The plants we generally refer to as bulbs have swollen, specialized roots, stems or modified leaves that store nascent buds, moisture and nutrients beneath the ground. Underground storage protects and allows them to become dormant – sacrifice their foliage when drought, extreme heat or cold conditions settle in to stay – then flourish again when growing conditions become favorable.

Tulips are a true bulb. They have an onionskin-like tunic that surrounds the bulb and helps prevent it from drying.

Photo Credit: ©2002 Dolezal Publishing/John M. Rickard

Crocus is a corm. Like the tulip, it also has an onion-like tunic, but its interior is more solid and uniform.

Photo Credit: ©2002 Dolezal Publishing/Donna Krischan

Windflowers grow from rhizomes.

Photo Credit: ©2002 Dolezal Publishing/John M. Rickard

Persian buttercup comes from a tuberous root.

Photo Credit: ©2002 Dolezal Publishing/John M. Rickard

Begonias are tubers.

Photo Credit: ©2002 Dolezal Publishing/John M. Rickard

There are five broad horticultural categories of bulbous plants: true bulbs, corms, rhizomes, tubers and tuberous roots.

True bulb

A plate-like base of modified stem tissue with attached, scale-like storage leaves and a central growth bud, often complete with an embryonic bloom, stem and leaves. Bulbs bear concentric rings if cut parallel to the base plate. They regenerate fully each year, often forming small offsets called “bulblets” along their basal plate, or forming seed or “bulbils” at either the site of their former flowers or, in the case of some lilies, at their axils – the junctions of stems and leaves.

Corm

A swollen, underground stem atop a plate-like base with one or more growth buds, usually bearing a flaky, dry, skin-like covering that preserves moisture. Corms appear solid, lacking visible structures inside. They’re depleted and wither as the plants grow, and a new corm forms on top of the old. Offspring – called “cormels” – form around the base of the new corm, taking 2-4 years to flower if detached and raised separately.

Rhizome

A modified thickened stem that lies horizontally atop the soil, partially beneath or completely underground, bearing scale-like leaves with nascent buds at each axil. Rhizomes branch outward to form new plants, each with independent roots and growth buds, as the parent root withers and dries.

Tuber

Depending on the species, either modified stems or roots lacking internal structures, with minuscule surface scales bearing eyes or growth buds along its surface. Tubers usually appear solid when cut. Like corms, many tuber species shrivel as the plants grow and nutrients stored within are consumed; simultaneously, they multiply by division to form new tubers. Others renew and enlarge their tuber each season.

Tuberous root

Engorged, fibrous, modified roots surround a crown – the junction of the stem with the root – bearing growth buds on the crown. True roots emerge from the tuberous root, and stems, leaves and flowers sprout from the crown. Over time, multiple crowns emerge from the expanding mass of tuberous roots, creating offspring plants.

Most bulbous plants are petaloid monocotyledons, meaning they sprout a single leaf as their seeds germinate, and they have petaled flowers. The rest are dicotyledons, specialized members of plant families that include both annual and perennial species.

The most common bulbs generally belong to three families – amaryllis (amaryllidaceae), iris (iridaceae) and lily (liliaceae) – and members of those widespread families can be found in virtually every geographic region except the poles. Fully a third of all the genera containing bulbous plants are represented in South Africa. Growers have yet to explore all of the possible disease- and pest-resistant varieties, vibrant colors and enticing scents of bulbs as they continue to hybridize wild species and create new cultivars.

Some popular genera, including Tulipa and Narcissus, have been divided further into formal horticultural classifications, termed divisions:

Narcissus divisions: 1) trumpet, 2) large-cupped, 3) small-cupped, 4) double, 5) Triandrus, 6) Cyclamineus, 7) Jonquilla, 8) Tazetta, 9) Poeticus, 10) species and wild forms, 11) split-corona and 12) other narcissus.

Tulipa divisions: 1) single early, 2) double early, 3) triumph, 4) Darwin hybrid, 5) single late (Darwin and cottage), 6) lily-flowered, 7) fringed, 8) veridiflora, 9) Rembrandt, 10) parrot, 11) double late (peony-flowered), 12) Kaufmanniana hybrids, 13) Fosterana hybrids, 14) Greigii hybrids and 15) other species, varieties and hybrids.

Bulbs vary according to their botanic classification and structural form. True bulbs hold a complete plant within them – a roots, stems, leaves and embryonic flowers – while the other bulb categories largely contain differentiated cells that will give rise to the mature plant’s parts clustered in growing points.

Bulbs have developed life cycles that permit them to grow and bloom under a wide range of conditions. Due to their diverse origins, there are bulbs suited to every garden, purpose, climate and soil. Each bulbous plant you’ll grow in your garden or indoors is fully adapted to a specific climate somewhere in the wild. Hardy bulbs capable of weathering the cold-winter climates are found in their native mountains of China, Japan, Russia and Turkey. Tender bulbs hail from central Asia, South Africa, Mediterranean climates such as Northern Africa and southern Europe. Tropical species require mild weather for planting outdoors and thrive indoors provided they receive adequate light and moisture.

Identifying a specific bulb for planting in your garden begins by knowing its two names: the common name, like “daffodil” or “jonquil,” and the scientific one – usually in Latin or latinized Greek and standardized by botanists throughout the world – like “Narcissus.”

Common names, while colorful and useful, vary by region and locale, are often used for plants of different species, and frequently change over time as horticulturists reclassify plants. To identify a specific cultivar, its variety name appears in single quotes after the scientific name, such as ‘Golden Boy’. When describing a bulb to nursery or garden center staff or a horticulturist, always use the scientific name to ensure accuracy.