In general, it is the need to reproduce that causes a flower to bloom. How that process is triggered differs from specie to specie. Flower buds form in reaction to different occurrences. The plant may be mature enough to put the energy required into flower and seed formation. The plant senses a coming winter in response to lengthening night periods. The plant experiences a cold period which stimulates growth toward sexual maturity. The plant senses the coming of spring by the shortening night periods. In some cases, the plant senses it will die and only then flowers and produces seed.
Theory holds that a flower-inducing plant hormone called floriden is the physical mechanism within a plant that causes flowering. There is strong evidence this theory is correct, but studies on it continue.
Annuals are plants that germinate, grow to maturity, flower and produce seed in a single season. The plant dies, leaving only its seeds behind to grow into new plants the following spring. Annuals grow quickly, and the flowers begin blooming as soon as the plant can provide the energy required to bloom and produce seeds.
The short life cycle of annuals causes the plants to undergo physical changes which force the flower to bloom, even if the plant has not reached full size. As the nights lengthen, the plant senses the coming winter and puts its energy into flower and seed production rather than growing.
Even though the plant may only manage to bloom a single flower, this may be enough to ensure that some seeds for the next year will grow into new plants. For annuals, maturity and lengthening night periods causes the plant to bloom.
Biennials, such as Sweet William, grow slowly the first summer, building a root system to survive the winter. In the spring, the roots put forth new shoots which grow very quickly.The shoots and leaves grow faster than the plant can produce more food, utilizing stores of energy from the previous year. Once the stems and leaves are mature, the flower buds are formed and opened. After pollination, seeds are produced and the plant dies. Biennials flower in response to maturity which is achieved the second year of their lifespan.
Environmental conditions and maturity play a large part in causing a perennial plant's flowers to bloom. These plants usually do not flower in the first year after germinating from seed, although some do. Perennials spend their first summer establishing themselves as plants that will live more than 2 years. Sensing the coming winter by the lengthening night periods, they go dormant. After the days begin to lengthen again and temperatures warm, they break dormancy and begin growing again.
When a perennial plant reaches maturity, it flowers and puts some energy into the process. Seeds form, fall to the ground or are carried off by animals. For perennials, flowering may be put off if conditions are not exactly right. They can afford to be a little patient and bloom at exactly the right time.