Today we have an answer!
I stopped by Cactus Jungle a few months ago asking about Edithcolea grandis. At the time, you said you didn’t have any because you always get it from a supplier. When I asked about propagating, you said that it’s hard to grow from seed, and you have never been able to get it to root. Since then, I’ve been doing some experimenting, and wanted to share some successes.
First of all, I don’t know how people grow it from seed; I couldn’t do it at all.
But, after my main plant started to rot and I took a few cuttings, I was able to experiment with rooting. What worked (with 2 separate cuttings) was to use rooting hormone, put pots in a warm-ish place that’s accessible to sunlight – nothing drastic, just a windowsill or an inner covered patio), keep the soil moist, but not wet, and keep the pot covered with a plastic bag. I just had regular 2-inch plastic pots covered by a ziplock bag. I live in the foggy part of San Francisco, so it was never especially warm or cold. What did NOT work was keeping pots uncovered, keeping them in the greenhouse (probably too much temp variation?), or keeping the soil too dry or too wet.
I hope this is helpful to you, and thanks for the beautiful nursery and great plants.
Thanks for the update! Do you have pictures of the new little plants?