What is a Perennial Plant?

“What is a Perennial Plant?” and “How can a Daffodil be the same sort of plant as a Beech tree and a Cactus?” are frequent questions from both new gardeners and those who are still feeling a bit confused. I understand your confusion, and that whilst you don’t want the full-on botanical reasons, the very simple definition isn’t always helpful enough. My aim here is to find a middle ground.

What is a Perennial Plant?

A perennial is a plant which survives for three years or more. Compare this with annuals that live for a year, like Cornflowers, and biennials that live for two years, such as Sweet Williams. See Plews Gardening Glossary for fuller definitions.

Generally speaking, we use a lot of perennials in the ornamental gardens and flower gardens, but fewer in the vegetable plot. Perennials may be: –

short lived, such as Rudbeckia and Tulip
long lived, an Oak or Elder tree for example

And just to add to the fun, liverworts and mosses are also perennial plants, as are mushrooms. But we won’t be looking at these types here due to lack of space.

Now, describing perennial plants as being either long lived or short lived can be a tricky one as the classification isn’t universally accepted. This is partly due to the fact that not all plant trials continue for long enough for a plant’s lifespan to be fully evaluated. But we’ll go with the following, which works well for the majority of gardeners.

Short-lived Perennial Plants

These are perennial plants with an approximate lifespan of 3-10 years. Most herbaceous perennials and bulbous perennials are in this ‘group’, as are many sub-shrubs and some shrubs. These are discussed in the sections below.

Long-lived Perennial Plants

The fact that there are Yew trees, Taxus baccata, in Ireland and Scotland which are over 2000 years old suggests that they are a long lived species. And there is a consensus that the vast majority of trees are long lived perennials, although not all of them reach such a great age! Many shrubs are also long lived.

What is a Perennial Plant?

However, there are further classifications of perennials that we need to be aware of. Not everyone uses all of these, but I thought it helpful for you to know of them. They are: –

Woody perennials
Sub-shrubs
Herbaceous perennials
Evergreen perennials
Small perennials
Bulbous perennials
Tender perennials
Aquatics

Whether a perennial is evergreen, semi-evergreen or deciduous doesn’t affect it’s main categorisation, but some will subdivide the above on the basis of whether the plant retains its foliage all year.

NB Climbing plants and herbs may come under any of these categories so I haven’t mentioned them separately

Let’s look at these one by one with definitions and examples.

Woody perennials

Woody perennials are those plants which maintain a woody structure of branches and stems above ground at all times. The ‘woodiness’ of their stems, branches and trunks develops over time as a result of lignin. Lignin is an essential part of the cell structure as it aids in the transportation of water through the different parts of the plant. It helps to give the tree or shrub its strength to grow tall. Ecologically its important too as it is where woody perennials hold most of the atmospheric carbon that they absorb.

The majority of long lived perennials have a woody structure and they may be evergreen, deciduous, or semi-evergreen. This latter is most often seen as a result of environmental or weather factors rather than a ‘true’ semi-evergreen, so is not generally used as a description.

Evergreen woody perennials

Buxus
Camellia
Laurel
Lavender
Thuja
Yew

Deciduous woody perennials

Ash
Cherry tree
Hawthorn
Hibiscus
Potentilla
Rose

Sub shrubs

There is also a category of woody perennials within shrubs called sub shrubs, which includes plants such as Penstemon, which maintain a soft stem, ie not lignified, over winter. This isn’t universally accepted, and some prefer to call these plants herbaceous perennials as they may die back over winter in cooler temperate zones.

There are also some long lived perennials, such as Palms, which do not develop a woody structure. You may find them put into this group, or in with the herbaceous perennials.

Herbaceous Perennials

Herbaceous perennials are termed short lived perennial plants. “But they die off so they can’t be perennials” I hear you say. This could be because the top growth, that above the ground that you can see, has died back overwinter. It will regrow in the following spring (or possibly summer, depending on the actual plant). Or it may be that the plant you bought is not suited to overwinter in a northern climate in wet soil. This is because, to make a gardener’s life more fun, there are –

hardy herbaceous perennials,
half-hardy herbaceous perennials,
tender herbaceous perennials

Hardy plants are frost tolerant, half hardy plants are tolerant of some frost whereas tender plants are not generally frost tolerant (although it may be cold and wet that kills them off rather than just cold). Gardens have their own microclimates so a half hardy perennial may be happy and survive a hard winter in one garden but not in another.

What all herbaceous perennial plants have in common is soft leafy growth rather than a permanent woody skeleton. This soft foliage often dies back over winter, but all perennial plants have some form of underground storage capability which enables the plant to survive through the winter. For example, you may have noticed that depending on where you live, your Geraniums maintain a good leafy mound all winter, or they may shed their foliage.

Hardy herbaceous perennials

Alchemilla mollis
Aquilegia
Geranium
Heuchera
Hosta
Japanese anemone

Half-hardy herbaceous perennials

Cinneraria
Gerbera
Helenium

Tender herbaceous perennials

Heliotrope
Pelargonium (aka pot geranium)

Evergreen Perennials

These are not so much large evergreen trees and shrubs as the smaller plants. Think of plants such as Bergenia, aka elephants ears and Lamium, deadnettle. They form a clump or spread across the ground and are very useful as year round weed suppressing ground cover plants.

Bergenia
Dianthus (cottage garden pinks)
Vinca (perwinkle)

Small Perennials

What is a perennial plant can get even more complicated here as there is some crossover between evergreen perennials and small perennials. For example, Lamium tends to be evergreen but sometimes it isn’t. Perhaps if you think of these as being small as in mainly low to the ground (in clumps or trailing) it might help. Another feature is that they generally have woody stems or foliage above ground unlike the herbaceous perennials.

Ceratostigma plumbaginoides
Primula
Viola

Bulbous Perennials

The term bulbous perennials includes bulbs, corms, tubers and rhizomes. There are many of these and, like the herbaceous perennials they can be sub-divided into –

hardy bulbous perennials
half-hardy bulbous perennials
tender bulbous perennials

The half-hardy and tender ones are those which are unlikely to or won’t survive wet soil in winter. They will need a warm start or frost protection, depending on the plant and some may only be grown outdoors during the summer months.

Hardy bulbous perennials

Allium
Narcissus
Tulip

Half-hardy bulbous perennials

Acidanthera, Gladiolus Murielae
Freesia
Ranunculus

Tender bulbous perennials

Dahlia
Nerine sarniensis
Tigrida pavonia, tiger flower

Aquatics and semi-aquatics

Most of the plants grown in British garden ponds and bog gardens will be hardy, unless you’re a pond plant enthusiast. Ones such as water lilies, Nymphaea, dies back over winter. others such as Juncus, sedge, are evergreen.

What is a Perennial Plant – Tender perennials

However to complicate matters, many of the plants we treat as annuals, using them as summer bedding plants, for example, are in fact perennials. In temperate zones they will be tender perennials, both woody and herbaceous. These are the more exotic plants which we treat as annuals. This means that whilst they are native in warmer climates, bringing them into gardens in Western Europe and the USA conveys them into climates with colder winters and freezing conditions, and wetter winter weather than they are used to. Hence they tend not to survive without some help from the gardener.

Pelargoniums, also known as ‘pot geraniums’, ‘bedding geraniums’ or sometimes just as ‘geraniums’, are a good example. Many people treat these as annual summer bedding; as do many public garden planting schemes. However, some gardeners keep their pelargoniums for many years as they bring them into a frost free area such as a greenhouse for the winter months. Pelargoniums are native species of warmer climates such as Madeira, Africa and the southern Mediterranean.

It’s interesting to consider that until we started to import plants from warmer climates in sufficient quantities for a number of people to cultivate and collect, we didn’t need to know whether a perennial was hardy or tender. In fact the description was irrelevant. What we now call a hardy perennial would be native to the area it was growing in. A tender perennial such as a Canna, a tropical native, would simply not be found in an English Elizabethan knot garden.

Have I answered the Question: What is a Perennial Plant?

Hopefully my explanations and examples have helped you understand the difference between the various groupings of perennials. And why its not always as straightforward as it could be!

For further gardening advice and inspiration, check out Plews Potting Shed blogs, including the selection below. The blog has been going for over 10 years, so there are plenty of gardening topics to ‘scratch your gardening itch’

You could come and find us on your preferred social media too – Instagram Facebook Pinterest

Or if you’d like a more personal approach, why not have enjoy a gardening course with your own garden as the classroom? ? Because where better to learn about what’s relevant to you as a gardener? Plews Gardening Courses and Lessons are bespoke to you and your garden and combine hands on confidence giving with necessary ‘why we do this’ reasons.