Gardeners have been growing seedlings indoors for generations, without the need for additional lighting. In the past, however, they enjoyed more light than many modern gardeners, as houses were often equipped with cold frames, a kind of mini-greenhouse resting on the foundation on the south side of the house. In this way, as soon as the seedlings had risen and hardened off, they matured in full sunlight.
Nowadays, cold frames are rare. What’s more, modern windows, insulated with inert gases, let in less light than in the past. As a result, our poor seedlings have a harder time of it. Yes, you can grow seedlings without extra light, especially if you have large south-facing windows, but they’re likely to be a bit stunted.
Growing in front of a single window is especially risky for seedlings that spend a lot of time indoors. It’s no problem to sow morning glories (Ipomoea) or lettuce plants indoors with less-than-perfect lighting, as they are only sown in late April or May, but for begonias and celery, which are often sown in February, the effect of several months of irregular and insufficient lighting is very harmful and sometimes even fatal. That’s why a fluorescent lamp can be very useful.
Growing Seedlings Under Artificial Lighting
LED lamps are often preferred to fluorescent lamps for growing seedlings. They are more energy-efficient, longer-lasting and give off less heat, which reduces the risk of burning plants. What’s more, they offer a full spectrum of light, essential for plant growth, and allow height adjustment to prevent plants from burning or drying out.
Any fluorescent lamp is suitable for seedlings… but the small lamps used to replace incandescent bulbs only light a few pots at a time. Long, straight fluorescent lamps cover much more space: under a standard 120 cm lamp with two tubes, you can grow several hundred seedlings.
The best tube for seedlings is Cool White. It gives off mainly blue rays. Avoid horticultural tubes and Daylight or Warm White tubes, which are enriched with red rays.
For LEDs, prefer a full-spectrum lamp.
Installation
Place your lamps 15 to 20 cm from the seedling trays. As the seedlings germinate and grow, raise the lamp so that it is always 15 to 20 cm from the top leaves. As not all seedlings grow at the same speed, it may be necessary to raise some trays on inverted pots so that their foliage is at the required distance.
Run the lamps for around 14 hours a day. A timer is useful for this. Note that since plants grow faster under artificial lighting than in front of a window, you’ll need to keep a closer eye on watering.
Finally, it’s important to acclimatize your seedlings to outdoor conditions before transplanting them into the ground, whether or not they were grown under fluorescent lights.
Go ahead, make the leap to growing under artificial lighting this year. It’s an investment that pays off!
Larry Hodgson published thousands of articles and 65 books over the course of his career, in both French and English. His son, Mathieu, has made it his mission to make his father’s writings accessible to the public. This text was originally published in Le Soleil on March 8, 2008.