In horticulture, a bulbous plant is an ornamental plant, herbaceous or perennial species which produce fleshly storage organs including true bulbs as well as corms, tubers, rhizomes, and tuberous roots.[1][2] These subterranean organs store moisture and nutrients and are used by bulbous plants to survive winter or other adverse conditions such as summer drought and heat (estivation) in a dormant state. The resting period may be in winter or summer, depending on the species under consideration.[2]
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A bulb is an underground vertical shoot that has modified leaves (or thickened leaf bases) that are used as food storage organs by the plant. A bulb's leaf bases generally do not support leaves, but contain food reserves to enable the plant to survive adverse conditions. The leaf bases may resemble scales, or they may overlap and surround the center of the bulb as with the onion. A modified stem forms the base of the bulb, and plant growth occurs from this basal plate. Roots emerge from the underside of the base, and new stems and leaves from the upper side.[3] Most true bulbs have their scales closely packed together (Hyacinthus) but some types, such as the Lily (Lilium), they are loose and swollen. Most popular bulbs have a papery skin, the tunic, whose function is to protect the tissues within. Some true bulbs such as the Lily do not have a tunic and are therefore easily damaged by rough handling. Cardiocrinums dies after flowering but other bulbs are perennial. Vegetative reproduction in true bulbs is by means of offsets called bulblets.[2] Examples of species with true bulbs, apart from the already mentioned, are Muscari, Tulipa, Narcissus and Allium.
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 petiole sheaths, formed from the leaves produced the year before, which 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 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 that are modified leaves. As a result, when a corm is cut in half it is solid, but when a true bulb is cut in half it is made up of layers.[3]
A rhizome is a horizontal stem that growth underground, often 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. For many plants, the rhizome is used by farmers and gardeners to propagate the plants by a process known as vegetative reproduction. Examples of plants that are propagated this way include irises, Lily of the Valley, Cannas. [3]
A stem tuber forms from thickened rhizomes or stolons. The tops or sides of the tuber produce shoots that grow into typical stems and leaves and the under sides produce roots. They tend to form at the sides of the parent plant and are most often located near the soil surface. The below-ground stem tuber is normally a short-lived storage and regenerative organ developing from a shoot that branches off a mature plant. The offspring or new tubers, are attached to a parent tuber or form at the end of a hypogeogenous rhizome. In the fall the plant dies except for the new offspring stem tubers which have one dominant bud, which in spring regrows a new shoot producing stems and leaves, in summer the tubers decay and new tubers begin to grow. 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 are long lived such as those of tuberous begonia but many tuberous plants have tubers that survive only until the plants have fully leafed out, at which point the tuber is reduced to a shriveled up husk.[3]
Adaptation to tough environments makes bulbous plants invaluable in a garden setting. Many are suitable for the kind of thin, dry and nutrient deficient soil found at the foot of a hot and sunny wall. Others can cope with dry shade and are therefore ideal for planting under deciduous or evergreen trees, where nutrients and soil moisture are deficient.[2]
Spring-flowering, summer-flowering and autumn-flowering bulbs provide the opportunity to add extra color to any garden. They can be planted under shrubs or between perennials in the flower border or can be naturalized informally in lawns and orchards, or formally in seasonal bedding display. For splashes of color all around the garden, bulbous plants can also be planted in pots.[2]
Bulbous plants ring the seasonal changes throughout the year with glorious flower display. Some possess handsome foliage, others are valued for their flagrance, but above all, their blooms are the most essential. They offer a wide variety of color and form, from bright, primary shades to delicate, pastel hues.[1]
Traditionally, bulbous plants are cultivated in parterres and herbaceous borders. The choice of the species depends on a number of factors, among them, the site in the garden (shady or sunny) and the type of soil, the color and the effect that is pursued in the garden and the part of the year when the flowering is wanted. Some examples of bulbous plant genera and their flowering season are given below.[1]
Many species of bulbous plants have woodland or shady places as their natural habitats. In the garden they are specially valuable, ornamenting places which may otherwise be difficult to plant. 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.
A rock garden, also known as a rockery or an alpine garden, is a type of garden that features extensive use of rocks or stones, along with plants native to rocky or alpine environments. It is possible to cultivate a great number of species in it. The only limitation is the size of the plants, they must be small. Some of the genera of bulbous plants with species well suited for the rock garden are:[1] Allium, Anemone, Anthericum, Bulbocodium, Chionodoxa, Cyclamen, Eranthis, Erythronium, Galanthus, Ipheion, Muscari, Ornithogalum, Oxalis, Romulea, Rhodohypoxis and Scilla.