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Plant types

There are many different ways to describe plants and, since plants are so diverse, no single system is exact yet wide enough to provide an easy way of defining the diversity. Below are three different ways in which plants are defined and some examples of the terms used.

Definition by form

Woody plants

This describes plants where the stems have a rigid outer structure. This is formed as a result of ’secondary thickening’ of the stem, ie it grows outwards as well as upwards. This process adds a substance called lignin into the stem cells which strengthen them. This woody growth is maintained all year. Plants which we consider to be trees and shrubs are all woody plants.

Shrubs vs trees

A shrub is a woody perennial plant which naturally grows from two or more stems originating at ground level. It is different from a tree because of the multiple stems; trees generally only have one main stem (ie the trunk).

There are lots of grey areas to cloud the issue. For example, opinion is divided as to whether Acer palmatum is a tree or a shrub, due to the many different forms in which it grows. You often get shrubs which have been trained to grow with a single stem, such as Cornus alba shrubs which have been pollarded, or standard roses.  Equally, trees with attractive bark such as Betula utilis var. jacqumontii are often grown with multiple stems to show off the whiteness of their trunks.

In reality, it doesn’t really matter whether you call a plant ‘tree’ or ’shrub’. There are more important things to worry about when describing woody perennials, such as whether this lovely little plant will eventually grow into a monster and swamp your garden – if that happens you really won’t be worried about whether the plant towering above you naturally grows with one or several stems!

Conifers

Conifers are cone-bearing plants which, along with cycads, ginkgos and gnetophytes, differ from all other trees because they are ‘gymnosperms’, which literally means ‘naked seed’. Their seeds are exposed on the surface of structures rather than being enveloped within a fruit formed from the ovary (which occurs in ‘angiosperms’, meaning ‘a seed borne in a vessel’). In total there are only around 840 living species of gymnosperms (of which about 630 are conifers), compared to over 300,000 species of angiosperms. Nevertheless, in many parts of the world conifers dominate in number over wide areas.

Their leaves are in the form of needles and generally (but not always) evergreen, being retained for between two and four years. They do not bear flowers, unlike angiosperm trees, instead they usually have male and female cones; the male cone releasing pollen to pollinate the ovules on the female cone.

Conifer seeds (usually winged) are held on scales which make up the female cones, although in some cases the cones are berry-like (such as the juniper ‘berry’) or the seeds are surrounded by fleshy cups called ‘arils’ (eg Taxus species). The scales (and therefore the seeds) are usually shed about a year and a half after the initial appearance of the cones and pollination, which is why cones seem to remain on conifers for such a long time.

Conifers are of great economic value as they include plants such as pines, firs and spruces, which are particularly sought after for their timber.

Sub-shrubs

Sub-shrubs are generally low growing plants which are entirely woody or which have woody growth at the base but soft, herbaceous growth above. Lavenders are often considered to be sub-shrubs. Some people don’t use the term ’sub-shrub’ and will consider instead that it is either a shrub or a herbaceous perennial.

Herbaceous plants

The term ‘herbaceous’ is used to describe any plant which does not have any woody growth, so all the stems of the plant remain flexible throughout its life. You often come across ‘herbaceous perennials’, which are plants living for more than 2 years which do not become woody. Many of these plants will die back over winter, although some are evergreen (such as grasses).

Many types of plants are herbaceous, here are just a few examples:

Definition by whether it keeps its leaves all year

A deciduous plant is one which does not keep its leaves for the whole year. Generally this means that the leaves will fall, or the whole plant dies back, over the autumn/winter months (ie at the end of the growing season). Conversely, evergreen plants retain their leaves throughout the year.

These terms can be used to refer to any type of plant. For example, the oak (eg Quercus robur) is deciduous because its leaves fall in autumn. Equally the herbaceous plant Achillea millefolium ‘Cerise Queen’ is deciduous as all the above-ground parts of the plant die back over winter. Helleborus orientalis is an evergreen herbaceous plant, while Chamaecyparis obtusa ‘Nana Gracilis’ is an evergreen tree.

There are also intermediate plants, known as ’semi-evergreen’ or ’semi-deciduous’, which may retain some or all their leaves at the end of the growing season, sometimes depending on environmental conditions (such as a warm winter).

Definition by lifespan

Perennials

Plants which live for more than 2 growing seasons. Eg Dahlia ‘Bishop of Llandaff’ and Acer palmatum ‘Atropurpureum’.

Biennials

Plants which live for 2 growing seasons (generally having vegetative growth in the first year and flowering in the second year). Eg foxgloves (Digitalis purpurea).

Annuals

Plants which completes their lifecyle within a single growing season. Eg Limnanthes douglasii.

Ephemerals

Plants which have more than one lifecycle in a growing season; these are often weeds as they increase in number rapidly. Eg groundsel (Senecio vulgaris).