The short version: An annual goes from seed to seed in one growing season and then dies: lettuce, beans, zucchini, cosmos, sunflowers. A biennial grows leaves and a fat root in its first year, waits through a cold winter, then flowers, seeds and dies in its second: carrots, beetroot, parsnips, onions, foxgloves, hollyhocks. A perennial lives for years and flowers again and again: asparagus, rhubarb, rosemary, chives, echinacea. The labels shift with climate, though. Tomatoes and chillies are perennials that frost turns into annuals, kale and silverbeet can run for a year and a half in a mild garden, and a cold snap can trick a biennial into bolting in its first year. Knowing which cycle you are dealing with tells you how to sow, when to expect flowers and how to save seed.
Every seed packet is a promise about time. Some pay out in six weeks and are finished by autumn. Some ask for two years. Some take a season to settle and then feed you for twenty. The three words that describe those promises are annual, biennial and perennial, and they are the most useful bit of jargon in gardening.
Annuals: one season, all in
An annual germinates, grows, flowers, sets seed and dies within a single growing season, sometimes within a few weeks. Because it has one chance to reproduce, it puts everything into flowering and fruiting, which is why annual vegetables such as zucchini, beans and corn crop so heavily and why annual flowers such as marigolds, zinnias and cosmos bloom for months if you keep picking. The gardener's side of the deal is that you sow them fresh every year, and that a single sowing gives a single harvest window, so the trick with annuals is succession sowing: small batches every two to three weeks for a continuous supply.
Annuals split by season. Cool-season annuals such as peas, broad beans, lettuce, coriander, poppies and calendula are sown in autumn or late winter to grow through the mild months and finish before the heat. Warm-season annuals such as beans, cucumbers, basil, zinnias and sunflowers wait for the soil to warm after the last frost. Older books also grade them by frost tolerance: hardy annuals shrug off light frost, half-hardy annuals take cool weather but not frost, and tender annuals need warm soil and are killed by the first frost.
Biennials: leaves this year, flowers next
A biennial spends its first season building a rosette of leaves and a storage root, sits through winter, then uses the stored food to send up a flower stalk, set seed and die in its second season. Most of our root vegetables are biennials, and we eat them halfway through the cycle: carrot, parsnip, beetroot, turnip, swede, onion, leek, celery and celeriac, along with cabbage, kale, silverbeet and parsley. The classic biennial flowers are foxglove, hollyhock, sweet william, Canterbury bells and honesty, sown in one year to flower the next.
The switch from leaves to flowers is thrown by cold. A period of low temperatures, which botanists call vernalisation, tells the plant that winter has passed and it is safe to flower. Roughly speaking, celery needs a couple of weeks below about 10 degrees and carrots need many weeks, up to two or three months, near 5 degrees, though the exact figures vary by crop and variety. That mechanism explains two things gardeners see every year. A biennial vegetable in a frost-free climate may never flower at all, which is why seed savers in Brisbane can struggle to get carrots to flower. And a long run of cold weather after an early sowing, once the seedlings are a few weeks old, can convince a young carrot or onion that it has already had its winter, so it bolts to seed in its first season with nothing worth eating underneath. Bolt-resistant varieties exist for exactly this reason, and so does the advice to sow biennial roots at the right time for your zone. Our guide to why coriander bolts covers the other trigger, heat and long days, which sends annuals to seed early.
Perennials: plant once, harvest for years
A perennial lives for more than two years and, unlike the other two, does not usually die after flowering. Most flower and seed every year for the rest of their lives; botanists call those polycarpic. A few, such as some agaves and bromeliads, grow for years and then flower once and die, which is called monocarpic. Perennials come in two habits: herbaceous ones die back to the roots in winter and reappear in spring, and woody ones such as rosemary and bay stand year round.
In the vegetable garden the perennials are the plants that reward patience. Asparagus is not cut at all in its first year and only lightly in its second, then can produce for around twenty years. Rhubarb stays productive for many years and is divided when it slows. Globe artichoke stays productive for a few years and is renewed from offshoots. Sorrel, perennial leeks, bunching onions, chives and most Mediterranean herbs (rosemary, thyme, oregano, sage) sit in the same class. Perennial flowers grown from seed, echinacea, delphinium and lupin among them, usually give little in their first year and a proper display from the second, and delphiniums are short-lived even then. Bedding rudbeckia (Rudbeckia hirta) and many salvias are the exceptions and flower in the season they are sown. Our guide to perennial vegetables goes through the crops worth the wait.
Where the lines blur
- Tender perennials grown as annuals. Tomato, capsicum, chilli, eggplant and basil are perennials in their tropical homelands. Frost kills them, so in temperate and cool Australia they are sown fresh each spring and pulled in autumn. In Darwin, Brisbane and coastal Queensland, where frost is rare or absent, gardeners commonly report chillies and capsicums cropping for three or four years.
- Biennials grown as annuals. Almost every root crop. We harvest carrots, beetroot and onions in their first season and never see the flower unless we leave one in for seed.
- Biennials grown as perennials. Kale and silverbeet in mild climates can keep giving leaves for a year or more before they finally bolt, depending on climate and variety, so they behave like short-lived perennials.
- Annuals that behave like perennials. Cosmos, calendula, nigella, alyssum, borage and coriander self-seed so readily that the patch comes back every year from dropped seed, even though every individual plant lived a single season. Our guide to self-seeding flowers lists the reliable ones.
- Day length as well as age. Onions bulb in response to day length, not just time in the ground, which is why varieties are sold as short-day, intermediate and long-day and why a wrongly timed sowing can bolt or never bulb.
Planning a garden by life cycle
- Annuals go in the beds you rework every season and get sown in succession. Deadhead annual flowers and pick annual vegetables young to keep them producing; the moment they set mature seed, they are done.
- Biennials get sown at the right time for your zone so that winter arrives at the right stage, and one or two plants get left in the ground if you want seed. For root crops that means lifting the best roots in autumn, storing or replanting them, and waiting for the second-year flower.
- Perennials get a permanent bed with the soil prepared properly once, away from the annual beds you dig over, and a year or two of patience before they pay back.
Seed saving follows the same logic. Annuals are easy: let a plant mature and collect. Biennials need the second season, which in a small garden means committing a bed for two years or overwintering roots in a pot. Perennials are usually multiplied by division or cuttings rather than seed.
A word on foxgloves and rhubarb
Two of the plants in this article deserve a safety note. Every part of a foxglove is poisonous if eaten and the sap can irritate skin, so keep it away from where small children play, and note that foxgloves that set seed are declared pests in Tasmania, where they cannot be sold or propagated, so we cannot send foxglove seed to Tasmanian addresses. Rhubarb stalks are food, but the leaves contain enough oxalic acid to be dangerous and must never be eaten. If a child eats any part of either, call the Poisons Information Centre on 13 11 26.
Life cycles across Australia's climate zones
- Tropical (Cairns and far north Queensland, Darwin and the Top End of the Northern Territory, the Kimberley in Western Australia): no frost, so tender perennials such as chilli, capsicum and eggplant live for years, while true biennials often never get the cold they need to flower.
- Subtropical (Brisbane and coastal Queensland, Coffs Harbour and northern New South Wales): chillies and capsicums commonly overwinter; kale and silverbeet run long; cool-season annuals are sown in autumn.
- Arid and semi-arid (inland Queensland, Broken Hill and Dubbo in New South Wales, Mildura in Victoria, inland South Australia and Western Australia, Alice Springs in the Northern Territory): cold winter nights vernalise biennials reliably, and summer heat ends most annuals early, so timing is everything.
- Temperate (Sydney and New South Wales, Melbourne and Victoria): the textbook pattern. Tender perennials are grown as annuals, biennials behave, and perennial beds thrive.
- Mediterranean (Perth and southern Western Australia, Adelaide and the Adelaide Plains in South Australia): mild winters mean many cool-season annuals and some biennials run on into spring, and Mediterranean perennial herbs are at home.
- Cool and alpine (Hobart and Tasmania, Canberra and the ACT, Ballarat in Victoria, Orange and Armidale on the New South Wales tablelands, and the alpine country of the Snowy Mountains and the Victorian high country): hard frosts end every tender perennial each autumn, biennials get all the cold they need, and perennials such as asparagus and rhubarb are at their best.
These are broad groupings. Your own last and first frost dates decide which perennials survive and when biennials will bolt.
Frequently asked questions
Is a tomato an annual or a perennial?
A perennial by nature that frost turns into an annual. In frost-free gardens a tomato can live more than a year, though most gardeners replace it because the second season crops poorly and carries disease.
Why did my carrots flower in their first year?
A long cold spell after an early sowing convinced the plants that winter had already happened, so they skipped the root-building year. Sow at the recommended time for your zone and try a bolt-resistant variety.
Do perennial flowers grown from seed bloom in the first year?
Usually a little or not at all. Echinacea, lupin and delphinium typically make a proper show from their second year, so grow them alongside annuals for the first season.
Can I save seed from a biennial vegetable?
Yes, but it takes a second season. Leave a few of the best plants in the ground, or lift and replant the roots, and collect seed after they flower the following year.
Sort your packets into the three cycles and the calendar writes itself: sow the annuals in waves, time the biennials to the cold, and give the perennials a bed of their own. ABC Gardening Australia's guide to perennial vegetables is a good next read.
Related growing guides
- Perennial vegetables: plant once, harvest for years
- Self-seeding flowers that come back every year in Australian gardens
- How long do seeds last? Storing seeds and testing if they still grow
Something for this season and something for the next ten: browse our flower seeds, asparagus seeds and cosmos seeds.