Mammillaria standleyi
Standley's Mammillaria!
I obtained a specimen of this Mammillaria years ago. It was an older plant with offsets, so I gently removed several heads and planted them in the landscape for trial. In the ground in Hempstead, with wet winters and lows down into the upper teens, the plant did fine.
The plant produces hemispherical bodies that reach about 4" in diameter and are green to grey-green in color. It is typically solitary in youth but will cluster with age. A white wool is produced in the axils on the upper 1/3 of the plant. Small, pea sized, purplish flowers occur early in the year. I sure wish I had not left my potted specimen out in the extreme cold because this was one of my favorite Mammillarias.

