Everything you need to know about different flower bulbs to grow them successfully

Bulbs, corms, rhizomes… How to tell them apart?

Let’s start with a brief vocabulary note. The expression “plante à bulbes” generally covers all plants that have fleshy underground storage organs. This is what allows them to feed, develop and flower. But in reality, not all have a true bulb.

The bulb is generally swollen and rounded in shape. It is in fact a plant at an embryonic stage that has a stem and whose modified leaves (scales or tunics) cover the central part. The bulb multiplies by producing bulbils under the soil (sometimes called bulblets), next to the main bulb. If you cut an onion in half, you will indeed notice the stem and the surrounding scales, as well as the base from which the roots emerge. In bulb plants, the foliage will die back a few weeks after flowering. But it will have enabled the plant to build up nutrient reserves, carefully stored in the bulb, to restart the life cycle the following year.

Above, hyacinth bulbs. Bottom left, tulip bulbs, and on the right, narcissus bulbs

Corms are also close to bulbs, but have a flatter face. These are also storage organs that allow the plant to store nutrients. However, it is in the swollen scales, formed from modified leaves, that these nutrients are kept. A corm does not have multiple layers of scales like a bulb; it is solid. This is the case for crocuses or gladioli.

Above, gladiolus corms; bottom left, crocus corms; bottom right, ranunculus tubers

Rhizomes, for their part, are in fact large fleshy, swollen roots, as seen in lily of the valley, iris or daylilies. They grow horizontally and bear roots. Again, this allows the plant to create nutrient reserves. Ginger is also one of these rhizomatous plants, as are anemones. Rhizomes are generally running, which allows them to spread quickly, sometimes colonising whole areas. This is what makes certain bamboos invasive, even destructive.

Above, canna rhizomes; bottom left, iris rhizomes; right, ginger rhizome germinating

Finally, let’s consider tubers, which are in fact pieces of stem or root that have swollen to become storage organs. This is what we find in the potato. New shoots develop directly on the tubers, which makes it easy to divide them by separating them from the parent plant. Dahlias also have tubers.

Dahlia and Colocasia tubers

To plant them correctly, read our article: “Which direction should bulbs be planted?“.

Why plant flower bulbs?

Bulbous plants have many advantages:

1. They multiply very easily. Many varieties naturalise, meaning they come back reliably each year and spread within a given area without any intervention from the gardener. A simple way to obtain new plants naturally, capable of flowering quickly. By planting bulbs you can see their numbers double or even triple over the years. This can create impressive floral carpets. It is also possible to divide them to multiply them yourself, to plant elsewhere in the garden or in a pot.
2. Bulbous plants offer an incredible diversity of colours: from the softest pastels to the most vivid, intense shades, there is something to suit every taste. Flower forms are equally varied: clusters, trumpets, bells, stars, cones, and more.
3. Bulbous plants also vary in size and habit. The smallest, which do not exceed about 20 cm, can be grown even in small gardens or in pots on a balcony, terrace or courtyard. The largest, sometimes exceeding 2 m, as with some varieties of cannas or giant dahlias, fit beautifully into lush flower beds. Bulbous plants are also perfect for dressing the base of trees and shrubs, brightening a rockery, a border or a lawn that is a little too restrained. They easily fill empty spaces.
4. Some are highly fragrant, ideal for adding a scent to the garden or near paths. They are indispensable in a scented garden.
5. Bulbous plants are easy to combine in any garden style: romantic, exotic, contemporary, Japanese-inspired, bohemian, naturalistic, etc. They generally do not hinder growth of other plants.
6. By planting different types of bulbs, it is possible to have the garden in flower almost all year round, from late winter to late autumn.
7. They are generally easy to grow, even for beginner gardeners. Simply plant the bulbs in the ground and let them get on with it. No need for fertiliser or extra care. Their main requirement is well-drained soil in which water does not stagnate, as this can cause bulbs to rot. The most tender types are simply lifted before winter and stored dry, such as dahlias or cannas.

Dahlias

Main spring bulbs

Spring bulbs are, logically, those that flower just after winter. Some are particularly early, showing their first flowers while winter is still here, as early as January, such as snowdrops, cyclamens, crocuses or the must-have narcissi. A little later in spring you will also see hyacinths, muscari, tulips, anemones and irises. At the end of spring, even at the start of summer, the ornamental allium takes over with its very graphic, spherical and colourful flowering. Less well known but equally interesting are the scillas and their bell-shaped flowers, the camassias with their spikes of star-shaped flowers, and the erythroniums with their charming lantern-like flowers.

They thus poetically herald the imminent return of warmth and light, while bringing a welcome touch of colour and vitality during the least lavish season in the garden.

Plant these bulbs in autumn.

Muscari, Erythronium, allium and tulips

Main summer bulbs

As you will have realised, summer-flowering bulbs bloom during the warm season. Among the best known are obviously dahlias, lilies, arums, cannas, amaryllis, agapanthus and alstroemerias. Let’s not forget crocosmias, which resemble small gladioli and show vivid colours, perfect for adding a touch of the exotic. Also worth mentioning are tulbaghias, with lanceolate foliage and graceful white or pink tubular flowers. For richly scented blooms, turn to certain lilies, freesias with their jasmine‑like fragrance, or ornithogalums.

One of the few drawbacks of these floriferous, colourful bulbs? They require cool soil (that never dries out completely), at a time of year when water shortages affect many regions. Especially in southern gardens, it is therefore important to site them in a spot sheltered from drying winds and scorching sun. Applying mulch is also recommended to reduce natural evaporation and better retain moisture in the soil. Summer bulbs are also often the least hardy. Some are downright tender and will need to be lifted before winter in regions that experience frost.

Crocosmia, alstroemeria, dahlia and freesia

Main autumn bulbs

Autumn-flowering bulbs take over from summer blooms once summer ends. They help prolong garden interest and are superb in woodland settings, adding a splash of colour. Among must-haves are ivy-leaved cyclamens and colchicums, capable of forming vast carpets. It’s also the flowering period of the famous Crocus sativus, whose flowers yield the precious red spice: saffron. Less well known, the Sternbergia lutea or false autumn crocus produces bright, cup-shaped yellow flowers between September and October. Also mention the nerines, whose late flowering occurs between September and November, revealing umbels of flowers with curled petals.

In temperate climates, they are planted at the end of summer and have the advantage of flowering a few weeks later. In warmer, drier regions, they are planted in autumn and will flower the following year.

Crocus sativus, ivy-leaved cyclamen, Sternbergia and nerines

Choosing the right flower bulbs

As with all other plants, bulbs will be chosen according to your growing conditions. So take into account :

• soil type (heavy, light, fertile, poor…), to adapt planting conditions (lighten soil, feed it…) ;
• exposure (sunny, dappled, part‑shaded…) ;
• available space (small border or planter, large bed, lawn…) ;
• climatic conditions (sea spray, heat, humidity, strong winds, severe frosts…).

For example, large flower bulbs, such as gladioli, do not tolerate excessively windy exposure, which can cause their flower spike to bend. Therefore choose a sheltered position or provide suitable staking.

In garden centres, bulbs are often available in packets. Make sure you choose only healthy-looking bulbs, that are firm and show no signs of mould or damage. Note that size refers to the bulb’s circumference, usually given in centimetres. This gives an indication of the eventual flower size: a snowdrop of size 4–5 cm will logically produce a smaller flower than a tulip bulb of size 10+.