Plants And Light: Do Seedling Plants Need Darkness To Grow

Image by Clara_Gabrielli

By Liz Baessler

Do seedling plants need darkness to grow or is light preferable? In northern climates, seeds often need to be started indoors to ensure a full growing season, but this isn’t only because of warmth. Plants and light have a very close relationship, and sometimes a plant’s growth, and even germination, can only be triggered by extra light.

Do Plants Grow Better in Light or Dark?

This is a question that doesn’t have just one answer. Plants have a quality called photoperiodism, or a reaction to the amount of darkness they experience in a 24-hour period. Because the earth is tilted on its axis, the periods of daylight leading up to the winter solstice (around December 21) get shorter and shorter, and then longer and longer leading up to the summer solstice (around June 21).

Plants can sense this change in light, and in fact, many base their yearly growing schedules around it. Some plants, like poinsettias and Christmas cacti, are short-day plants and will only bloom with long periods of darkness, making them popular as Christmas gifts. Most common garden vegetables and flowers, however, are long-day plants, and will often go dormant in the winter, regardless of how warm they are kept.

Artificial Light vs. Sunlight

If you’re starting your seeds in March or February, the length and intensity of the sunlight is not going to be enough to make your seedlings grow. Even if you keep your house lights on every day, the light will be diffused throughout the room and the lack of intensity will make your seedling plants get leggy.

Instead, buy a couple of grow lights and train them directly over your seedlings. Attach them to a timer set to 12 hours of light per day. The seedlings will thrive, thinking it’s later in the spring. That being said, plants do need some darkness to grow, so make sure the timer also turns the lights off.

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