain.

Tie it to something. An empty covered shelf unit is remarkably light and catches wind like a sail.

Skip it if: you want to grow plants to maturity. This is a propagation space and a frost blanket, not a growing house.

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.

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.