Why do plants flower when they do?

Mostly because of day length, which is the only environmental signal reliable enough to time a life cycle by — temperature varies wildly from year to year, but the length of the night on a given date never does.

How plants measure it. Pigments called phytochromes exist in two interconvertible forms that switch depending on the light they absorb. Red light converts one to the other; darkness slowly reverts it. The balance therefore encodes how long the plant has been in darkness. Crucially, experiments showed that plants are measuring the length of the uninterrupted night rather than the day — a brief flash of light in the middle of a long night prevents the response entirely.

The three categories:

Short-day plants flower when nights exceed a threshold — chrysanthemums, poinsettias, many autumn flowers. This is why a poinsettia must be given long uninterrupted darkness to colour up, and why a streetlight or a lit room prevents it.

Long-day plants flower when nights are short — spinach, lettuce, many summer flowers.

Day-neutral plants, which respond to age, size or other cues instead — tomatoes among them.

The other signals that interact with it:

Vernalisation, a required period of cold. Many biennials and winter cereals will not flower until they have experienced winter, which prevents a plant germinating in autumn from flowering immediately and being killed.

Age and size, since a plant must be developmentally mature — the reason a young fruit tree does not crop regardless of conditions.

Stress, which can trigger early flowering as an emergency reproductive response — this is bolting.

Temperature, modifying the exact timing within a season.

Why this matters practically: lettuce and spinach bolt in summer because long days tell them to; forced bulbs and out-of-season flowers are produced by manipulating light and cold artificially; and planting a short-day crop at the wrong time of year produces a plant that will never do what you wanted, however well you feed it.