I have found them to be very hit or miss from seed here in Phoenix, under my rough and ready outdoor conditions. The majority of hook-spined, Baja/Sea of Cortez mammillaria seem very challenging from seed. They love to rot. Once they get established, after a few years, they are more resilient. I bet I would have much better success if I could provide much more carefully controlled conditions for the first few years.
Late July into August is always the time when "Ferocactus" hamatacanthus sinuatus puts out bright lemon yellow flowers, very pretty. I have two of these from seed. The larger one is from the southern Texas area and the smaller, but I think better looking one is from Tamaulipas MX. I think they really belong in Hamatocactus or maybe even Thelocactus.
from Tamaulipas, 10 years old. one gallon pot.
001.JPG (223.36 KiB) Viewed 264 times
002.JPG (172.29 KiB) Viewed 264 times
Larger plant from Texas, 15 years old, 2 gallon pot.
some Echinofossulocactus seedling pics by request and a few other bits.The seedlings have recently just gotten hammered by several heavy rains, hope they don't rot. I am experimenting with intentional, slight etiolation this year, just to bulk them up more before winter. The plan is to tighten them up in stronger sun gradually next "spring" (February-April here).