Encyclopedia of Life Sciences - Vol. 2

By Anne O’Daly | Go to book overview

BIENNIAL PLANTS

Biennial plants are plants that complete their life cycle over two growing seasons, after which they die

Foxgloves (Digitaws
purpurea) are a biennial
species imported to the
United States from
England
.


CONNECTIONS
Biennials go
into a state of
DORMANCY
between their two
growing seasons.
Some biennials are
ROOT AND
TUBER CROPS
.

BIENNIALS AS GARDEN ORNAMENTALS

The advantages of biennial plants over perennials are that they develop and
flower earlier and the flowers are often bigger, brighter, and more numerous.
However, perennials last longer, and constant replanting is unnecessary.
Biennials also need to be sown from seed, and there is always the chance that
the seeds may fail to germinate. Many perennials have some means of
reproducing vegetatively, and cuttings from a parent plant can therefore be taken
to produce new plants. Some perennials, such as sweet william (Dianthus
barbatus
), pansies (Viola tricolor), and wallflowers (Erysimum cheiri), are often
treated as biennials because they are at their best in the first flowering year.

A CLOSER LOOK

There are about 250,000 different species of flowering plants, and one way of classifying them into easily recognizable groups is by the length of their life cycle. Some plants, including many weeds, complete their lives within one growing season, less than a full year, and are called annuals. At the other extreme are the perennials, including many herbaceous plants (those that die down over the winter and produce new shoots come spring), which live tor three or more growing seasons. In between are the biennials, which complete their life cycle over two growing seasons. They germinate from seed, grow and develop in the first year, then flower and produce seeds in the second year, and then die.


Producing offspring

While annuals devote all their energy into maturing early and producing as many offspring as they can w ithin one growing season, biennials spend their first growing season developing and preparing for the following year. Annuals and biennials expend much of their energy on flower and seed production. Perennials continue to grow and develop from year to year, a process that uses most of their energy, but because they live longer, they have more time over which to produce offspring.


Colorful biennials in gardens

Biennials last only about 18 months from the day they are sown until they reach the compost heap, but for the gardener wanting masses of color, annual and biennial species are the obyious choice. Although these plants are only temporary residents, they produce the brightest, most colorful flower displays. Popular biennials grown for their flowers include the foxglove (Digitalis purpurea), Canterbury bells (Campanula medium), pansies (Viola tricolor), hollyhocks (Alcea rosea), and forget-me-nots (Mjosotis spp.). They are best sown in early summer so that the plants have a chance to develop fully before the winter sets in.


Biennial vegetables

Many vegetables also have a biennial life cvcle. Beets (Beta vulgaris), cabbage (Brassica oleracea), and turnip (Brassica rapa) are all biennial vegetables; they are normally picked after their first year of growth, before they have the chance to produce flowers, when they usually become inedible. Many biennials, such as the carrot (Daucus carota) and parsnip (Pastinaca sativa), store food in their roots to enable them to survive their period of dormancy between the two growing seasons. This food store is then used the following year to provide the energy for flower and seed production.

Some biennials, such as the lettuce (Lactuca sativa), may flower early in their first growing season and effectively become annuals if they are grown in poor soil or in drought conditions. They are unlikely to survive until the following year.

The shape of some biennial plants may change markedly in their second growing season. For example, the familiar leafy rosette of lettuce becomes a tall, slender spire, bearing small yellowish flowerheads. In this form it is scarcely edible and therefore of little use commercially.

B. BROWN

See also: ANNUAL PLANTS; PERENNIAL PLANTS.


Further reading:

Armitage, A. M. 2001. Armitage’s Manual of Annuals, Biennials, and Half-Hardy Perennials. Portland, Oreg.: Timber Press.

Grey-Wilson, C. 2000. Annuals and Biennials. American Horticultural Society Practical Guides. New York: Dorling Kinderslcy Publishing.

-179-

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Encyclopedia of Life Sciences - Vol. 2
Table of contents

Table of contents

  • Title Page i
  • Contents iii
  • Useful Information 148
  • Artificial Life 149
  • Asthma 153
  • Atmosphere 154
  • Atp Production 158
  • Autism 161
  • Autoimmune Disorders 164
  • Bacteria 167
  • Bats 173
  • Beetles 177
  • Biennial Plants 179
  • Biochemistry 180
  • Biodiversity 183
  • Bioethics 187
  • Biogeography 192
  • Biological Control 195
  • Biological Warfare 197
  • Biology, History and Philosophy of 200
  • Bioluminescene 205
  • Biomes and Habitats 208
  • Bionics 212
  • Biorhythms 216
  • Biosphere 220
  • Biotechnology 221
  • Birds 228
  • Birds of Prey 234
  • Blood 238
  • Bone 248
  • Botanical Gardens 251
  • Botany 253
  • Brain 255
  • Bulbs and Corms 265
  • Butterflies and Moths 267
  • Cacti and Succulents 272
  • Caecilians 275
  • Calcium 277
  • Calorie 279
  • Camouflage and Mimicry 281
  • Index 287
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