You do not need glass. You need light, and that is the part most people get wrong.
A seedling kept warm but dim stretches towards whatever light there is and ends up pale, floppy and beyond saving. A seedling kept cool and bright stays short and stocky. Given the choice between a warm dark shelf and a cold bright windowsill, take the windowsill every time.
A windowsill will start Tomato Red Cherry in February and Cucumber Supermarketer in March. South-facing if you have it. Turn the tray daily or everything leans. Pot on once, into something deeper, as soon as the first true leaves appear.
A cold frame is not for starting seed, it is for the fortnight between indoors and outdoors. Skipping that fortnight is why plants that looked perfect on the windowsill collapse in the bed.
A cloche — even a cut plastic bottle — raises soil temperature a few degrees and keeps the rain off. That is enough to carry an October sowing of Green Peas Petit through winter in a well-drained bed, and to keep Lettuce Mix cutting from November to February.
