Many of my cacti are planted in dish-gardens with good quality potting soil mixed with an equal amount of sand. My experience shows this is superior to pure potting soil. These cacti are for display on the balcony, so I display them properly with rocks and pebbles mulch. Most of my containers are non-glazed terra-cotta. I have let many of the seeds drop near their parent plants. Most of the seed-producers are Mammillarias. I grow them in a mediterranean climate, so there must be occasional irrigation in summer. When I removed the pebbles mulch, I noticed there are many seedlings. I decided to keep them and cover them back, and here they pop-out through the mulch the next year. Not as many as before, but they do survive the mediterranean climate.

Pebbles mulch, after all, increases survival chances of seedlings. It offers protection from direct sunlight and retains some humidity. Just like with any other plant that can be propagated in the open, I expect that chances of fungal and other infection will be lower than inside a high-humidity and low-air-circulation greenhouse. Seeds sown in the open do not need a sterile soil, you can just toss them in the garden. The same garden soil will spell disaster to seeds inside a nursery.

Anyone has experimented with on-spot seed germination? (Germinating the seeds outside and let them grow where they are). I use non-glazed terra-cotta containers, to allow the soil dry more quickly during winter rains. In summer, this is still important because the stone mulch may retain the water for too long and reduce air circulation. The dry stone mulch at the top may deceive you and let you think the soil is dry. I did not find any need to irrigate the seedlings more than I do irrigate the mature cacti. In other climates I may need to irrigate them a little bit more.

Experimenting with types of mulch: Transluscent (Silicate-based, including sand) Vs. dark and opaque (Pumice, dolomite) and different colors within-reach (Preferrable natural stone colors, not dyed pebbles!!!). Size of pebbles matters too, but I don't have such a great variety within reach. When I find the seedlings, they were then growing under a layer of coarse pumice pebbles. I then replaced the pebbles with the more aestetically appealing and smaller beach pebbles (mostly white-colored calcium-based sea-shell fragments and small sandstones). Looking back, I may have needed to increase irrigation, but that still seems to work.

Anyone tried?