Before I had a greenhouse, I used coldframes a lot. Mine were basically concrete blocks, 2 courses high, stacked on concrete slabs which I laid on sand. I divided the space up into 3 bays, each one about 1.2m square. I used a variety of 'lights' - mostly a couple of old windows with timber frames, plus some polycarbonate sheet. The whole thing was a bit sketchy but I used it for around 5 years or so, raising a load of seedlings and cuttings. Mostly, it kept the birds off the pots and provided a dry space for seedlings (which I sowed direct into 4inch square pots.

The back walls were a course higher so that the lights were laid on top at an angle (for water drainage and extra light. It makes a difference how you orient the frames - mine ran across the site, north to south, so the sun hit the surface of the frames all day. It was waterproof but still open to mice so I did protect my sweet peas (for example) with mesh netting or even covered seed trays.

Cold frames have a long history of effective use. Although mine has since been filled with soil and is used as a nursery bed, it was a really useful solution to protecting young plants over winter.

I have seen little wooden coldframes with polycarbonate lids for sale on the interweb but honestly, this is one of those projects which is easy to build from scrap materials. I guarantee you will be able to knock up a robust and generous frame for a fraction of the cost of buying some shoddy effort off Amazon. My allotment site is littered with cheap garden kit, ranging from plastic cloches, rubbish -plant supports, flimsy edging, plastic raised beds and so on. Nearly all of it will fall apart during its first season.

i built a "hot frame" in the spring of 2021 which never heated enough so I think of it as a cold frame. But it was very successful in growing some seedlings; other than peppers. Rather than repeat I'll give you a link to what I posted in an old thread.

I think the hot frame would have been better had I taken both 2x5 foot sashes instead of only one. It's a shame because 2 of them were priced the same as one. In the spring of 2022 I didn't use the frame because a heart condition kept me from hauling the fresh horse manure

Thanks very much, rosaprimula and John D. Good stuff to learn.

How do you think a cold frame would be for sowing delphinium seeds and growing the seedlings? Presumably they wouldn’t germinate until March or April. Now I sow the seed indoors in mid-January, and then grow the seedlings under grow lights. But I have so many that doing the hardening off process in April—bringing them out and in, in and out, every day for 7-10 days—is getting to be SUCH a chore. My thought was to move the process outdoors under a cold frame, though I presume that by May when I want to plant them, the plants will be much smaller than those grown indoors

Delphs were ideal candidates for cold-frame raising. Like the cold, germinate well (and surprisingly quickly) and grow away through winter (slowly). I seem to recall always covering my seeds with pieces of welsh slate cos they seem to prefer to germinate in the dark. I am also growing d.nudicaule and d.semi-barbatum this year.

I don't know delphiniums but I plant tomato seeds in mid March for indoor growing. If I plant them in the cold frame I can plant them in mid April and they're still very much larger. i think it's mainly because the seedlings get full sunlight from sunrise on.

Rosaprimula, did you germinate the delphinium seeds right in the pots in your cold frame? I believe you’re in the UK with warmer winters than what we have here in Massachusetts. I’m curious when I could plant them outside and reasonably expect them to germinate and live through the winter. I’m trying to imagine a small seedling trying to make it through our frigid January and February days and nights, and it seems the only way that could happen is if the cold frame really provides some steady heat thanks to sunny days during the winter.

I think a coldframe provides light and protection from rain/pests, more than any heat, although it does stay a couple of degrees. warmer because of insulation. I have added the extra protection of fleece for seedlings which are already up and growing (if there is a big temperature drop), but for seeds, I let them get on and trust to the plant DNA. I have never been very successful using growlights, heat mats and such - mostly because I cannot keep track of all the different plantlets. I a prefer to leave this to the innate genetics of the seeds.

If you sowed seeds now, they will sit until the conditions are optimum...which would possibly be later than a UK winter. Makes no difference when the seeds are sown...only that the gradually returning sun triggers the seed to germinate at the point of opportunity. Why not try an experiment - sow some seeds now, outside under some cover to protect from winter rains and foraging beasties, and keep some back to do what you have successfully done in the past.? IME, delphs germinate at quite low temperatures (have had seeds trigger while in the fridge).

This year, I have sown a bunch of seeds and left them to get on with it...while I am also attempting to be a bit more scientific, following stratification recipes and moving things in and out of the fridge, keeping notes and timetables. Remains to be seen how everything fares.

Thanks, rosaprimula. I'm curious why you mention that a coldframe provides light and protection from rain and pests more than heat. Isn't the normal and proper functioning of a coldframe to gather the sun's heat through the polycarbonate or other material and use and store it to warm the plants inside? I ask out of pure ignorance, as I've never done it before.

There isn't much thermal mass to store heat in a cold frame so winter temperatures are maybe a degree or 2 higher. Same in the greenhouse. By the time when temperature differentials are an issue, my seedlings are out in the world. I am sure there is some scientific explanation but I am a technical dimwit. I do use the greenhouse all year round (believe me, ventilation is not a problem) but by April, the covers are off the cold-frames.