At our store, we are asked by customers frequently “do I really need a grow light?” If you want to keep growing your small veggies, lettuce, and herbs indoors over the winter, the answer is yes!
The reason why we need lights is the quality and number of hours of sunlight that we get at this time of year in the northern hemisphere won’t cut it for your plants. Indoors, the lumens (amount of light) are a fraction of what your plants would get outdoors in the summer. If you just put them on a windowsill, they’ll stretch their stems looking for light and become leggy.
A grow light is like a little sun for your home. The sun produces electromagnetic radiation, some of which is visible to us as colours. This is what plants use for photosynthesis.
What kind of bulb do I need?
It really depends on what you want to grow. For seedlings, cool crops (like lettuce), microgreens, wheatgrass, and herbs, get a full spectrum compact fluorescent bulb, such as the one pictured to the right. It produces light by bouncing gas off of the metal powder inside the bulb. This sort of bulb is great for beginners and will provide enough lumens for these crops. These bulbs are cool and efficient, but the bulbs need to be replaced periodically (about every 6 months).
If you want to continue growing your greenhouse crops indoors (such as your tomatoes, cucumbers, peppers, peas, beans, etc) you’ll need a higher intensity light. The light from compact fluorescent bulbs won’t be able to penetrate the leaf canopy (the light stops at the first leaf it encounters). The high-intensity lights can penetrate the canopy like real sunlight. They work by shooting high-voltage electricity into the bulb, making the gas spark. This means they run hotter than the compact florescent bulbs as well, so you’ll need additional venting, cooling fans, and ducting. They also require ballasts to control the power source in addition to the bulb and fixture.
These bulbs produce a yellow-reddish light that’s ideal for mature flowering plants. Young plants do require light from the cool/blue end of the spectrum, so HPS lights are mainly used to supplement the natural light in a greenhouse or area which receives some natural light.
Metal Halide (MH)
MH bulbs produce a more balanced blue-ish light, and are used generally when the lamp will be the only source of light for the growing area.
Light-Emitting Diodes (LED)
These bulbs convert electricity to light and run at a lower voltage than the HPS and MH bulbs. This technology is not yet mature and not readily available to retail consumers. We are in the process of comparing the LEDs to other forms of lighting and so do not carry them in our store yet.
How much light will I get from each lamp?
|150-175||2′ x 2′|
|250||3′ x 3′|
|400||4′ x 4′|
|600||6.5′ x 6.5′|
|1000||8′ x 8|
Having grow lights near young seedlings (4-6″ away) prevents the stems from elongating, so the plants develop strong stems. The lights need to be this close because of the inverse-square law of light. If your lights need to be farther away because of how you are using your space, you’ll need stronger bulbs. In general, plan to place the lower wattage systems 12-24″ away from the plants, the medium wattage 12-36″, and the high wattage 18-48″.
When you are shopping for lights, the package will tell you how many lumens the bulb emits. For reference, the lumens on a clear summer day will be about 20,000, and about 100,000 under full direct sun at noon. For comparison, a bright office will be about 500 lumens. This should really drive home the need for lights in the winter.
How long do I run the lights?
This is where you open up your trusty search engine and do some research. Your plants will react to the hours of light and temperature provided as if it were actually that time of year outdoors. For example, warm temperatures, long days and short nights will cause lettuce to bolt, but tomatoes and peppers will love those conditions. Expect to run your lights for at least 10 hours per day.
Do I need anything else?
Other accessories you might use are timers, circulating fans, ducting, and reflectors to bounce the light back towards the plants.