growing seedlings under lights Seeds & Seed Starting

Growing Seedlings Under Indoor Grow Lights


growing seedlings under lightsAnyone who’s tried growing plants from seed knows that proper lighting is critical to producing an abundance of stocky, green seedlings. For those of us lucky enough to have a south-facing window with 12+ hours of full sun, lighting isn’t an issue. But for the rest of us, an indoor lighting system of some kind is a necessity. This primer on lighting will help you choose the options that work best for you.

Light Color

Sunlight contains the complete spectrum of light, including all colors of the rainbow. Although plants use the full spectrum for photosynthesis, red and blue light seem to be most critical. Red light stimulates vegetative growth and flowering (but if a plant gets too much, it will become tall and spindly). Blue light regulates plant growth, which makes it ideal for growing foliage plants and short, stocky seedlings (but too much will result in stunted plants).

Light Intensity

The intensity of light that a plant receives is determined by the wattage of the bulb and distance between the plant and the light source. Different plants have different light intensity needs, but most seedlings grown for the garden will need higher intensity light to germinate and flourish. In general, the leaves should be about 2 – 4 inches away from the light source (assuming use of a fluorescent bulb – see below).

Duration of Light Exposure

Most vegetables and garden plants require at least 14 to 18 hours of light each day; without enough light they get pale and leggy. However, be sure to give plants a nightly rest. Respiration, which is an important part of the growth process, only takes place when it’s dark. To ensure that plants are getting the right amount of light and darkness, put the lamp(s) on a timer.

Types of Bulbs

You can choose between incandescent, fluorescent, and high-intensity discharge (HID) bulbs, each of which has its own pros and cons. Choose the grow light that works best for the type of plants you want to grow and where you plan to grow your seeds.

Incandescent Bulbs

These include halogen bulbs and are the type of light bulbs still used in most homes. They are a good source of red light, but a poor source of blue, meaning that plants will likely become spindly when grown under incandescent light. Incandescent bulbs, and especially halogen bulbs, produce a lot of heat in relation to the amount of light they give off; plants growing too close to the bulb can be easily burned.

Fluorescent Bulbs

These types of bulbs produce two to three times more light than incandescent bulbs for the same amount of energy and are the most inexpensive lights for indoor gardening. However, they usually require bulky external ballasts (like, for example, overhead shop lights) so aren’t as easy to work with as incandescent and LED bulbs.

Cool white bulbs are a good source of blue and yellow-green light, but are a poor source of red light. Plants grown under cool white bulbs will be stocky or even slightly stunted. Warm white bulbs emit plenty of orange and red light, but less light in the blue and green spectrum. These bulbs, when used alone, result in tall, spindly plants. If  you are growing seedlings under two-bulb fluorescent fixtures, you can usually achieve a good color balance by combining one cool white and one warm white bulb.

Full-spectrum fluorescent bulbs produce a balance of cool and warm light that replicates the natural solar spectrum, although these are less energy efficient than other fluorescent bulbs and tend to produce more heat. But, given the wider range of light frequencies emitted by these bulbs, they are a good choice for growing seedlings.

Related Review: SunLite Garden full-spectrum grow lights and stand

LED Grow Lamps

Unlike other bulbs which produce light across a broad spectrum, light-emitting diodes (LEDs) give off light within a narrow band. LEDs designed for growing plants emit light in the two bands that plants need – red and blue. The result is a purple glow that not everyone likes. Although these bulbs are long-lived and very energy efficient, they cost considerably more than fluorescent bulbs.

Research is still ongoing to determine which combination of light frequencies are best for plant growth and how LED grow lights compare to fluorescent bulbs in producing healthy seedlings. For now, the jury is still out …

High-Intensity Discharge Lamps

These lamps are used by commercial growers and serious horticulturalists. These energy-efficient bulbs generally emit twice the amount of light (lumens) as a fluorescent bulb. However, the bulbs and special fixtures are considerably more expensive than those needed for incandescent or fluorescent lights. They also tend to be high-wattage bulbs, so you need to be sure your electrical system can handle the load. Some of these lights burn so brightly that they must be located in a special room and you’ll need to wear eye protection when working around them.

Metal halide lights emit an intense, bluish-white light that is excellent for growing plants. The foliage stays green and vigorous, and plants are usually stocky and strong. Metal-halide lights are currently the number one choice for serious indoor gardeners. Mercury vapor lamps emit a bluish, relatively well-balanced, high-intensity light. High-pressure sodium bulbs are usually used to promote flowering and fruiting but, when used exclusively, they produce leggy, weak-stemmed plants.

Given all the choices, it’s easy to ensure that your plants get the quality, intensity and duration of light they need to stay in peak condition.

Growing Tips

  • Rotate your plants each week. The light from a fluorescent bulb is more intense at the center of the bulb than it is at the ends.
  • Replace fluorescent tubes when the ends darken. That means the tube is old and the light output may be less than half of a new bulb.
  • Clean your fluorescent bulbs each month. Dust and dirt can dramatically decrease the amount of light emitted.
  • Place your hand where the light hits the foliage. If you feel any warmth, the light is too close.

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