This is a very slow growing succulent species. It is also endangered, so the more people that cultivate this species, the better chance that good genetics can continue to exist and the species not go extinct. Mature specimens can sell for 100's of dollars online. I am grafting about half of my seedlings, and let the other half grow bare root.
I tried two methods- One using my standard succulent seed starting mix, and another with homogeneous medium grade (for cement) sand. Both either had an humidity dome, or the pot was placed in a plastic bag, respectively. Surprisingly (due to its simplicity), the sand condition worked the best. I had the highest germination on sand, and higher survival. This was surprising, but a clear result- so recoment planting these seeds on pure sand.
These are all grown under artificial LED grow light. I am using a "medium" light level for these seedlings. I will use brighter light once they are bigger and stronger.
UPDATE: ~40 days in sand. Maybe 1 mm in diameter.
I am starting to add some moisture around 1x a week by spraying the side of the bag with boiled water. But otherwise keep the bag closed.
UPDATE: I opened the bag to stay open, but left the cup in the bag at ~60 days. I would occasionally add water to the bag to water the cup from the bottom, and continue to spray the inside from a water sprayer when the surface looked dry and starting to crack. I continue to only use boiled water at this time to limit algae growth.
UPDATE: I added a bit of diluted cactus liquid fertilizer and coffee at ~80 days.
UPDATE: 8 months after germination. I have kept the cup in the bag, but left the bag open. I spray regularly (~1-2 weeks) with H2O, and keep under my LED Grow lights. They are now a whopping 2mm in diameter!
UPDATE: 18 months post-germination. One is really taking off! That bigger one is a little over 7mm. I have taken some of the other seedlings for grafting- which I will show when they have started to take off.
Great notes, Chris! I really appreciate the timing and photo posts, as well as the dual growing medium component. I'm about to sow a bag of these seeds, and understanding the conditions you succeeded in is a true boon. The added benefit of target dates/actions very much helps my expectations and formulation of actions/treatments. Currently I have managed around 200 Ariocarpus and Lophophora seedlings for over a year, and recently branched into Copiapoa. So, I guess, why not Strombocactus?! I also found the high sand-low loam-limestone mix to work best for me with high success. The hardening to a drier environment has been long and slow, as seems yours. I truly appreciate your post. Thank you.
Thx a lot Chris, i am about to sow this cactus. I ll use pur sand in one of them following ur advice and for the other one i ll put 5 mm of sand on top and regular mix with a bit of limestone to fill the bottom part. My hypothesis is that once the roots pass the sand, they wil get more oxygene and nutrient going down the pot.