I’ve had these Astrophytums (first pic is A. myriostigma and the second is A. coahuilense) for almost 4 years now and they’re the slowest growing cacti I own. These ones are special to me because they started my fascination with spineless cacti.
The bright part of the plant shows exactly how much they have grown in the 4 years I’ve had them. Normally, the new growth just blends in, but I kind of had an accident with mine. Just after I bought them, I mistakenly placed them under an aphid-infested tree and they were covered in sticky aphid poo. The poo then acted like agar, which is the perfect nutritious meal for black sooty mold. I now had two sticky, mold infested cacti. Because Astrophytums are hairy, I couldn’t remove all of the soot mold and my plants have looked dirty ever since then. The new growth on the top of the plants is perfectly clean, and it now acts as a way for me to check on them and if they’re growing like they should. And it looks kind of cool, too.