Stenocactus obvallatus

Stenocactus obvallatus is a small cactus native to central Mexico. It belongs to the cactus family, Cactaceae. Its range includes Durango, Hidalgo, Oaxaca, Querétaro, and nearby states. It grows in desert or dry shrubland. In Mexico, it has the name Tepenexcomitl. Its identity and range still need more study. Some plants placed under this name may belong to other species.

The stem forms a ball or a broad egg shape. Most plants have one stem, though some develop two. They reach about 0.20–0.33 feet (0.06–0.10 metres) tall, sometimes more. Usual width is 0.20–0.36 feet (0.06–0.11 metres). Some reach 0.49 feet (0.15 metres) across. The grey-green or blue-green stem has a sunken crown. It bears 25–50 thin, wavy ribs that widen at the areoles. Each rib holds two to four of these white pads. Each pad bears about five to twelve spines. The longer central spines rise or spread from the stem. The upper spine is broad, flat, and reddish brown. Shorter white spines surround the central group.

Flower buds nest in wool at the stem tip. Their tips may show before growth starts in early spring. Several flowers can open together during a month-long display. Each bloom spans about 0.10 feet (0.03 metres). Flowers range from pale yellow or white to pale pink. Purple-red stripes mark the petals, and the centre looks reddish. Good conditions may bring another flush later in the year. Its compact form and flowers suit a potted cactus collection.

Give this cactus plenty of light and good air movement. Use a free-draining cactus mix with little organic matter. Choose a small pot with drainage holes, and repot every other year. Water during summer, then let the mix dry before watering again. Keep it dry through winter and avoid damp air during rest. Wet compost can kill roots and cause rot. Too much shade and water can also distort its shape. During growth, use feed low in nitrogen, with more potassium and phosphorus.

Frost can harm this species, though dry plants may withstand brief cold near −5°C. A winter minimum of 5–8°C gives a safer margin. Growers usually raise new plants from seed in spring. Seeds can sprout within one to two weeks at 21–27°C. Give young plants air, but keep them out of full sun. Wait until roots develop well before moving seedlings into separate pots.