Annuals are the horticultural fireworks. They are planted, quickly give a dazzling display, and then die away leaving you (or nature) to dispose of the dead flower, leaves and roots. Biennials take two years (or growing seasons) to complete this cycle. In the first year you may get a lovely bit of foliage, but it is not until the second season that you are rewarded with flowers. Having flowered they, like the annuals, completely die away. Perennials are stayers. Though most don't bloom in their first year, from the second year on they will bloom year after year for the lifetime of the plant. Over the winter the top part of the plant will die away, but many keep their leaves for the whole year providing "winter interest" and groundcover. The lifetime of a perennial will vary according to the plant itself, ranging fom five to twenty-five years.
Sometimes a perennial can behave like an annual due to the local climatic conditions. If the conditions are warmer than the plant's native conditions, it will shoot up, provide a dazzling bloom in the first year, and then die back too exhausted to make it through to the next year. Similarly, if the conditions are colder than the plant would ideally like, it may make a great display when planted, but be unable to make it through the winter frosts. These would generally be called "Tender Perennials", whereby the ease of growing, relative cheapness and first year blooming make it worthwhile to grow as an annual.
In short, an annual quickly gives a beautiful display but will need replacing each year, a biennial will take twice as long to provide a bloom and then need replacing, and a perennial, though usually slower to provide flowers, repays your patience with a display year after year.
As seed sellers we are occasionally asked "why grow annuls when perennials give so much more for the money? The converse of this is "Why grow perennials when you can have a different display with annuals every year? Annuals are often brighter, flashier than perennials. They offer a far greater range of colours and shapes. Additionally, they tend to germinate more reliably and offer you the chance to change parts of your garden each year. If a colour combination doesn't quite work, plant something else next year. Perennials can provide a sense of stability in the garden and more importantly, they often provide year round interest. Effectively having to start your beds and borders from scratch each year is labour intensive and can be expensive.
As in most things, gardens need balance; often perennials provide the structure and annuals the patches of glory. Most importantly, we often choose a plant for its unique beauty, height, colour, whether it likes shade or direct sun and so on. The fact that it is an annual, biennial or perennial is secondary. For instance, I would love it if the digitalis that I use to great effect under my sycamore tree was either an annual, for blooms in the first year, or a perennial so that I didn't have to keep replacing them. But it isn't, it only comes as a 'biennial' and so I happily keep sowing each year knowing that, though only half my digitalis are in bloom in any one summer, this is vastly outweighed by the pleasure that they give me.
How hardy is it?
There are numerous excellent and comprehensive classifications of hardiness. At Seedaholic we have kept it simple, using just 'Hardy', 'Half-Hardy' and 'Tender'. The hardiness label indicates how cold a plant will tolerate:
|Hardy:||Will tolerate temperatures down to -15°C|
|Half-Hardy:||Will tolerate temperatures down to 0 °C (freezing)|
|Tender:||Only happy above 5°C. Will usuallly need overwintering in a greenhouse or inside.|