Grow Lights for Greenhouses: The Ultimate Guide


Here you will learn how to create the best greenhouse climate for your crop with the ultimate guide to greenhouse grow lighting.

Grow lights are designed to give plants the light they need for vital chemical reactions for healthy growth when the sun’s rays just are not enough. They are essential for plants in greenhouses so they can carry out the process of photosynthesis, while protected from the rain, wind, animals, pests that damage the plants, and cold temperature.

How do you choose the best grow lights for your greenhouse? Keep reading on to find out.

Understanding Greenhouse lighting

Greenhouse grow lighting is a lot more complicated than first meets the eye. Greenhouse farmers searching for the best lighting for their greenhouses should consider the following three factors: what type of crop is being grown, what time of year it is, and how much sunlight is naturally available.

  • Greenhouses typically need six hours of direct or full-spectrum light every day.
  • When this is not possible naturally, supplemental grow lights must be installed.
  • Supplementing lighting inside the greenhouse with multiple high-intensity grow lights will promote crop growth and yield.
  • Farmers use them to maintain growth and extend the growing season, and boost yields and profits.

Just as important as supplemental grow lights are photoperiod control lighting. For example, photoperiod light is the number of hours that plants receive light in 24 hours.

Let say the sun rises at 6 am and sets at 8 pm; a 14-hour photoperiod has passed. Using Photoperiod control lights helps simulate long days. This triggers early flowering or delays flowering, depending on the plant’s needs.

Greenhouse farmers have a wide selection of greenhouse grow light options to choose from, so it is vital to understand all the different lighting styles.

Let’s look at the uses and advantages of different lighting types.

Here’s a quick video about grow lights to get started, with a lot more info below:

High-Pressure Sodium Grow Lights

High-pressure sodium greenhouse lights give off more orange and redder spectrum light but look golden-white to the human eye.

These lights promote budding and flowering and are generally used later in the growth cycle of the plant. Sodium grow lights are around seven times more efficient than incandescent lights and work well when used together with natural daylight. Sodium grow lights are a great option for greenhouses. 

These lights also allow a 10% increase in intensity and photosynthetically active radiation (PAR.)

Remember to give sodium grow lights about 4 to 5 minutes to warm up and one minute to cool down. They are not ideal for greenhouses where the lights are turned on and off frequently. Another important point is to be aware of placement; high-pressure sodium lights should be installed 30 to 36 inches above the plants for best results.

Programmable and Fixed Spectrum LED Grow Lights

LED grow lights are the longest-lasting option that has a typical lifespan of 50,000 hours. The diode of an LED light will not burn out as fast as standard light bulbs. This gives LED grow lights an incredibly long lifespan. LED grow lights are more efficient than standard grow lights because more power input goes to light than heat. 

For example, incandescent bulbs are only 20% efficient because most of their input power produces heat.

The biggest advantage of using LED grow lighting in your greenhouse is the massive energy savings. LED lights are easily installed into any greenhouse and allow up to 70% savings.

No warm-up time is required for LED grow lights, and they are free of mercury, making disposal more environmentally safe than other light fixtures. LED grow lights provide superior functionality when used as a single source of lighting, making them an attractive option for many greenhouse growers.

Ceramic Metal Halide Grow Lights

Ceramic metal halide grow lights are used because of their blue light but look bright white to our human eyes. Ceramic Metal Halide grow lights can be used as the primary light source.

They have an average lifespan of 8,000 to 15,000 hours. Metal halides grow lights are 3 to 5 times more efficient than incandescent bulbs and make an excellent option for greenhouses that do not get regular natural sun. 

Metal halide lights need to warm up for 5 minutes before they can give out full light. They also need to cool down for 5 to 10 minutes before you can restart them again. They are not recommended for greenhouses where the lights are turned off and on often.

  • Ceramic metal halide lights need to be hung 30 to 36 inches over the plants. 
  • They provide darker leaves and overall healthy-looking plants. 
  • Ceramic metal halide lights are the best solution for greenhouses and hydroponic applications. 
  • Greenhouse farmers often use these lights at the early start of a plant’s life when seeds are in the vegetative growth phase.

These lights have dimmable ballast that allows growers to get the perfect lighting for their plants. Metal halides are completely silent, so there is no annoying humming, buzzing, or high-pitched noises.

T5 Greenhouse Grow Lights

T5 grow lights are the most efficient and most popular fluorescent greenhouse grow light option. T5 lights use less energy than any traditional lamps and last up to 50,000 hours. 

They are environmentally friendly and sometimes have aluminum reflectors for maximum efficiency. T5 is ideal for greenhouses, hydroponics, warehouses, and barns. These grow lights are often used from the initial phase of seed starting to full term plant growth.

The tubular shape of the lamp denotes the “T,” and the number 5 indicates its diameter in eighths of an inch. These lights are slim, only 5/8″ of an inch in diameter, which makes these fluorescent tubes more efficient than standard fluorescent lights

T5 Fluorescent Lamp gives off an extremely high lumen output and full-spectrum light that is great for plants from the seedling phase up to full term growth. T5 has minimal heat output, which means it can safely be mounted within 6 to 12 inches close to plants. There is technically no such thing as too much light; however, it is vital not to use too powerful grow lights in a small space, which will result in overheating of the plant’s leaf area.

Here’s a quick video with info about the T5 grow lights:

What Problems Can You Face with Greenhouse Grow Lights?

When you purchase a grow lighting system for your greenhouse, you get several benefits; however, there are also some problems that you might encounter. Let’s look at a few.

Not Enough Light

Your greenhouse plants’ growth can be jeopardized when you do not have enough lights installed in your greenhouse. Generally, plants are adaptive, and they will stretch to reach the sunlight.

  • All the plant’s energy that should normally be used for seedlings, blooms, or fruits will be wasted as the plants try to reach more sunlight to carry out crucial chemical activities.
  • Because of this, you will have top-heavy plants that are prone to diseases and will not flower or bloom as they should.
  • You should make sure that the grow light systems you install have enough wattage for your plants.
  • It should be installed close enough to the seedlings during the growth period, which is usually about 1 to 2 inches away.

Many flowering houseplants need to be 12 inches away from the light source to be able to carry out the essential photosynthesis process. All kinds of foliage plants can be placed 36 inches away from the grow lights. When mainly growing vegetables, you require more light exposure. It stimulates the flowers to bloom and produce fruit.

Too Much Light

When there is too much light, it can harm the plants. Typically, plants need light and water to make oxygen and starch for food. During the nighttime, plants change starch to sugar and store it.

Most greenhouse growers make the mistake of switching on the grow lights all the time to stimulate faster plant growth; however, this can affect the plants’ health. When the plants get too much light exposure, it can burn the plants and make them go pale. At least 8 hours of total darkness are needed to guarantee the best plant health.

  • Always pay attention to the wattage of the grow light and the distance between the plant and the light. 
  • When the light has a high wattage, it may be too high for your greenhouse plants, affecting their health. 
  • The light should not be too close, so it can not burn the plants.

Some plants have different requirements when it comes to light intensity. Plants that usually grow in dry areas with abundant sun exposure need more grow lights to thrive. The kind of plants that grow in shady or tropical forests and do not get enough natural access to the sun do not need many grow lights.

The Wrong Kind of Light

Make sure you pick the kind of light that works best for your plants. Some kinds of grow lights produce blue light that promotes bushy growth, while the red light will be best for flowering plants.

Picking the correct HIDs or LEDs is crucial for the best plant growth. You should avoid HID row lights when you have smaller or weaker plants, as this light’s heat output can be too intense. LED grow lights are more versatile because you get an adequate light spectrum. Fluorescent tube grow lights are more cost-efficient and cooler than other sources.

Uneven Distribution

Even when you have the best grow lights, your greenhouse plants can suffer if you do not have the lights installed correctly. Some plants require different light exposure. Generally, the light should be distributed at 20 to 40 watts per square foot of the greenhouse area.

Also, make sure that light reaches all the plant’s leaves. Spacing your plants under the lights evenly is important to avoid the bigger plants not keeping the smaller plants in the shade.

  • How big your greenhouse is will also have an impact on the type of grow lights you require. 
  • If your greenhouse is large, a good idea will be to install HIDs, as these lights have a wider coverage area.

Fluorescent grow lights also emit enough light to cover a larger area and usually produce less heat. Make sure they are installed evenly to cover the whole area in the greenhouse. For bigger greenhouses LED grow, lights work well if you distribute them evenly.

How long should you keep the greenhouse grow lights on?

Light is essential for plant growth and health, but all plants need rest. Darkness is crucial for plants to carry out the chemical processes they need. Plants do not require lights to be switched on 24 hours. It is a good idea to install a timer that controls how long your grow lights are switched on.

When it comes to daily light requirements for plants, there are three categories. Short-day plants need 12 hours of light or less to promote blooming. When these plants receive too much light, it can harm them.

When growing azaleas or begonias in a greenhouse that has enough sun exposure, it might not be necessary to install grow lights at all.

Long-day plants need 16 and 18 hours of light exposure for healthy growth. Commonly, vegetables and garden flowers are long-day plants and require more light when seedlings are growing.

Day-neutral plants like geraniums and foliage plants need 8 and 12 hours of light all year round. This means you might switch on the grow lights only when the day is shorter when your greenhouse does not get enough sunlight during the winter season.

Conclusion

There are many automated greenhouse growth lighting options available to cater to different plant types and stages of growth. The range of supplemental growth to photoperiod control grow lighting options we mentioned above will make your greenhouse lighting a simple, efficient process.

Robert Sampson

I'm Robert Sampson and I live in Colorado where I spend a lot of time in the backyard with my family either grilling, playing games and sports, or working on a project to make our backyard a better place to be.

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