Plants And Light: Do Seedling Plants Need Darkness To Grow

Seedling Sprouting Out Of The Soil
seedling light
(Image credit: Clara_Gabrielli)

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. Since the earth is tilted on its axis, the periods of daylight leading up to the winter solstice (around December 21st) get shorter and shorter, and then longer and longer leading up to the summer solstice (around June 21st). 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.

Liz Baessler
Senior Editor

The only child of a horticulturist and an English teacher, Liz Baessler was destined to become a gardening editor. She has been with Gardening Know how since 2015, and a Senior Editor since 2020. She holds a BA in English from Brandeis University and an MA in English from the University of Geneva, Switzerland. After years of gardening in containers and community garden plots, she finally has a backyard of her own, which she is systematically filling with vegetables and flowers.