Thimble mammillaria Mammillaria vetula

The thimble cactus is a small, slow-growing, succulent subshrub found in dry shrublands or rocky slopes in the Mexican states of Hidalgo, Guanajuato and Queretaro. They are frequently found in colonies and are grown as ornamentals. This cactus has a clumping habit and grows from 3 to 6 inches tall and 4 to 5 inches wide. It has cylinder-like stems that are covered with white, hair-like spines. The origin of the common name, thimble cactus, results from the stems looking like tiny little thimbles. It is a member of the Cactaceae family. The epithet is Latin for "little old one."

This plant grows best in full sun or bright direct light and appreciates some afternoon shade in hot weather. They prefer sandy soil or a well-drained cactus mix and require good drainage. Adding grit or perlite for extra drainage helps, but they are tough plants that tolerate a variety of conditions. It is hardy in temperatures ranging 20 to 100 degrees Fahrenheit. It also prefers slightly acidic to neutral pH.  The plant is drought tolerant, and It is best to allow the soil to dry out completely between waterings. Stop watering in the winter and move to a colder location to encourage flowering in the spring. Repotting may be required when the plant outgrows its container or about every 2 to 3 years. It is propagated by offsets, stem cuttings, and seeds. 

The thimble cactus stems are 3 to 6 inches long, bluish-green, and cylindrical and are 0.5 to 1.5 inches wide with rounded tips. The stems is covered with tubercles that are small and mound-like. Each tubercle has 11 to 16 stiff, white radial spines and less than 5 white or dark brown central spines. Central spines are usually found on mature plants. The flowers are small, yellow, funnel-shaped, and about 0.5 inches in diameter. The petals have a long, pink to brownish stripe in the center. The fruits are pinkish-white and mature to a fleshy red with numerous tiny black seeds. 

The thimble cactus is easy to grow and popular as an ornamental. It is frequently used as a houseplant or in rock gardens or xeriscapes. 

Insects, Diseases, and Other Plant Problems:  This plant is generally pest-free. Monitor for mealybugs, scale insects, and spider mites. Avoid overwatering to prevent root rot and provide good air circulation to prevent fungal infections. 

VIDEO Created by Elisabeth Meyer for "Houseplants, Succulents, and Cacti", a plant identification course offered in partnership with Longwood Gardens.   

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