Transplant production: Light considerations

Light is essential for plant growth and providing the right type, in sufficient amounts, is critical to ensure healthy seedlings.

Ideally, sufficient natural lighting is available for germinating seeds. This provides several advantages:

  • Growing plants will be more acclimated to full solar exposure
  • Natural light provides the complete spectrum necessary for plant growth
  • It’s free           

However, when starting seeds early in the season, or when greenhouses are not available for transplant production, artificial lighting may need to be used.

There are several types of artificial lighting that can be utilized when starting seeds. When selecting a light source, it is important to consider cost of equipment, light spectrum, electrical requirements, heat output and coverage.

The primary types of artificial lighting used for seedling production are fluorescent, metal halide (MH), high pressure sodium (HPS), and light-emitting diode (LED).

Fluorescent
Usually the least expensive and most readily-available, fluorescent light fixtures are a commonly used lighting system for home gardeners and small commercial operations. They come in a variety of sizes and configurations, but are rated using the letter ‘T’ and a number, which is greater for bulbs with a larger diameter. Most commonly used are T5 and T8 bulbs, which are five-eighths inch and one inch in diameter, respectively.

These bulbs come in a variety of spectrum options, often denoted by their rating in kelvins (K). The color temperature ranges from 2700K, which is a warm white color, to 8000K, which is a cooler, brighter white. Often, growers will utilize multiple bulbs on opposite ends of the color spectrum to try to provide the largest array of light wavelengths for growing seedlings. For example, a two-bulb unit might operate a 2700K bulb and a 6500K bulb to provide both warm white light and cool, bright light.

Fluorescent bulbs tend to put out minimal heat, which is important because they need to be relatively close to growing seedlings for maximum effect (see Table 1).

''Table 1 Comparison of common light options including incandescent, compact fluorescent, fluorescent, high pressure sodium, and metal halide.

Metal halide/High pressure sodium

MH and HPS bulbs are oftentimes used in commercial greenhouse and nursery operations, both as primary artificial light sources, as well as supplemental light in the early season. They vary in their light output and electrical draw, but typically begin at 250W and can exceed 1000W for a commercial horticultural application. MH bulbs tend to provide light on the blue and green spectrum, while HPS bulbs offer more red and orange light. This is important to consider, as blue/green light is more often used to promote vegetative growth, while red/orange light tends to promote flowering.

MH and HPS light fixtures require a ballast that matches the bulb wattage to run effectively. These fixtures and bulbs put off much more heat than a fluorescent fixture and also require more power to operate. However, MH and HPS bulbs tend to provide more usable light than fluorescent bulbs and can be situated further away from growing plants, providing greater coverage per bulb (see Table 1)

Light-emitting diode

LED lights are the most modern addition to artificial lighting technologies available to growers. While the fixtures tend to be more expensive, they operate on very little electricity, can provide a wide spectrum of light, and have a long operating life.

Whether artificial light is needed as the sole light source for emerging seedlings, or it is being used to supplement natural light in a greenhouse setting, growers have a wide range of options to match the specific needs of their operation. By balancing cost, light spectrum, and coverage, growers can provide appropriate lighting to grow healthy, productive transplants.

To read about the light research being done in the MSU Floriculture Department, visit their publications page.

Be sure to keep an eye out for the next article in this series: Transplant production: Greenhouse systems.

Collin Thompson is the Farm Manager of The North Farm at the Michigan State University Upper Peninsula Research and Extension Center in Chatham, Michigan and a Community Food System Educator with Michigan State University Extension.