Bulbs, Corms, Rhizomes and Tubers - What's the Difference?

Whether they’re flowering in your garden or cramming the shelves in garden centres, bulbs are everywhere right now. Looking at the packets on the rails, you’ll see “10 bulbs for £4.99” or “5 corms for £1.99”, and we use the general term ‘bulbs’ to cover bulbous plants (technical term, geophytes), but what is the difference?

A bulb has a modified stem, called a basal plate from which leaf bases, often called scales, grow. Imagine peeling an onion, and each layer is one of these modified leaves. Growth comes from the middle of the bulb, and roots grow from the basal plate—examples: daffodil, allium, lily.

Corms possess a swollen vertical stem, which, unlike bulbs, is solid. A fresh corm replaces the old one each growing season, with a ‘tunic’ of old leaf bases protecting it—examples include crocus, crocosmia, ixia.

Think potatoes. A tuber is a swollen underground stem or root used as a storage organ and usually has multiple growing points—examples include dahlia, begonia, cyclamen (often marketed as a corm).

Thickened underground stems or roots, not quite as big as a tuber. Rhizomes and tubers are easily propagated compared to bulbs and corms and can be invasive—examples include lily of the valley, Solomon’s seal, bearded iris.

Formed by orchids, these sit on the growing medium and are a thickened part of the stem. Although you’ll see them on many indoor orchids, like Cymbidium and Oncidium, you’ll generally only see the hardy(ish) Pleione and Bletilla for sale as dry bulbs.