Bulb, corm, tuber or rhizome?
All are underground food stores that let a plant sit out winter or drought. Whether the store is a stem, a root or a bundle of leaf bases changes how you plant and divide it.
Bulb
A short stem (the basal plate) wrapped in fleshy scales that hold the food store, with the next flower folded inside.
How it grows. Flowers on the food in the scales. After flowering the leaves feed the bulb for next year, so let them die down naturally.
How to propagate. Offsets (small bulbs at the side), scales (lilies), or bulbils. Divide when dormant or just after the leaves die.
Examples. Allium (ornamental onion), Camas, Crown imperial, Daffodil, Dog's-tooth violet, Dutch iris.
Corm
A swollen, solid stem base. No layers inside, just starchy tissue, with a papery tunic and buds on top.
How it grows. Each year the old corm shrivels and a new one forms on top of it, often with small cormlets around it.
How to propagate. Remove cormlets and grow them on for two to three years. Large corms can be cut only if each piece has a bud.
Examples. Colchicum (autumn crocus), Crocosmia (montbretia), Crocus (spring-flowering), Freesia, Gladiolus, Ixia.
Tuber
A swollen piece of underground stem that carries buds (eyes) scattered over its surface.
How it grows. Shoots come from eyes on the surface. The tuber is a swollen stem, so it can be cut into pieces that each carry an eye.
How to propagate. Cut into pieces with at least one eye (two for potatoes), let the cuts dry, then plant.
Examples. Anemone blanda (Grecian windflower), Anemone coronaria (poppy anemone), Climbing lily, Hardy cyclamen, Jerusalem artichoke, Lords and ladies.
Rhizome
A stem that grows sideways at or just below the soil, with nodes that make shoots above and roots below.
How it grows. The stem creeps sideways and branches. Growth comes from the tips and from nodes along it.
How to propagate. Cut sections that each have a node or a fan of leaves. Replant at the same level, often with the top of the rhizome at the surface.
Examples. Agapanthus, Asparagus, Bearded iris, Calla lily, Canna, Ginger.
Tuberous root
A true root swollen with food. It has no buds of its own: new shoots come only from the crown where the roots join the old stem.
How it grows. The food is stored in swollen roots. New growth comes only from buds on the crown where the roots join the old stem.
How to propagate. Divide so every piece keeps part of the crown with at least one bud. A swollen root with no crown will not grow.
Examples. Dahlia, Herbaceous peony, Ranunculus (Persian buttercup), Sweet potato, .
Side by side
| Bulb | Corm | Tuber | Rhizome | Tuberous root | |
|---|---|---|---|---|---|
| What it is | Stem base plus fleshy leaf scales | Swollen stem base, solid | Swollen stem tip | Horizontal underground stem | Swollen root |
| Inside | Layers | Solid, starchy | Solid, with eyes outside | Segmented, with nodes | Solid, no buds |
| Buds | Inside, at the centre | On top | Eyes over the surface | At nodes and the tip | Only at the crown |
| Lasts | Many years | One year, replaced by a new one | One year, or grows larger | Spreads indefinitely | Clump enlarges |
| Divide by | Offsets, scales | Cormlets | Pieces with an eye | Cut at nodes | Pieces with crown |
| Examples | Tulip, daffodil, lily, garlic | Crocus, gladiolus, freesia | Potato, cyclamen, tuberous begonia | Iris, canna, ginger, turmeric | Dahlia, peony, sweet potato |
Why it matters in the garden
Depth: bulbs and corms go in at two to three times their own height, while a bearded iris rhizome sits with its top at the surface. Division: a potato tuber can be cut into pieces with eyes, but cutting a daffodil bulb in half does not give two plants. Storage: layered bulbs dry out fast, solid corms tolerate drying, and dahlias are lifted with the crown intact.
Pets: several of these plants are unsafe for cats and dogs, and the bulb is often the most toxic part. Each plant page quotes what the ASPCA lists.