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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.