Ornamental bulbous plant

Bulb of a red onion, cut vertically

A true bulb (i.e. a bulb in the botanical sense) is an underground vertical shoot that has modified leaves (or thickened leaf bases) that are used as food storage organs by the plant. The bottom of the bulb is made up of a short section of stem forming the basal plate. Storage leaves are produced from the top of the basal plate, roots from the underside.[4] Genera with true bulbs are Muscari, Allium, Tulipa and Narcissus.[5]

Crocosmia corm, showing solid construction with outer tunic and shoots emerging from the top

A corm is a short, vertical, swollen underground plant stem consisting of one or more internodes with at least one growing point, with protective leaves modified into skins or tunics. The thin tunic leaves are dry papery, dead sheaths, formed from the leaves produced the year before. They act as a covering that protects the corm from insects and water loss. Internally a corm is mostly made of starch-containing parenchyma cells above a more-or-less circular basal node that grows roots. Corms are sometimes confused with true bulbs; they are often similar in appearance to bulbs externally, but corms are internally structured with solid tissues, which distinguishes them from bulbs, which are mostly made up of layered fleshy scales.[4]

Dried ginger rhizome

A rhizome is a horizontal stem that often grows underground, usually sending out roots and shoots from its nodes. Some plants have rhizomes that grow above ground or that lie at the soil surface, including some Iris species. Usually, rhizomes have short internodes; they send out roots from the bottom of the nodes and new upward-growing shoots from the top of the nodes. Examples of plants that grow in this way include irises, lily of the valley (Convallaria majalis) and cannas.[4]

Root tubers of Dahlia

A stem tuber may form from thickened rhizomes or stolons. The tops or sides of the tuber produce shoots that grow into typical stems and leaves, and the undersides produce roots. Such tubers tend to form at the sides of the parent plant and are most often located near the soil surface. A below ground stem tuber is normally a short-lived storage and regenerative organ developing from a shoot that branches off a mature plant. The new tubers are attached to a parent tuber or form at the end of an underground rhizome. In the autumn the plant dies except for the new offspring stem tubers, which in spring regrow one or more new shoots producing stems and leaves. Some plants also form smaller tubers and/or tubercules, which act like seeds, producing small plants that resemble (in morphology and size) seedlings. Some stem tubers, such as those of tuberous begonias, are long lived, but many tuberous plants have tubers that survive only until the plants are in full leaf, at which point the tuber is reduced to a shrivelled up husk.[4]

Roots can also form tuberous structures (tuberous roots) that are in some ways similar to stem tubers, but of a different anatomical origin. Ornamental plants with tuberous roots include the Persian buttercup, Ranunculus asiaticus,[6] and dahlias.[7] When sold in the dry form, dahlia "bulbs" consist of a cluster of tuberous roots attached to one or more stems. Only the stems produce buds, from around the "collar" close to where the roots are attached. A tuber without any attached stem will not grow.[8]

Tubers may form from the hypocotyl of the young seedling, as in Cyclamen.[9] Since the hypocotyl is a region between the stem and the roots, such tubers are variable in their anatomy and growth habits. Thus the roots of Cyclamen graecum grow from the base of the tuber, suggesting it is a stem tuber, whereas those of Cyclamen hederifolium mostly grow from the upper surface of the tuber, suggesting it is a root tuber.[10]

Crocus cartwrightianus flowering on 15 November in Greece

Most bulbous plants are adapted to hot, dry summers and cooler, wetter winters. They are dormant through the summer and grow during the autumn, winter and spring.[12] Within this group, there are variations, largely determined by how cold the winter is. Many tulips (Tulipa species) of Asian origins, for example, have adapted to an extreme continental climate, with dry, very hot summers, very cold winters and springs with short showers. They grow mainly during the spring.[1] In cultivation such tulips may be planted in late autumn (e.g. November in the northern hemisphere).[13] In regions where winters are milder, some species, such as Crocus cartwrightianus, flower in the autumn, either at the same time as the leaves appear or before.[14] Others, such as Arum creticum, produce leaves in the autumn which last through the winter until the plant flowers in the spring.[15]

Summer drought occurs particularly, but not exclusively, in those regions of the world with a Mediterranean climate, which are rich in bulbous plants. Such regions include the Mediterranean itself through to Central Asia, the south west of South Africa, the south west of Australia, parts of the western United States, such as California, and parts of western South America, particularly Chile.[11]

A bluebell wood in Kent, England

Many species that grow in the understory of deciduous woods or forests are also summer dormant. They use their stored reserves in order to grow rapidly and complete their annual growth-cycle at the beginning of spring before the developing tree canopy blocks out the sun's light.[1] North America is home to many such woodland bulbs, including Erythronium, Trillium and some lilies, such as Lilium pardalinum.[16] The common bluebell, Hyacinthoides non-scripta, is native to western parts of Europe, but particularly the British Isles,[17] where it carpets the floor of some woods. The woodlands of Asia, including China and Japan, are home to bulbous plants such as arisaemas and the giant cardiocrinums.[16]

Some bulbous plants grow in communities that are adapted to recurrent fires during the dry season (for example, many Iridaceae species). During these periods the plants are dormant and in this way can survive the heat of the fire. The fires clean the surface vegetation, eliminating competition and also supplying nutrients to the ground from the ashes of the burnt plants. When the first rains fall, the bulbs, corms and rhizomes rapidly start to shoot, starting a new period of growth and development sustained by the reserves accumulated in their storage tissues during the previous season. Various South African species from the genus Cyrtanthus, for example, are well known for their ability to flower rapidly after natural grassland wildfires, and for this reason several of these species are known as "fire lilies". In fact, some species, such as Cyrtanthus contractus, only flower after a wildfire.[18]

Eucomis comosa, a winter dormant bulbous plant

A second category of bulbous plants are those adapted to dry, generally cool winters and warmer, wetter summers. They are dormant through the winter and grow in spring, summer and autumn. Regions of South Africa and Lesotho with this type of climate include the East Cape, and the Drakensberg mountains in the north east of Western Cape province, which are particularly rich in bulbous species,[19] including plants such as gladioli, Eucomis and Rhodohypoxis. Other areas with similar winter drought include parts of Central America, such as Mexico where the tiger flower (Tigridia pavonia) is found.[20]

In some areas of the tropics, rainfall is interspersed with periods of dryness; more than one wet/dry cycle may occur in a year. Bulbous plants in these areas are adapted to warm, wet periods followed by warm, dry periods. They typically flower near the beginning of the rainy season. Such a climate occurs in Kenya, which has wet conditions in October to December and then again in February to May. Glory-lilies (Gloriosa) and Crinum species are examples of bulbous plants adapted to these conditions. Tropical Asia has similarly adapted bulbous plants, such as species of Hedychium (gingers).[21]

Bulb species are traditionally planted in flowerbeds (parterres) and herbaceous borders in parks and gardens. The selection of species to plant depends on various factors, such as the soil type, the position (sunny or shady location), the colour or effect that is required and the season of the year when the plants are required to flower.

Border of tulips planted among pansies

Some examples of bulbous plant genera and their flowering season are given below:

• Spring flowering bulbs.[27][28] Spring is the most typical season for bulbs to flower. Some examples are: Allium, Arum, Asphodelus, Camassia, Convallaria, Crocus, Cyclamen, Eranthis, Freesia, Fritillaria, Galanthus, Hyacinthus, Hippeastrum, Iris, Ixia, Leucojum, Muscari, Narcissus, Ornithogalum, Ranunculus, Scilla, Trillium and Tulipa[29] and Zephyranthes.
• Summer flowering bulbs:[30][31] Achimenes, Agapanthus, Allium, Alstroemeria, Amaryllis, Anemone, Begonia, Calochortus, Canna, Crinum, Crocosmia, Dahlia, Dierama, Eucomis, Galtonia, Gladiolus, Gloriosa, Haemanthus, Hymenocallis, Lilium,[32] Oxalis, Pancratium, Paradisea, Polianthes, Sprekelia, Tritonia, Watsonia, Zantedeschia.
• Autumn (fall) flowering bulbs:[33] Crocus, Colchicum, Cyclamen, Nerine, Sternbergia.
• Winter flowering bulbs: some species from the following genera: Galanthus, Crocus, Cyclamen and Eranthis.

Some species of bulbous plants grow naturally in shady or woodland areas, and thus are well suited to areas in a garden that have similar conditions. Some species for shade are Allium ursinum, Anemone blanda, Anemone nemorosa, Arum italicum, Convallaria majalis, Corydalis flexuosa, Cyclamen purpurascens, Disporum flavescens, Erythronium, Fritillaria pallidiflora, Galanthus, Hyacinthoides non-scripta, Iris douglasiana, Leucojum vernum, Lilium martagon, Ranunculus ficaria, Sanguinaria canadensis, Smilacina racemosa, Trillium and Uvularia grandiflora.

In large parks it is possible to plant some species so that they multiply spontaneously and grow amongst the grass or under trees. This practice, the naturalization of a species, is widely used in northern Europe and requires that the species' ecological requirements be satisfied. The most obvious advantage of this cultivation method is that it minimizes the attention the plants require once they have been naturalized. The plants that are suitable for naturalization are those that are sufficiently small but able to compete with the surrounding grass, they must be robust and able to withstand year after year of inclement weather and they must be prolific in order to spread rapidly.[34][35]

Naturalized crocuses in Victoria Park, London

Some of the bulbs that are suitable for naturalization in parks include: Allium, Anemone, Arum, Colchicum, Crocus, Cyclamen, Endymion, Fritillaria, Galanthus, Ipheion, Leucojum, Lilium, Muscari, Narcissus, Ornithogalum, Scilla, Sternbergia and Tulipa.

A rock garden is a garden that uses a combination of rocks and small plants. The plants are often chosen for their suitability to rocky terrain. Some of the bulb genera that are most suitable for rock gardens include:[1] Allium, Anemone, Anthericum, Bulbocodium, Chionodoxa, Cyclamen, Eranthis, Erythronium, Galanthus, Ipheion, Muscari, Ornithogalum, Oxalis, Romulea, Rhodohypoxis and Scilla.