Tips for Controlling Light Intensity in Your Greenhouse

Light intensity is one of the most important contributors to the success of your greenhouse. Too much light can cause the plants to dry out, while insufficient light will deprive them of essential energy and stunt their growth. These seven tips will give you total control over the light intensity in your greenhouse and create an ideal environment for your precious plants.

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  • Know Your Plants

First and foremost, you need to know what type of light intensity your plants prefer. The general rule of thumb is an intensity of 20 to 40 watts per square foot for greenhouses or indoor gardens, but there are exceptions for certain species. These are some of the common plants that prefer light with high, medium, and low intensity:

  • High light: Succulents, sunflowers, cosmos, ficus, monstera, lantana, marigold, geranium, salvia, sedum, fruit and vegetable plants
  • Medium light: Ferns, palms, ivy, violet, begonia, alocasia, dracaena, calathea, anthurium, aspidistra, maranta, philodendron
  • Low light: Snake plant, spider plant, peace lily, anthurium, aglaonema, fittonia, nephytis butterfly, zamioculcas, pothos


If your greenhouse is all about growing food, you need light fixtures with high intensity. When sun-loving fruit and vegetable plants don’t get enough light, their leaves turn yellow and fall prematurely. Their flavor and nutritional value will also suffer from light deficiency, leaving you with a bad crop. The proper light intensity increases the chance of a good harvest.

  • Incorporate Full-Spectrum Bulbs

Natural sunlight includes all colors of the rainbow, which is why it’s usually better than artificial grow lights. However, full-spectrum light bulbs are viable alternatives to sunlight. They also possess the entire light spectrum, mimicking the sun with a balance of warm and cool light all plants can enjoy.


Your greenhouse doesn’t have to use full-spectrum bulbs exclusively, but they should be the primary fixtures. T-5 bulbs are the most popular full-spectrum models because they use less power than typical incandescent lights and last longer. You can also incorporate these types of grow lights that play specific roles:


  • Light-emitting diodes (LEDs): The most energy-efficient type of grow light, requiring 60% less energy than other bulbs and coming in both red and blue colors.
  • High-pressure sodium (HPS) bulbs: A type of high-intensity discharge (HID) bulb that emits strong red light to encourage flowering and budding.
  • Metal halide bulbs: The second type of HID bulb that emits blue light to stimulate bushy plant growth.


You should make a point to avoid incandescent and fluorescent grow lights. They have poor energy efficiency, tend to overheat quickly — which can burn your plants — and don’t last as long as LED or HID lights.

  • Determine How Many Lights You Need

Next, you need to determine exactly how many lights your greenhouse needs. You can always adjust the number later if you get it wrong on the first try. Start by calculating the greenhouse’s surface area, then compare the number to how much surface area each fixture can cover. Wattage is usually a strong indicator of a light model’s surface area coverage.


However, just because your lights have high wattage doesn’t mean you should cover the whole greenhouse with them. Too much power can overheat the place and burn your plants. That’s partly why LED and HID lights are the best options. Their low energy consumption allows them to shine at a low temperature while covering a large surface area.

  • Adjust the Light Placement When Necessary

Once your light fixtures are in place, you must closely monitor and adjust them when necessary. You’ll have to move them as your plants get bigger, but you must also consider the season and weather conditions. Here are some general guidelines for how far away your lights should be:


  • 100-200 watts: 8-12 inches away
  • 200-399 watts: 12-20 inches away
  • 400-599 watts: 20-30 inches away
  • 600+ watts: 30+ inches away


You can experiment with various distances and record each plant’s response for the best growth results. When a plant looks like it’s trying to stretch toward the light, bring it closer. If it appears scrunched up and shows abrasions, move the light farther away. It could take months to discover the most ideal distance, but gardening is slow work.

  • Utilize Shading Devices

One effective way to control your greenhouse’s light intensity is by using shading devices. Climate screens are excellent choices because they reflect and diffuse sunlight to keep greenhouses cool and prevent overheating. They’re also available in different light transmission levels, allowing you to control light exposure down to the exact percentage.


Similarly, greenhouse shading paint is becoming an increasingly popular choice. This method can be optimal for eliminating light pollution from external sources in communities that are heavily lit at night. Black paint can be used to absorb extra heat, while white can be used to reflect it away as needed.

  • Create a Lighting Schedule

Now that all your light fixtures and shading solutions are taken care of, you need to create a consistent lighting schedule. Even the most sun-loving plants need a break every day. You should only provide eight to 10 hours of full-spectrum light daily and make incremental changes based on the plants’ response.


For example, you might have to increase the full-spectrum light exposure time in winter when days are shorter. Make a point of mimicking the amount of natural sunlight each day, but always err on the side of caution. Keeping your plants in the dark for an extra hour is better than overheating them

  • Change the Lights as Your Plants Grow

As your plants grow and mature, they develop different environmental preferences. You might have to change your grow lights to accommodate these needs.


For example, if you invested in high-pressure sodium bulbs with red light, you’ll eventually have to swap them out for full-spectrum bulbs or metal halide bulbs for that extra blue light. Remember — plants prefer more blue light when they’re young, while fully mature plants prefer red light that helps with flowering and seed production. 

Take Full Control of Your Greenhouse

Light is one of the trickiest parts of gardening. When you think you’ve figured out the ideal environment for your plants, they start drooping or showing burn marks. You won’t have to deal with these problems anymore. With the above light-intensity tips and tricks, you can take complete control of your greenhouse and unlock its full potential.

Author Bio

Jack Shaw is a freelance writer specializing in home improvement, gardening, and caring for the outdoors. He’s the senior writer of Modded.com, and has contributed his advice through sites like CAD Details, House 2 Home Organizing, Log Cabin Hub, and more.