I planted some peruvian torch seeds about 3 months ago in a warm cupboard with a single 60 watt lightbulb and soil made from equal parts sand and potting compost. Most of them germinated within a few weeks and grew to about 5mm tall. I then moved them to a greenhouse and since then they havent grown any bigger. They have become much darker in colour and grow lots of spines, but just don't look too healthy.

The greenhouse gets fairly warm on sunny days, doesnt get much direct sun and gets cold at night. I was planning on keeping the cacti there until autumn when they would move to a windowsill indoors.

Ive been watering them from the top using a fine mist.

Any tips?

A picture could help to guess what's happening.

When seedling are this young, they don't like sun or too high temperature.

Ive tried attaching a picture here but cant seem to do it. Im using a phone, not a laptop, maybe thats why? Anyway, they cant be getting too hot where they are and no direct sunlight either. They grew quickly at first and then stopped at about 5mm high, went from bright green to dark reddish green and have been growing spines on their tops.

Ok I see how to do it now.

Well... I'd say to much light as well. Just based on the symptoms. This little guys can't take the heat or really any unfiltered light. They just dont have the mass to deal with it. Red/purple baby cacti usually means to much heat or light.

I hope they make it! Just dont water too much, maybe place a fan nearby (being careful not to completely dry out the soil either)

All what Madcap said but for me the soil look too dry for those babies with tiny roots. At this stage, mine are still in the nursery box i use to sprout them, to keep humidity not too low. Of course, soggy soil must be avoided.

Warning : I'm not a cacti pro, just an amateur with 2 years of experience so i could be wrong. Eventually, some expert will chime in.

I never managed to to propagate cacti in a greenhouse for the first year. I think it is because of the temperature swings. It easily can get quite warm on a sunny day, yet at night the temperature drops significantly.

The only ones that survived, were the ones which were kept inside nearby a north facing window. Then slowly adapt them to the greenhouse by covering them first with shadow cloth.

Thanks for the advice. Think Ill try putting them back in the airing cupboard with the single lightbulb where they first germinated and see if they recover.

One more thing, the soil/sand mix doesnt retain much moisture. Is it better to soak from underneath and then leave to dry out or just mist from above once a day, which only really wets the top of the soil?

Just to clarify, my nursery box is a simple transparent plastic box with a cover so natural light is enough (no direct sun !).

Your problem is probably they didn't like to switch their environment (artificial light to natural light and closed space to open space) too fast, gradualy is better.