The Best Cold Frames

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.

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 bed and a crop cover in one box, which is the cheapest way to stop cabbage moths, birds and a light frost at the same time.

The cover is the product here. Cabbage white caterpillars can strip a row of kale in days, birds will take strawberries the moment they blush, and a late frost can end a first tomato planting. A hooped mesh over the bed handles all three.

Fitting the hoops to the bed from the start avoids the usual improvised arrangement of sticks and clothes pegs, and they fold back rather than having to come off entirely.

Remember to open or remove the cover once fruiting crops flower. Netting that keeps moths out also keeps pollinators out, and an unpollinated squash plant is a lot of leaf for no dinner.

Skip it if: you already own a bed. A separate hoop-and-net kit over what you have costs far less than buying the whole thing again.

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.