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I bought two packets of Browningia seeds: candelaris and caineana, and a packet of Astrocactus bertinii. I sowed all three in takeaway containers with a 50/50 mix of coarse sand and fine seedling mix and put on the lid. At the same time I sowed about 20 other varieties of seeds. All were in separate containers

That was almost twelve weeks ago. Every other type has germinated to some degree, but I've gotten nothing from theses three seeds.

Around six weeks back I opened all the containers up and added a bit more water as they looked a bit dry, but still nothing. I even put the containers in the fridge for a few days to see if some "cold weather" followed by warmth might trick them into thinking it's spring. Still nothing.

Is there something special I need to do with these species? I do have gibberellic acid but I've not used it yet. Maybe I should though?

No experience with these, but I saw that Austrocactus seeds are very difficult to germinate, so on the French site Semeurs de cactus (only in French) it gives a range of % of success with various methods, and a bath of hydrogen peroxide scores the highest, so you could try.
http://semeurs-de-cactus.fr/index.php/E ... re/key/126
Or scarification, or just chip the seeds. I had great success doing this on Pediocactus seeds, here's the link, including a youtube video :
https://www.cactiguide.com/forum/viewtopic.php?t=44006
You could try different approaches and see what works

Jangaudi wrote: ↑Wed Apr 14, 2021 9:20 am No experience with these, but I saw that Austrocactus seeds are very difficult to germinate, so on the French site Semeurs de cactus (only in French) it gives a range of % of success with various methods, and a bath of hydrogen peroxide scores the highest, so you could try.
http://semeurs-de-cactus.fr/index.php/E ... re/key/126
Or scarification, or just chip the seeds. I had great success doing this on Pediocactus seeds, here's the link, including a youtube video :
https://www.cactiguide.com/forum/viewtopic.php?t=44006
You could try different approaches and see what works

Any idea what they mean by hydrogen peroxide? What strength, how long etc?

Edit: The nail clipper idea is good.

Download wrote: ↑Wed Apr 14, 2021 10:24 am Any idea what they mean by hydrogen peroxide? What strength, how long etc?

Nope, been trying to decipher this website for a while as my French is not that great Sometimes extra information is added, like % or time, but not always. However, I think hydrogen peroxide is usually used pure, as it is mostly sold as a 3% dilution. But maybe someone can corroborate.

I have sowed few species seeds after Hydrogen peroxide treatment and the results are satisfactory till now. I dilute 3% Hydrogen peroxide with water (1 part Hydrogen peroxide and 3 parts water). I then soak my seeds in it for sometime and rinse it again with plain water. Also I normally wet my seed potting mix with diluted hydrogen peroxide rather than plain water. It works in two manner - it first kills all pathogens and secondly provides extra molecule of oxygen for further root development.

Some desert cacti also need some longer time before to be able to germinate, sometimes several years. I would store those Browningia seeds and try them again after some months or a year.