Gardening glossary
Photoperiod
Photoperiod is the botanical mechanism behind day-length sensitivity. Although gardeners talk about "day length," plants actually measure **night length** — the uninterrupted dark period — using a light-sensing pigment called **phytochrome**. A single flash of bright light in the middle of a long night can completely reset a plant's flowering response, which is how commercial chrysanthemum growers force out-of-season blooms.
The three classical photoperiod categories:
- **Short-day plants (SDPs)** flower when night length exceeds a critical threshold — usually nights longer than 10–12 hours. Examples: chrysanthemums, poinsettias, soybeans (most varieties), short-day onions, rice, cotton, ragweed. They naturally bloom in late summer, fall, or near the equator.
- **Long-day plants (LDPs)** flower when nights are shorter than a critical threshold — usually nights shorter than 8–10 hours. Examples: spinach, lettuce (bolting), radish, dill, wheat, barley, long-day onions, most temperate-zone wildflowers. They naturally bloom in late spring and summer at mid-to-high latitudes.
- **Day-neutral plants** flower based on age, temperature, or accumulated growth — photoperiod doesn't directly trigger flowering. Examples: tomatoes, peppers, cucumbers, sunflowers (most modern varieties), maize, day-neutral strawberries, dandelions.
There are also **facultative** versions of each category, where photoperiod accelerates or delays flowering but isn't strictly required. Most modern crop varieties have been bred toward day-neutrality or wide photoperiod tolerance to extend their growing range across latitudes.
Practical implications:
- Bolting in spinach, lettuce, and cilantro in early summer is a photoperiod response — not just heat. Look for "slow-bolt" varieties or sow in fall. - Indoor cannabis growers manipulate photoperiod (12/12 light/dark) to trigger flowering in short-day strains. - Onion seed catalogs list the latitude band each cultivar is bred for — pay attention. - Some perennials need a specific photoperiod sequence to break dormancy in spring; if you bring them indoors under constant lighting, they may never reset properly.
If you remember one rule: when a crop bolts, flowers, or bulbs "too early," check photoperiod before you blame your watering.