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Understanding Photoperiodism in Plants: How Day and Night Affect Plant Blooming

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Plants use the daily cycle of light and dark (the photoperiod) as a calendar cue to time seasonal events like flowering, dormancy and leaf drop. Photoperiodism is the physiological response to the relative lengths of day and night, enabling plants to flower at the proper season. For example, many flowering plants bloom together in spring or autumn to maximize pollination.

In classic 1920s experiments Garner and Allard showed that a tobacco variety (“Maryland Mammoth”) would only flower when nights were long – demonstrating that night-length (not day-length) is usually the controlling factor. In practice, it is the uninterrupted dark period that determines whether a plant will initiate blooms.

Many plants fall into three photoperiodic categories:

Short-day (long-night) plants: Flower only when nights exceed a critical length (typically late summer or fall). Common examples include chrysanthemums, poinsettias, Christmas cactus and soybeans. These species naturally bloom in spring or fall when days are short. (If exposed to longer daylight, flowering is inhibited.)
Long-day (short-night) plants: Flower only when nights are shorter than a critical length, i.e. when days are long (usually late spring/early summer). Common garden examples are lettuce, spinach, clover, and onion, as well as ornamental flowers like asters, coneflowers and California poppies. These bloom in summer when the photoperiod lengthens.
Day-neutral plants: Flowering is insensitive to day length. Examples include tomato, corn, cucumber and most common beans. These bloom based on maturity or other cues, not photoperiod. (Some “everbearing” strawberry varieties are effectively day-neutral, whereas most commercial June-bearing strawberries require short days to flower.)

So, to summarize, a short-day plant needs a long, uninterrupted night (e.g. >12–14 hours) to bloom, while a long-day plant needs relatively short nights (often <10–12 hours). Day-neutral plants bloom regardless of daylength, though many still flower better under favorable light. These classifications explain why some crops are limited to certain latitudes: for instance, the long-day crop spinach will not set seed in the tropics (days never get long enough), and the short-day ragweed fails in far-north climates (frost comes before days are short enough).

How Photoperiodism Works

Plants perceive daylength through light receptors (photoreceptors) in their leaves and use an internal circadian clock to measure night length. The best-studied receptors are phytochromes, which sense red vs. far-red light. In sunlight (which is rich in red light) phytochrome converts to its active P_fr form; in darkness it slowly reverts to the inactive P_r form. Thus at dusk almost all phytochrome is P_fr, and during a long night it gradually converts back to P_r. For many short-day plants, a threshold amount of P_r must accumulate before a flowering signal is released. This signal is often called florigen – a mobile hormone produced in the leaves that travels to the shoot apex (tip) to trigger the production of flowers.

In practical terms, this means a short-day plant will only flower if the night is long and completely dark. For example, studies with the cocklebur (a classic short-day plant) showed it requires at least ~8.5 hours of unbroken darkness each night to flower. Even a brief flash of red light (as little as a few seconds) during the night instantly converts P_r back to P_fr and resets the photoperiodic clock, preventing flowering. This is why streetlights or indoor lights at night can stop short-day plants from blooming. Soybeans are infamous for not flowering and cropping in areas with outdoor lighting at night.

Long-day plants work on the opposite principle. They typically require their circadian clock to coincide with daylight late in the day. A well-known mechanism (studied in Arabidopsis) involves the clock-gene CONSTANS (CO): under long days CO accumulates and induces the FT gene (florigen), triggering flowering, whereas on short days CO does not persist and flowering does not occur. In simple terms, long-day plants need enough light in late afternoon to activate flowering genes, while short-day plants need long, complete darkness.

Long-day plants rely on a specific molecular mechanism to regulate flowering based on day length. A well-known mechanism studied in Arabidopsis thaliana, shows that a key component is the CONSTANS (CO) gene, which encodes the CO protein. This protein promotes flowering by activating the FLOWERING LOCUS T (FT) gene, which produces florigen—a mobile flowering signal which is a protein. Under long-day conditions, CO protein accumulates late in the day due to synchronization between the plant’s internal circadian clock and extended daylight. This accumulation enables CO to activate FT gene expression, producing florigen which triggers flowering. Under short days, CO protein is degraded before it can induce FT, so flowering does not occur. In essence, long-day plants require light in the late afternoon or evening to stabilize CO protein and initiate the flowering process, whereas short-day plants depend on long periods of uninterrupted darkness to suppress it.

In addition to flowering, photoperiodism controls other seasonal processes. In temperate trees, shortening days trigger dormancy: the extended dark hours cause phytochrome signals that induce abscisic acid and other hormones, stopping growth and hardening buds for winter. When days lengthen again, the inverse signals release dormancy and resume growth. Thus photoperiodic sensing helps plants synchronize their life cycles with the seasons.

Examples of Photoperiodic Plants

Many familiar plants exemplify each category. Short-day plants (requiring long nights) include autumn-flowering ornamentals and some tropical crops. For example, the poinsettia (a tropical Euphorbia) will only develop its colored bracts when nights exceed about 12–14 hours. In greenhouses, poinsettias must be kept in complete darkness each night (often by covering) starting in autumn to produce the Christmas display color. Other classic short-day examples are autumn chrysanthemums, Christmas cactus, autumn-colors of some bulb plants, soybean, and rice. Many weeds (e.g. cocklebur, ragweed) are also short-day, ensuring they bloom in fall rather than midsummer.

Long-day plants flower when the photoperiod lengthens. These include spring or early-summer bloomers and cool-season vegetables. Lettuce, spinach, radish and onion (among vegetable crops) typically bolt or flower in long days. Ornamentals like Phlox, Lupine, lilies and poppies also require long days to bloom. Even potato tuberization and flower-setting in some tuberous plants are long-day processes. Certain trees and grasses (e.g. fruit trees, ryegrass) will keep growing if days are long.

Day-neutral plants are largely indifferent to daylength. Common examples include tomatoes, cucumbers, sweet corn, and strawberries labeled as “day-neutral.” These plants initiate flowering based on age or developmental stage rather than photoperiod. (In practice, many so-called “day-neutral” varieties may flower more abundantly under certain light conditions, but they are not strictly dependent on daylength.) It’s worth noting that strawberries are broadly classified into two photoperiod types: short-day cultivars, which initiate flowering as days shorten (typically late summer to autumn in temperate climates), and day-neutral cultivars, which can flower and fruit throughout the growing season when temperatures are suitable. The term “June-bearing”—commonly used in North America—refers to short-day strawberries that fruit heavily in late spring to early summer but is not widely used internationally.

Practical Implications of Photoperiodism for Gardeners

Understanding photoperiodism lets gardeners manipulate blooming times. By controlling light exposure, you can induce out-of-season blooms or prevent unwanted flowering. Common techniques include:

Shortening days (forcing short-day plants): To trigger flowering in short-day species (e.g. chrysanthemums, poinsettias), give them long nights with no light interruption. Growers often drape opaque cloths or move plants into dark rooms for at least the critical dark period each day. For example, autumn chrysanthemums naturally bloom under autumn’s long nights, but a spring grower can force them to bloom earlier by covering the plants nightly (for ~12 hours) over several weeks. It’s crucial that the dark period be unbroken – even a brief streetlight glow or a flash from a camera can negate the effect. (An experiment showed <2 foot-candles of light at night is enough to inhibit poinsettia flowering, so blackout curtains must be very dark.)
Lengthening days (forcing long-day plants): For long-day plants (e.g. summer crops or spring flowers), gardeners can use supplemental lighting to extend natural daylight. Simple incandescent or LED grow lights left on for a few extra hours after sunset will trick the plant into perceiving a longer day. For instance, to have Easter or Valentine’s blooms from long-day ornamentals (like certain lilies or poppies), nurseries expose them to additional light (often from 6–10 pm) for several weeks. In a home setting, placing a lamp on a timer to give 14–16 hours total light can induce early flowering in lettuce, spinach, or long-day flowers.
Scheduling: Both approaches require weeks of consistent treatment. Most photoperiodic plants need about 4–8 weeks under the new daylength before flowers appear. For example, to produce a fall bloom of chrysanthemums in June (summer in the northern hemisphere), one must cover them nightly for roughly six weeks. Likewise, to have early lettuce or alliums, one would start light extension several weeks before the desired flowering date.
Examples: Poinsettias illustrate these tactics: in the wild they turn red as natural days shorten, but in greenhouse production growers ensure plants get ~14 hours of darkness each day starting in mid-September. As a result, the bracts color up in time for December sales. Similarly, home gardeners can prolong lettuce harvest by giving seedlings additional evening light in winter, or force spring bulbs (like Siberian squill) by mimicking longer cold-dark periods – though bulbs often have additional cold requirements beyond photoperiod.

By leveraging photoperiodism, gardeners and growers can time flowering to their needs. Whether it’s blocking light to force a poinsettia’s holiday display, or adding a lamp to coax seedlings into bloom, understanding day–night cues gives precise control over plant development.