Defining plants isn’t easy, even for an expert. However, it’s necessary for us to be able to correctly identify plant material in our gardens or perhaps a plant seen elsewhere that we would like to grow. This two-part guide is to help beginners define the types of garden plants and so make identification easier. This week I outline the basic plant groups and the most distinct characteristics of each one. Next week I will look at the large group of monocotyledons, which includes grasses, flaxes and sedges. We will also look at plants’ other key features, such as leaf shape and colour, fruits, seeds and the seasonal cues that can aid identification.
The first step, though, is to have a clear idea which family of plants you are looking at. Then you can home in on such details as colours, form, stem structure and seasonal changes that will help you pinpoint the plant you want. Often, you’ll need people with experience to make the final call, but by providing accurate information, you can make their task a lot easier and the identification more accurate. There are also many books available that assist the process.
Trees are the largest-growing members of the plant kingdom. The main difference between a tree and a large shrub is the trunk. A tree generally has a single woody trunk from which its branches grow.
Shrubs have multiple stems, but are generally smaller and more compact than trees, although some shrubs can reach heights of about 6m. A shrub is also woody and can be deciduous or evergreen, but unlike herbaceous perennials, which may be reduced to surviving underground during the winter months, it is always visible.
A palm is defined by the form of its leaf, which is fan-shaped. Many palms have a tree-like form with a single trunk that supports multiple fan-shaped leaves. All palms descend from a single common ancestor and are generally from tropical areas with warmth and high rainfall. New Zealand has only one, the nikau (Rhopalostylis sapida), which is the world’s southern-most palm.
Often mistaken for palms or ferns, cycads are one of the most primitive forms of plant, having changed little since the Jurassic period when they were most common. Rather than being fanlike, their leaves are pinnate or feather-like in form, although usually quite leathery, firm or sometimes even spiky.
They are best characterised by their method of reproduction whereby their unfertilised seeds are exposed to the air rather than concealed, as they are in most of the plant kingdom. These seeds appear like a pine cone in the centre of the plant and are probably its most distinctive characteristic. They are most commonly fertilised by a specific beetle. Cycads may live up to 1000 years. New Zealand doesn’t have any native cycads.
Succulents are defined by their ability to store water in their leaves or stems. The family are normally from arid areas, which is why the leaves or the stems could be described as swollen, and when cut are thick and glistening. Cacti are succulents, but it is generally the stem that is swollen and performs photosynthesis, with most varieties having transformed their true leaves into spines to discourage grazers and to protect their precious water stores. The native ice plant, Disphyma australe, which has pale-pink and white flowers through summer, is a great succulent for a coastal garden.
Ferns are most easily identified by the frond that unfurls from the centre of the plant in a characteristic koru or spiral. They do not have seeds or flowers, instead reproducing with spores, which grow on the underside of the fronds as small dots in a broad range of colours, such as black, white, brown or green.
The roots are normally fibrous and are not dissimilar to those of seeding and fruiting plants. New Zealand has many native ferns suited to a range of growing conditions. Tree ferns such as Cyathea dealbata or Cyathea medullaris are particularly popular. They are defined by the trunk that holds the fronds off the ground.
All trees and shrubs are perennial by definition, but the term perennials in gardening most commonly refers to herbaceous ones, which include many popular traditional flowering plants. They are best defined by the soft fleshy nature of their stems, which renew each year. Bananas are an example of a large-growing perennial, whereas roses are shrubs; though deciduous, their woody material is constant. Common herbaceous perennials include penstemons, salvias and echinacea.
Annuals are plants that survive one season only, then set seed for the next cycle. The family includes many food crops as well as bright flowering plants. Although true annuals, such as peas, wheat or marigolds, will only grow for one season, often perennial or biennial (those with a two-year cycle) plants that are susceptible to cold will be treated as annuals so as not to risk losing the parent plant during a harsh winter. One good example is the kumara, which is lifted annually, even though it’s a perennial.
A vine climbs other plants, rock faces, fences or even buildings. The advantage of this is that the plant does not have to invest the same amount of energy into a structure and instead can take advantage of an existing form as it strives to reach the light. This can make many climbers highly invasive and as troublesome as weeds.
Some climbers are also semi-parasitic. Mistletoes, for example, will climb a tree, then tap roots into its trunk so they can mine water and nutrients. Surprisingly, though, these parasitic plants are some of our most endangered. Our nine species of native mistletoe are very scarce and precious and if identified should be protected, especially from possums. This is most easily achieved by putting a simple metal band around the host tree’s trunk.