1.10: Classify Plants by Life Cycle

Learning Objectives

• Describe characteristics of plant life cycle classifications.

A plant will go through a sequence of stages from seed germination to seed production as a mature plant. For some plants, this sequence, or life cycle may take a few weeks while others continue to grow and flower repeatedly over many years. Plant life cycles are classified as annual, biennial, or perennial. Annuals complete their life cycle of germination from seed, growing, flowering, fruiting and dying within a single season of growth. Biennials require two seasons to complete their life cycle. In the first season, foliage production and storage of food reserves takes place followed by flowering, seed production and death in the next. Perennials typically flower annually once established and may live for several to a great number of years.

Types of Annuals

While the annual life cycle is completed within a single season of growth, the term annual or bedding plant may also be used to describe any plant that is grown outdoors in the spring and summer for one growing season.

Annual flowers differ in their tolerance to cold weather and frost. Hardy annuals are the most cold tolerant; they will take light frost and some freezing weather without being killed. In most cases, hardy annuals can be planted in the fall or in the spring before the last frost date. Examples of hardy annuals include Lathyrus (sweet pea), Viola (pansy), and Tagetes (marigold) cultivars. Most hardy annuals are not heat tolerant and usually decline and die with the onset of hot summer temperatures. Another type of hardy annual is the winter annual that germinates in the fall, overwinters as a rosette of leaves, and flowers in late winter and early spring. Species of Stellaria (chickweed) and Cardamine (snapweed) are examples of winter annuals.

Half-hardy annuals will tolerate periods of cold damp weather, but will be damaged by frost. Most half-hardy annuals can be seeded outdoors in early spring since they do not require warm soil temperatures to germinate. Seeds or plants are normally planted after the last spring frost. Examples of plants grown as half-hardy annuals are Cosmos (cosmos) and Tropaeolum (nasturtium). Some half-hardy annuals may decline in the midsummer heat but may re-bloom in late summer or fall.

Because most tender annuals are native to warm tropical regions of the world, they are sensitive to cold soil temperatures and are easily damaged by frost. Most seeds will not survive freezing soils temperatures and will not germinate when soil temperatures are below 15°C. It is recommended to wait two to three weeks after the last spring frost to sow seeds or transplant outdoors. Tender annuals include species of Begonia (begonia)and Impatiens (impatiens).

While some plants may be perennial in tropical regions, they are categorized as cool- or warm-season annuals when planted in colder regions. Cool-season annuals, such as Pelargonium (geranium), Petunia (petunia), and Antirrhinum (snapdragon), grow best when temperatures are in between 20° and 25° C. during the day. Best flower production is in the spring and fall; flower production tends to decline in the middle of a hot summer. Warm-season annuals, such as Zinnia (zinnia) perform well when day time temperatures are between 26° and 32°C. and night time temperatures are between 15° and 20°C.

Biennials

The life cycle of biennial plants is completed over two growing seasons. During the first season, they produce only leaves—usually in a rosette. Following a winter cold period, they flower in the second growing season, produce seeds, and then die. Popular biennials include Digitalis (foxglove) and Oenothera (evening primrose). Cultural practices are basically the same as for annuals, except that the plants are alive for two growing seasons.

Biennials present the obvious disadvantage of producing only foliage the first year. One solution is to sow biennial seeds in mid-summer so that the plants will develop during the summer and fall. After exposure to the winter cold, they will develop flowers in the spring.

Perennials

Perennial plants can be either short-lived or long-lived herbaceous or woody plants. Short-lived herbaceous plants such as Gaillardia (blanket flower) may live for only a few years, or they can be long-lived like Paeonia (peony). Woody plants also classify as perennials, though they are rarely referred to as such. Woody species have stems that continue to grow, developing a permanent structure that the plant cannot ‘replace’ once removed. Some woody plants live tremendously long lives, such as the 9500 year old Picea sp. (spruce) in Sweden and British Columbia’s 1000 year old Thujaplicata (western red cedar). Perennials that flower and fruit only once and then die are termed monocarpic. However, most perennials are polycarpic, flowering over many seasons in their lifespan.

Common hardy herbaceous plant families include:

Asteraceae – sunflowers

Brassicaceae – mustards

Crassulaceae – sedums

Liliaceae – lilies

Lamiaceae – mints

Poaceae – grasses

Ranunculaceae – buttercups

Review

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