The annual plant life cycle is influenced by temperature and the photoperiod. The photoperiod is the period of daylight in a day. As days lengthen, light triggers reactions in plants to come out of their dormant state and begin their annual cycle. Many factors affect how a plant will grow and whether it will complete its life cycle.
The annual life cycle of a plant is determined by the type. An annual plant, for example, completes its entire life cycle in one growing season. A biennial plant grows in two stages. Sometimes these plants will form a rosette the first year. A rosette is a circular cluster of small stems and leaves, which typically grows low to the ground. A perennial plant will live through many growing seasons, determined by the type of plant. The giant sequoia, for example, can live over 3,000 years, according to the National Park Service, making it one of the oldest living plants on Earth.
Plants can generally be characterised as cool-season or warm-season plants. The active growth period for cool-season plants is during the cooler spring months. Some plants such as skunk cabbage will begin growing even while snow is still on the ground. Warm-season plants wait until the late spring to summer to begin their active growing season. Some plants, such as big bluestem, require warmer soil temperatures in order for their seeds to germinate.
The function of the plant life cycle is to survive climatic and environmental conditions long enough to reproduce. Because of prevailing conditions, plants may reproduce through several methods. Flower and seed production are typical of flowering plants. Other plants, such as grasses, will also reproduce through the development of tillers. Tillers or runners are plant shoots that develop at the root.
Environmental stresses can affect the progression of a plant's life cycle. In order to produce food and energy for growth, plants require sunlight, water, and carbon dioxide. If drought conditions occur, plant growth may slow or stop altogether as it tries to conserve resources. During situations of severe environmental stress, reproduction is secondary. Many plants adapt to persistent environmental conditions so that they may still complete their life cycle during times of stress. For example, desert plants will undergo gas exchange through their leaves during the cooler night hours rather than risk water loss during the warmer, drier part of the day.
Other factors affecting the annual plant cycle are those caused by humans. Plants can be both positively and negatively impacted by human activity. Pesticide or fertiliser use can prolong a plant's life. Watering plants can mitigate the effects of drought-like conditions. Note, though, that the same pesticides and fertilisers that can help plants can also destroy them if used improperly.