Allium triquetrum thrives with consistent moisture during its active growing season, from late winter through spring. Ensure the soil remains damp but not waterlogged, as excessive wetness can lead to bulb rot. Once the foliage begins to yellow and die back in late spring or early summer, indicating the onset of dormancy, significantly reduce watering. During its summer dormancy, the plant requires very little to no water, relying on residual soil moisture.

Pruning for Three-Cornered Leek primarily involves removing spent flower stalks to prevent the plant from self-seeding, which is crucial for controlling its invasive spread. Dead or yellowing leaves can be tidied up for aesthetic purposes, though they will naturally die back as the plant enters its summer dormancy. No other significant pruning is typically required.

Allium triquetrum propagates very easily through several methods. It readily self-seeds, and its bulbs produce numerous bulbils that can be collected and planted. Clumps of bulbs can also be divided in late summer or early autumn during their dormant period. Due to its invasive nature, propagation should be managed carefully to prevent unwanted spread.

Three-Cornered Leek is not commonly grown in pots due to its vigorous and invasive nature, preferring to naturalize in garden settings. If grown in a container to restrict its spread, repotting should be done annually in late summer or early autumn. Divide the overcrowded bulbs and refresh the potting mix to maintain plant health and vigor.