Strombocactus disciformis aka Top Cactus

Strombocactus disciformis is a small Mexican cactus in the Cactaceae family. Its native range includes Querétaro and Hidalgo, with records from Guanajuato. It grows in desert and dry scrub. Wild plants occupy steep limestone slopes and cliffs. They also grow on mudrock banks beside streams. Some sites lie on near-vertical canyon walls. Illegal collecting for the plant trade has reduced wild populations.

This slow-growing cactus most often forms one rounded or flattened stem. It stands about 0.10–0.26 feet [0.03–0.08 metres] tall. Its width reaches 0.10–0.30 feet [0.03–0.09 metres]. Old plants can reach 0.66 feet [0.20 metres] tall. The blue-green stem has a grey cast. Brown, cork-like patches form around its base with age. A shallow dip and soft felt mark the crown. Firm, diamond-shaped tubercles overlap in spirals across the stem. Their tops look flat, while their lower sides form ridges. A thick, turnip-shaped root grows beneath the plant.

The upright spines have pale grey bases and darker tips. They reach about 0.04–0.07 feet [0.012–0.020 metres] long. Older parts of the stem shed their spines. Cream flowers grow from the crown from early spring through summer. Each bloom can stay open for several days. Flowers measure about 0.11 feet [0.035 metres] long and wide. Some have small marks near the tips or throat. Yellow anthers surround a white or yellowish stigma with eight to ten lobes. Small fruits split lengthwise and hold very small seeds.

In cultivation, this cactus needs a mineral mix that drains well. The mix should contain little organic matter. Strong sun or partial sun helps keep its form compact. Water sparingly from March through October. Keep it dry in winter, or when nights stay below 10°C. Growers keep plants on their own roots or use grafts. Its small size, patterned stem and cream blooms give it ornamental value.

Growers raise new plants almost entirely from seed. The seedlings grow slowly and need careful watering. They may take one or two years to reach a tiny size. Plants become easier to grow after about four years. Reaching flowering size can take eight to ten years. The supplied sources give no species-level food or toxicity assessment. They also provide no detailed account of pests or diseases.