Annual, biennial or perennial?

Annual, biennial or perennial?


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Annual, Biennial, or Perennial?  

Annuals.  One could argue that the whole life of a plant is geared toward one thing: reproducing. For annual plants, the production of flowers and seeds is the culmination of their very existence! Soon after annual plants produce mature seeds, they die, having exhausted themselves from sprouting, growing foliage and flowers, and finally producing viable seed in just one growing season. The mother plant dies, but she may have left hundreds, or even thousands, of seeds to carry on her legacy. For annual plants, one generation per year is the norm, so they have a life cycle of one growing season. Our bean seed, in the discussion above, is an annual; other common garden annuals include zinnia, cosmos, and broccoli.

Biennials.  Biennial plants complete their life cycle over two growing seasons. The first season they grow only foliage, commonly a low-growing rosette of leaves. The second growing season they form flowers and produce seeds; then, the mother plant dies. Common biennial flowers include foxgloves and Canterbury bells. But did you know that cabbage, kale, Brussels sprouts, carrots, and celery are also biennials? We usually harvest them in their first season of vegetative growth, so we never actually see the flowers.

Perennials.  Horticulturists don’t often talk about a perennial plant’s life cycle. Rather, you may hear about a particular perennial’s life span. For example, you might hear that columbines are relatively short-lived, so you should plan to replant every few years. Perennial plants continue to grow and flower for more than two years—and many will live for decades. However, if we wanted to talk about a perennial’s life cycle, we would need to look at how long it takes a particular plant to cycle from seed to seed.

As with annuals and biennials, perennials produce flowers that, if successfully pollinated, form seeds. The difference is that the mother plant doesn’t die after producing seed. If we define a life cycle as the time it takes a plant to go from seed to seed, you can see that perennials’ life cycles can vary widely. For many common perennials, completing a life cycle usually takes from two to perhaps five years. For example, if you plant a coneflower seed, you’ll get only foliage the first few years, with flowers and seed coming in subsequent years. If the first seed is formed the third season, then we would say the plant has a three-year life cycle, from seed to seed.

The word perennial is commonly used to describe long-lived herbaceous plants—those with green, non-woody stems. In temperate regions, most perennials die back to the ground in the winter, then sprout from the roots or crowns in the spring. Woody plants like shrubs and trees are also perennials, in that they grow for many years. However, in common usage a distinction is made between woody and non-woody perennials. The word perennial is reserved for herbaceous, non-woody plants. Woody plants whose aboveground parts persist through the winter are categorized as shrubs, trees, or woody vines.

Some weeds, such as dandelions, can produce more than one generation in a growing season. Their life cycle may be just a few months. On the other hand, the agave, or century plant, may need to grow for up to twenty years to accumulate the energy reserves necessary to produce its twenty- to forty-foot seed stalks. Even more remarkable, some types of bamboo are thought to have bloom cycles of over 100 years. It may take them 100 years to bloom, then they won’t bloom again for another 100 years. Of course a life span this long is difficult for scientists to verify.

Just to muddy things up a bit: Many of the plants northern gardeners grow as annuals are really perennials when grown in their native climates. Examples include petunias, geraniums, tomatoes, and peppers. And some plants like hollyhocks are really perennials, but most people treat them as biennials because after the second season they usually succumb to rust disease.

Now that we’ve looked at life cycles in general, let’s take a closer look at the details of reproduction.

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No Deadheads Allowed!
We all know it’s important to "deadhead," or remove spent flowers, from many garden plants to keep them flowering. And to continually pick "vegetables" like tomatoes and peppers for a longer harvest. But why is this so?

The production of mature, viable seeds on a plant triggers a chemical signal indicating that the plant has completed its life cycle, and no longer needs to expend the energy to produce more flowers. By removing flowers before they form seed heads, or harvesting vegetables before the seeds are fully mature, we can encourage the plant to produce more flowers and fruit. Because seeds haven’t been allowed to mature, the chemical signal is not triggered, and the plant continues to produce.


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Foxy Foxgloves.
If you love foxgloves and want to have flowers every year, be sure to sow some seeds each year. Biennial plants like foxgloves produce foliage the first growing season, and flowers the second season. By starting some seeds of these and other biennials every spring, you’ll ensure that half the plants will be at the flowering stage each year. (There are, by the way, some foxglove varieties that produce flowers the first season.)