How Much Lighting Do I Need In My Greenhouse?

Remember that the goal of grow lights is to mimic the sun! 

Not all greenhouses will need lighting, there are many plants that mature without cues from light, and many locations get plenty of natural light. However, there are many greenhouse lighting factors to consider. The first step to understanding your lighting needs is to read up on your crops and local climate data. This Sunrise/Sunset Calculator can also help you plan your crops around their sun exposure needs, and give you an idea of what time of year may lack the necessary natural sunlight to support crop growth. 

Greenhouse Lighting Factors

Your crop type and climate will have the biggest impact on how much lighting you need in your greenhouse. In addition to these factors, you will want to consider the effects on lighting from the shape of your greenhouse and your greenhouse plastic.

Greenhouse Lighting Factors: Crop Type

  • Long-day plants (carrots, kale, lettuce, spinach)
  • Photoperiodism neutral plants (auto-flower strains, cabbage, peas, tomato)
  • Short-day plants (the plant, chrysanthemum, poinsettias)

Greenhouse Lighting Factors: Climate

  • Latitude and longitude (length of day and seasons)
  • Cloud cover
  • Year-round vs seasonal growing

Greenhouse Lighting Recommendations

Hortitech's Senior Grow Expert Jerad Lauzier advises that a good place to start with lighting is to plan for one 1000 watt HPS grow light per 64 square feet of space. This should be sufficient for an 8ft x 8ft area, and multiplied to determine what will be a good starting point for your particular space. Jerad also notes that if only supplemental lighting is needed, you can use one HPS lamp per 10ft x 10ft area (100sqft). In climates with less winter sun, more lighting may be needed for year-round crops. You will want to consider the DLI for your location.

When Do You Need Supplemental Lighting?

  • Cloudy weather is common or frequent
  • Crops will be grown through winter when there is less natural light
  • Crops are not getting enough natural light
  • Used to increase daylight integrals (DLI)

Supplemental lighting needs to be the type of light that a plant can use to make energy. This includes metal halide lamps, high-pressure sodium lamps (HPS), or LED plant lights. The goal is to help the plants grow when nature is not giving them enough of what they need. Supplemental lighting can be used with long-day plants, short-day plants, and neutral plants. Supplemental lighting calculators can be used to help guide your lighting decisions, but the greenhouse glazing, structure, crops, and climate will also play a role.

When Do You Need Night Interruption Lighting?

Night interruption lighting is mostly needed for long-day plants. In some cases, it may be used to help keep short-day plants in vegetative growth. This lighting technique is more economical than lighting the greenhouse to extend the day. By interrupting a period of darkness, it makes the plant think that the night length is shorter. Many people utilize cyclic lighting routines and timers to achieve this.

When Do You Need Daylight Extension Lighting?

Unlike supplemental lighting, night interruption lighting, and daylight extension lighting does not need to be photosynthetically active. Even string lights can be used! Just be aware that only metal halide, HPS, and LED lights can emit light that plants can use for energy. The goal of daylight extension lighting is to accelerate flowering in long-day plants or keep short-day plants in a vegetative growth state.

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