(This plant comes in a 3.5" pot. You will receive a very similar plant to the one in the picture. It may or may not be blooming at the time of your purchase. The plant is shipped in its pot to prevent any damage to the roots).
Mammillaria duwei is a small solitary rarely offsetting plants that eventually forms tiny mounds. It is a small growing, but easily flowering, species that will make clumps given the best conditions.
Flowers: yellowish/cream to light yellow, about 20 mm long and in diameter.
Blooming season: Flowers readily, and for a long period from spring to the early summer.
Hardiness: Reputedly sensitive to frost, but less so if kept on the dry side prior to, and during, cold weather (hardy to -5° C for short periods). However warmth throughout the year will increase the grower's success (minimum 5° to 8°C during rest season).
Exposition: Outside bright sun, filtered sunlight or afternoon shade, inside it needs bright light, and some direct sun. Subject to sunburn if exposed to direct sun for too long. Tends to bronze in strong light, which encourages flowering and heavy wool and spine production.
Some of the information in this description has been found at desert-tropicals.com, llifle.com and cactus-art.biz