Mammillaria are not plants for landscaping around here, unless you live on the side of an arid cliff where it doesn't rain in the winter. Hewitt: "the plant needs to be dry when dormant in winter; it rots if over watered." Yep.
The longifolia which had done well ---filled up its 2" pot--- lived under an overhang where it didn't get rained on and was generally flourishing.Until, a victim of some demented Mary Poppins,it was seized, repotted,and put it where it got watered in winter. Total disaster--it rotted from the root.
Cactus have such strong personalities, like cats, that to lose one is not insignificant. And to lose one out of of careless ignorance is just plain tacky. Feeling awful, went to Home Depot to replace poor longiflora.
No mammillaria longiflora to be found. (longiflora refers to the striped pink and white flower; mammillaria means nipple. As a group the mammillarias are sometimes referred to as Nipple Plants.) What's being referred to is the cone shaped nodule that holds the spines.
You can see the cones more clearly in this mammillaria pringlei especially the right hand one.(The m. longiflora substitute was this group of three pringleii.)
Many mammillaria come from Central Mexico where it rains a lot from May to September Mammillaria can't handle water during those months----however, they need really sharp drainage Put some stones in the pot. Many are cliff dwellers. Save a life--move your mammillarias under shelter so they'll be protected from rain and cold.