Understanding Plant Classification

Understanding Plant Classification

Plant Category Definitions Explained

Have you ever wondered what defines a tree from a shrub, or bulb from a rhizome or a half hardy perennial from and hardy annual?  Plants are allotted to convenient categories according to their hardiness and the following descriptions should help you to understand these classifications.   All plants exhibit varying degrees of hardiness, or tolerance towards cold and damp, and this tolerance is directly related to the climates of their countries of origin.

We will explain the differences between trees, shrubs and climbers and divide the confusing bulb group into bulbs, corms, rhizomes and tubers.

A lot of the terms used to describe plant categories are usually abbreviated in catalogues and plant references, and we give these initials following the full name, where appropriate.

Hardy Perennials


This term denotes plants that will live and grow outside from year to year in all but extreme climates. Although this group includes shrubs and trees, the term ‘hardy perennial’ commonly refers to hardy herbaceous border perennials.

Herbaceous Perennials

These overwinter by using various forms of rootstock. The top growth dies down in the autumn and more new shoots emerge the following spring. This group of plants may be propagated from seed, although it is advisable to increase them by dividing the roots or taking cuttings. Familiar examples are delphinium, dianthus pinks, phlox and Geum (avens).

Half-Hardy Perennials


This group includes some of the well- known summer bedding plants, for example pelargonium and begonia. Dahlias, chrysanthemums and some bulbous plants like gladiolus are also represented here. Although some of these plants will survive outside during mild winters, they stand a much better chance of living if they are taken out of the ground and over-wintered in a dry place where temperatures remain above freezing.

Certain ‘tender’ or ‘greenhouse’ perennials also fall into this category. An example of this is Abutilon, which is often used as a ‘dot’ plant in summer bedding schemes, but originates in a warm climate and requires a high winter temperature to ensure survival. Other plants, such as antirrhinum (snapdragon) petunia and nemesia, although often referred to as half-hardy annuals and indeed treated as such by being raised from seed each year, are also in fact half-hardy perennials.

Tender/Greenhouse Perennials

Natives of hot and often humid zones, these plants require constant protected cultivation, although some of them can be placed outdoors in sunny position during warm weather. A few members of this group are used in summer bedding schemes



All annuals are characterized by completing their life cycle within a twelve-month period. They germinate, grow, flower and set seed and die, all during one growing reason.

Hardy Annuals

Hardy annuals are the familiar flow plants of the summer cottage garden. Plants such as calendula (pot marigolds, clarkia, godetia and centaurea (cornflower) may be grown from a spring sowing in open ground and will be flowering profusely in early summer. Seeds of hardy annuals may be sown during autumn and the young plants will stand the winter outside to give an extra early flower show during the following spring. Lathyrus (sweet pea) is often treated in this way.

Half-Hardy Annuals


The colourful bedding plants, half-hardy annuals, brighten many gardens in summer. They are natives of warm climates and so will not reproduce naturally outdoors in countries that experience cold, wet winters. The gardener has to intervene in the plant’s natural cycle and either collect seed in autumn and store it in a dry warm place in winter, or purchase fresh seed every spring. To obtain maximum showiness from these plants, seedlings should be raised in a warm environment, such as a greenhouse or heated frame, and planted out when all danger of frost is passed. Tagetes (African marigolds), zinnias and lobelia are familiar examples in this group. Seed of half-hardy annuals may be sown outdoors when all danger of frost is passed, but generally this method does not give such a good show of blooms.

Tender Annuals

Although generally requiring constant greenhouse cultivation, tender annuals can safely go outdoors at the height of summer when they are already in bloom. Schizanthus (butterfly flower), cineraria and celosia are all good examples.


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These plants grow one year to flower the next. Cheiranthus (wallflower), Dianthus barbatus (sweet William), bellis (daisy) and myosotis (forget-me-not) are all familiar spring bedding plants which are biennials. They are all hardy and seed is sown in early summer so that the plants will be large enough to plant out in their spring flowering quarters during the autumn. There are several greenhouse biennials, such as calceolaria. In addition, some tender perennials such as cyclamen are often treated as biennials.


These are usually defined as perennial plants that are woody, with one main stem (trunk), and a mass of branches and stems above. Trees usually grow over 4m (13 ft.) tall. They are generally sub-divided into two categories: the broad-leaved kinds which can have a variety of different shaped leaves showing a network of veins - such as the Tilia (lime), Aesculus (horse chestnut) and Laburnum - and the conifers, like Taxus (yew) and Picea (Christmas tree) which have needle-like or ‘scale’ leaves.

Trees can take many years to mature. They can also be either hardy, half- hardy or tender, and obviously it is only wise with such a long-term plant to buy a hardy type. Trees and shrubs are often referred to as evergreen (that is, they retain their leaves in winter) or deciduous (that is, they shed their leaves in autumn and produce new ones in the following spring).



Unlike trees, these are woody perennials that branch naturally from the base and have more than one main stem. They can grow as tall as 8m (26 ft.) or be only a few centimetres (1-2 in) high. Like trees, they can be hardy, half-hardy, or tender. Again it is sensible to plant only the hardy kinds outdoors. There are in-numerable examples suitable for gardens, such as Rhododendron, Camellia, Syringa (Lilac) and Buddleia. Shrubs are sometimes called bushes, and this term can also refer to a cluster of shrubs.


True climbers are a group of plants that grow upwards naturally and are able to support themselves against an object such as another plant, trellis, wall, string, netting, pergola and so on. They can do this by various means, such as tendrils (short twisted leafless growths), aerial roots (short stems with little roots), sucker pads (self-adhesive growths), leaf stalks which twist round the support, hooked spines, or by the twining growth of their stems.

Examples of some of the different methods are Lonicera (honey- suckle), by twining; Hedera (ivy), by aerial roots; clematis, by tendrils; par- Thenocissus (Virginia creeper), by sucker pads; Lathyrus (sweet pea), by tendrils; blackberry, by hooked spines; and convolvulus, by twining of the stems.

Climbers may be perennials or annuals, hardy, half-hardy or tender. In addition to the climbers, there are other plants, such as ‘climbing’ and ‘rambling’ Rosa (rose), Quince, Ceanothus, some Pyracantha (firethorn) and Cotoneasters that are basically shrubs but grow in such a way that they can be trained against supports, provided they are given a helping hand by twisting or tying in the shoots to the supports, and in some cases by careful pruning.



Many any people when they refer to bulbs also include plant organs which are similar in function (as food stores), but which should strictly be referred to as corms, tubers, or rhizomes. A true bulb is an underground ‘bud’ that has fleshy or scaly leaves around it, all growing upwards from a basal ‘plate’.

These are plant food storage leaves. Inside are the rudiments of flower, flower stalk and leaves. When conditions are right, roots grow down into the soil from the basal plate and the rudimentary flowers and leaves grow upwards to give the aboveground parts of the plant. The leaves and roots absorb plant foods during growth and feed these back into the bulb or a ‘daughter bulb’, before they die back at the end of their season. In this way, they prepare for the next growth period by again forming rudimentary flowers and leaves. Sometimes several bulbils or ‘daughter bulbs’ are formed around the original parent, and these can be used to increase the plants.

True bulbs include Lilium (lily), hyacinth, narcissus (daffodil), Tulipa (tulip) and Galanthus (snowdrop). All bulbs are easy to grow and have the advantage of rapid growth when conditions are suitable. Some are hardy and can be left in the ground all the year round (tulips and daffodils) whereas others (Nerine and Vallota) are half-hardy and are best lifted from the ground, dried and stored in cool, frost-free conditions (or well protected with straw outdoors). Yet others are tender and require greenhouse conditions throughout the year, such as Amaryllis and Lachenalia.



Tubers are thickened parts of underground stems or roots which store food and carry buds for leaves and flowers above ground, and roots below. Typical examples of these are dahlias and some begonias. Such plants are usually treated as half-hardy annuals and can be sown from seed. Usually, however, the tubers are stored in cool, dry conditions during the winter months, cut up into sections and started into growth in a cool greenhouse in spring, for planting out when hardened off.


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These are fleshy, swollen and solid underground stem bases with scales (outside leaves) that can be round (crocus) or flat (gladiolus). Like bulbs, they are food storage organs and contain embryo flowers and leaves that grow from the upper surface, while roots grow from a ‘disc’ below. They can be considered annuals because each year a new corm is formed above the old one.

Hardy corms, like crocus, can be left in the ground all year, but half-hardy ones, such as gladiolus, are best dug up, dried and stored carefully each autumn for planting the following spring. Propagation is by the small ‘cormlets’ also produced, but these may take a year or two before they flower.



A rhizome is a fleshy underground or creeping, root-like food storage organ from which leaves, stems and flowers grow upwards and roots downwards. It tends to be scaly and look ‘jointed’. Typical examples are flag irises, Convallaria (lily of the valley) and Polygonatum (Solomon’s Seal), all hardy plants.

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