Understanding these basic forms is more than an academic exercise, it's a powerful tool in getting the most from our plants. It comes down to knowing what a plant ‘wants’ to do and correlating this with what we want from a plant.
Annuals
Annuals live fast and die young. They last not so much for a year but a season. Their survival strategy is of avoidance.
The key to their life cycle is the seed. Annuals cope with drought, cold and heat by living in the good times and focusing on the seed as the mechanism for the plant getting through the bad times. In seed form they can survive for millennia. They are vulnerable plants and must reproduce as quickly and prolifically as possible. It is this "rock star" life cycle that makes them so valuable to us. They have no time to be coy, if they want their seeds pollinated and their fruit dispersed they have to get down to business and really flower and really fruit.
To get the most from annuals we need to keep the good times going for as long and consistently as possible. They are fair weather friends and at the hint of trouble, lack of water, a really cold or hot day they will bolt – go to seed.
So you need to know what they like and pander to that, especially if what you want are not seeds but leaves. Take coriander for example. It panics when it gets hot and runs to seed, its leaves become stunted and bitter. To keep it really leafy it's best grown in winter.
We can extend the useful life of annuals through dead heading, a delaying technique. However, if we are growing our annuals for seed or flower they need a bit of stress to trigger flowering. If we give them too much of a good thing, especially nitrogen rich fertilisers, they become all lush leaf and sappy, like Elvis with too many hamburgers.
The other thing about annuals is accepting that they won’t live forever. Unless you are seed saving, don’t let the sad fading display linger, get in and rip them out and start with the next planting. Nothing looks sadder than an aging rock star.
When they go to the dark side annuals become the ultimate bad boys – weeds. To deal with annual weeds we need to trick the seeds into breaking their dormancy then ruthlessly dispatch the seedlings before they can set the next generation of seeds. This progressively reduces the seed bank in the soil.
Biennials
Biennials, as the name suggests, last for 2 growing seasons. They are the middle distance runner as opposed to the annual sprinter. The line between annual and biennial can be fuzzy, depending on climate variation and breeding.
Breeders have tinkered with some foxglove cultivars, for example, to get them to flower within a season. Biennials, like annuals, are reliant on seeds for propagation but they usually have another means of coping with stress. Usually, this is in the form of a big root – think carrots and parsnips.
Biennials use their first season of growth to produce leaves and store energy that helps get them through the winter and provide for seed production the following spring/summer.
Parsley lets us know when its time is up. You can get two years out of happy parsley but the first season's growth will be leafy and in the second season it’s concentrating on stalks for seed heads. Some biennials we ruthlessly treat as annuals, silverbeet is an example, we want it for its youthful leaves not seeds. Likewise, onions we grow for the first season bulbs not the second year's flower.
Of course, if we are seed saving from biennials we need to be patient. If we are growing a biennial for its flowers however, we need to sow it a season before we want it to flower.
Biennial hollyhocks and foxgloves need to be planted as seeds the preceding spring or as seedlings before winter so they can go through their first season of vegetative growth to give them the power to flower. Even more than annuals we can be tempted into trying to get our biennials to last for ever, be bold and remove them and get on with the next season's planting.
Perennials
Perennials are plants with life cycles of more than two years. They sit out adversity rather than avoid it, they have various coping techniques. They might have waxy, furry or grey foliage to resist summer heat and drought. They might be deciduous to avoid the darkness and cold of winter. They are not solely reliant on the seed to propagate, they have bulbs, stolons, roots and shoots that become new plants. A bit like biennials when grown from seed, perennials will not really start maturing until at least the next growing season. Perennials are divided into herbaceous and woody perennials.
Herbaceous Perennials
Herbaceous perennials are the plants we associate with a perennial border. They all have soft annual stems and perennial roots and take various forms: ground covers, small shrubs, tufted lilies, ferns and grasses. They can be large – bananas are herbaceous perennials. Many popular herbaceous plants have evolved in prairies, steppes and Mediterranean regions where they are subject to grazing, drought, harsh winters and fire. Not only do they survive in these conditions – they thrive.
One common method of dealing with grazing or cold is producing new shoots at their base. Herbaceous perennials sacrifice their extremities. It is this survival technique that makes the constantly cut lawn a success. This ability to respond to cutting is a valuable tool for manipulating flowering with herbaceous perennials. By removing flowers or cutting plants to the ground we can extend or delay their flowering times
Don't let the delicate flowers of Delphinium Blue Sensation fool you – treat it mean after flowering and you will be rewarded with several repeat performances. Likewise many salvias, such as Salvia nemorosa, have a distinct clumping form and respond to well-timed hard pruning. Many herbaceous perennials go through a semi-dormancy in winter. This is why many of Diggers perennials will arrive in winter with an "I am dormant" tag. The plant is very much alive, it's simply sitting winter out below ground.
Sedums like Matrona and Autumn Joy dramatically demonstrate the winter die back. Many find the seed heads of sedums in winter to be sculptural and in areas with frost the old dead growth can provide protection to the base from the cold. So don't rush in to cut perennials too soon. The clumping form of many perennials can make propagating easy. By breaking up clumps we produce more plants, and reinvigorate the clump in the process.
Woody Perennials
Woody perennials are the true marathon runners of the plant – world they form the dominant structural plants of our gardens, the trees and shrubs.
Their wood or lignin allows these plants to grow large. They can compete with those fast living annuals by shading them and developing competitive root systems. When pruning woody perennials we need to be aware that this can have implications for many years to come. Likewise, because of their size in the landscape anything we do to them has implications for the plants that grow in their shade. The woody perennials are great carbon sinks, while we may be ruthless with annuals and herbaceous perennials we need to think long and hard before removing a mature tree or shrub.
Knowing the life cycle of the plants in our garden helps us to get the most from them. It also gives us confidence: no, you haven't killed that basil, it is naturally a one season wonder; be patient with your hollyhocks, they just need another season to flower, cut those salvias to the ground, they will come again.