What is the difference between a bulb, a corm, a tuber and a rhizome?

All four are underground storage organs letting a plant survive dormancy and regrow, but they are structurally different parts of the plant — and the difference determines how you plant, divide and propagate them.

A true bulb is a compressed stem surrounded by fleshy modified leaves — the scales. Cut one in half and the layered structure is obvious, with a basal plate at the bottom producing roots and a miniature flower already formed inside.

Examples: daffodil, tulip, onion, garlic, allium, snowdrop, lily.

They multiply by producing offsets — small bulbs beside the parent — which can be separated and grown on.

A corm is a solid, swollen stem base, with no layers. It looks superficially like a bulb but is uniform inside, with a papery tunic outside.

Examples: crocus, gladiolus, freesia, crocosmia.

The parent corm is used up each year and a new one forms on top of the old, often with tiny cormlets around it. This is why crocuses can gradually rise toward the surface.

A tuber is a swollen storage organ, and there are two kinds.

Stem tubers — potatoes and Jerusalem artichokes — have eyes, which are buds, distributed over the surface. This is why a potato can be cut into pieces and each piece with an eye will grow.

Root tubers — dahlias and sweet potatoes — are swollen roots and have no eyes. Dahlia tubers must be divided with a piece of the old stem crown attached, or they will not shoot. Dividing a dahlia clump into bare tubers produces nothing, which is a common and disappointing mistake.

A rhizome is a horizontal stem growing at or below the surface, sending shoots up and roots down along its length.

Examples: bearded iris, ginger, mint, bamboo, couch grass.

Bearded iris rhizomes must sit on the surface in sun, not buried, or they will not flower. And rhizomatous spread is exactly why mint and bamboo need containing.