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Anthesis is the opening of a flower, or the interval during which it is open and functional. It is a developmental stage within flowering, which also includes the earlier transition to making flowers. A flower bud can therefore be present before anthesis.^[ref]

The distinction is useful in an experiment: comparing two flowers three days after anthesis aligns their development more closely than comparing them three days after their parent plants were moved to a greenhouse. Opening does not by itself establish that pollination or fertilization has occurred. Those are separate events whose timing must be observed.

A flower bud is an undeveloped flower in which the floral organs are forming or remain enclosed before anthesis. It differs from a vegetative bud, which will extend a shoot with leaves. A flower bud is already reproductive even while its external coverings hide the future stamens and carpels.^[ref]

Dissecting a series of flower buds of increasing size can reveal the order in which floral structures appear. Opening the largest bud is not equivalent to observing the original switch to flowering: that developmental decision occurred earlier. This makes flower bud size a useful staging observation, provided the relation between size and internal development is checked for the particular plant.

Flowering is the transition to producing flowers, the reproductive structures of angiosperms. A meristem that previously produced vegetative organs begins a developmental program forming flowers. This transition can precede the visible opening of a flower, called anthesis, by a substantial interval.

Photoperiodism links flowering to seasonal day length. In Arabidopsis thaliana, the timing of CONSTANS gene expression and the stabilization of its protein by evening light allow long days to promote flowering. Experiments measuring CONSTANS abundance showed that a daily rhythm in messenger RNA alone did not explain the response: light-dependent protein degradation supplied another regulatory step.^[ref] Different plants integrate different combinations of day length, temperature and developmental state; flowering is not triggered by one universal environmental threshold.