It's Tricho season.
Many genera of cacti were lumped into genus Echinopsis. The formerly accepted genera were easily distinguishable by plant and flower size, upright versus sprawling habit, flowers opening by day or by night and by cultivation requirements.
Trichocereus were the plants with medium to large, upright stems, frequently clumping from the base, but sometimes growing like branching trees, with mostly evening-opening, colored flowers that stayed open into the day. This is an example of a hybrid Trichocereus. Stems are 5" in diameter and easily reach a meter tall. Most things included in Echinopsis, like this plant, clump from the base, so they are easily propagated by cuttings.
You can see a typical cactus flower: There is no firm difference between petals and sepals; numerous stamens in a ring from the throat; the pistil at the center.
Trichocereus hybrid
Linear Mode
