CORVALLIS, Ore. — If your lettuce bolts too soon or your mum won’t bloom, the problem may be day length.
The amount of light and darkness a plant receives — known as photoperiodism — determines when most plants form flowers, according to Oregon State University Extension Service horticulture experts.
Botanists once thought the number of daylight hours triggered flowering. Research has shown it’s actually the length of uninterrupted darkness that matters most.
Types of plants by day length
- Short-day (long-night) plants: bloom when days are shorter than about 12 hours. Common examples include chrysanthemums, poinsettias and Christmas cactus. If exposed to longer days, these plants won’t set buds.
- Long-day plants: bloom only when they receive more than 12 hours of light. Many summer bloomers fall into this group, such as asters, coneflowers, California poppies, lettuce, spinach and potatoes.
- Day-neutral plants: flower regardless of day length. Tomatoes, corn, cucumbers and some strawberries are in this category. Some plants, like petunias, bloom regardless of day length but flower earlier and more profusely in long days.
How gardeners can influence bloom
Horticulturists and home gardeners can manipulate day and night length indoors with lights or covers to encourage plants to bloom at times they normally wouldn’t.
- For chrysanthemums (short-day plants): Cover them for at least 12 hours daily for several weeks in late spring or early summer to simulate fall conditions and stimulate blooming.
- For long-day plants: Provide extra hours of light with grow lamps to bring them into bud earlier, before natural day length passes 12 hours.
This practice is common in the nursery and cut flower industry, especially for blooms timed around Valentine’s Day and Easter.
Previously titled What are short-day and long-day plants?
