Whether you need additional grow lights in your greenhouse depends on several factors. Greenhouses allow more natural sunlight to enter than indoor growing spaces, but the amount of light transmission depends on the glazing material, angle of the sun, time of year, and greenhouse structural components. Supplemental lighting may be needed to achieve optimal growth, especially in winter at higher latitudes. The crop types and your desired production schedule are also key considerations. This article explores when grow lights become necessary in greenhouses to support healthy plant growth.
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How Much Light Reaches Greenhouse Plants
The main benefit of a greenhouse is allowing sunlight to penetrate, unlike an indoor growing area. However, greenhouses do still block some of the sun’s rays. The percentage of total solar radiation that passes through the greenhouse covering is known as the light transmission coefficient.
| Glazing Material | Light Transmission Coefficient |
|---|---|
| Glass | 90% |
| Polycarbonate | 80% |
| Polyethylene Film | 80-90% |
| Fiberglass | 80% |
As shown in the table, glass has the highest light transmission, allowing about 90% of sunlight to pass into the greenhouse. Plastics like polycarbonate and polyethylene films allow 80-90% light transmission. Fiberglass is generally around 80%. So even with high transmission glazing like glass, some natural sunlight is still filtered out.
The angle of the sun is another factor determining light levels. As the sun moves across the sky, the intensity of direct beams entering the greenhouse changes. At sunrise and sunset, light becomes more indirect and weaker. Of course, the amount of daylight also varies significantly by season, with fewer hours and lower angles in winter.
Greenhouse structural elements like support beams, pipes, hanging baskets, and internal walls can also block some incoming light, creating shadows. The shape and orientation of the structure impacts sunlight capture as well. Curved walls and roofs allow better light diffusion than straight walls. An east-west orientation maximizes exposure when the sun is most overhead.
Light Requirements for Plant Growth
Both the intensity and duration of light affect plant growth processes like photosynthesis, flowering, and fruiting. Insufficient sunlight can result in spindly, weak plants with poor yields. Most plants need a minimum of 10-12 hours under lights of adequate intensity to thrive. Intensity is measured in lux, foot-candles, or PAR (photosynthetically active radiation). Different crops have varying optimal light levels:
| Plant Type | Minimum Light Intensity |
|---|---|
| Leafy greens, herbs | 5000 lux / 500 fc / 200-500 PAR |
| Fruiting plants | 7500 lux / 750 fc / 400-600 PAR |
| Flowering plants | 10,000 lux / 1000 fc / 600-800 PAR |
As a guideline, most plants require at least 10,000-15,000 lux of light for proper growth. Getting sufficient intensity usually necessitates supplemental lighting even in greenhouses.
Seasonal Changes in Natural Light
The large seasonal shifts in daylight hours and sun angles mean greenhouses typically receive adequate natural light in the summer but not winter. Exact numbers vary by location and greenhouse specifications, but here are some typical figures:
| Location | Summer Solstice Day Length | Winter Solstice Day Length |
|---|---|---|
| Chicago | 15 hours | 9 hours |
| Los Angeles | 14 hours | 10 hours |
| Miami | 14 hours | 11 hours |
The summer solstice has the year’s longest days, while the winter solstice has the shortest. Even in warm southern locations like Miami, the winter day length is 25% less than summer. With fewer daylight hours and lower sun angles, winter light levels are likely too low without supplementation, especially at higher latitudes.
Greenhouse Location and Glazing
Greenhouses positioned at more northern or southern latitudes face much bigger light variation across seasons compared to equatorial regions. For example, a greenhouse in Canada may receive 16 hours of daylight in summer but only 8 hours in winter. The same greenhouse in Costa Rica would have a relatively consistent 12-13 hours of sun all year.
Double poly greenhouses use an inflated layer of greenhouse plastic film and maintain higher interior temperatures during winter. But the second layer cuts down on light transmission, making supplemental lighting more critical. Single layer poly and glass greenhouses allow more light for less energy, but also lose more heat in winter.
When to Use Grow Lights in a Greenhouse
Winter Months
Greenhouse grow lights are most likely needed from late fall through early spring when natural light is most limited. While equatorial greenhouses may still get 12 hours of winter sun, intensities are lower. At higher latitudes, both the duration and intensity of winter light are insufficient for many plants, making supplemental lighting essential.
Cool weather crops that tolerate lower light levels, like lettuces, kale, and chard, may grow fine without added lights. But fruiting vegetables, flowering plants, and heat-loving crops will require assistive lighting over winter. Leafy greens may become leggy and lower-yielding without enough light intensity.
Some greenhouses rely entirely on natural light year-round. But most operations use supplemental lighting from late fall until early spring to maintain plant growth. The exact months depend on location and crop selection. Greenhouses farther from the equator generally need a longer supplemental lighting period.
Cloudy Periods
Consistent gray overcast days can reduce the light in a greenhouse below optimal levels even in spring and fall. While duration may be sufficient on cloudy days, intensity is often too low for adequate growth, especially for fruiting plants. Adding grow lights on persistently dark days can help maintain higher intensity for better plant health and yields.
Occasional one-off cloudy days don’t necessarily require supplemental lighting. But if gloomy weather persists for a week or longer, the reduced sunlight will slow photosynthesis. Flowering and fruit production may also be impaired. Turning on grow lights for a few hours midday can help compensate for stretches of low natural light due to heavy cloud cover.
Morning and Evening
Turning grow lights on in early morning and evening extends the effective photoperiod or number of “daylight” hours plants receive. Adding a few hours of supplemental lighting before dawn and after dusk mimics longer summer days. This technique can encourage flowering in some long-day plants and keep harvests higher year-round.
Photoperiod manipulation using supplemental lighting works well for crops that respond strongly to day length. Examples include African violets, chrysanthemums, poinsettias, and strawberries. Illuminating plants from 5-7am and 7-10pm can sustain fruit and flower production that would otherwise slow or stop in winter.
Year-Round Light Boost
Some greenhouse crops like tomatoes, cucumbers, and peppers need consistently high light levels, making year-round supplementation beneficial. While glass and poly coverings transmit over 80% of sunlight, the diffused light is rarely adequate on its own to maximize productivity.
Also, marginal weather like overcast stretches is common enough even during spring through fall that relying solely on natural light results in periodic slowdowns in growth and lower overall yields. Maintaining supplemental lighting daily provides optimal intensity and uniformity for the highest production.
Maximizing productivity may require supplementing natural sunlight throughout the year, especially for fruiting vegetables and flowering crops. Daily grow light operation with seasonal adjustments to durations can promote vigorous, healthy growth regardless of outdoor conditions.
Supplemental Greenhouse Lighting Options
There are several types of electric grow lights used in greenhouses to deliver extra intensity or extend photoperiods. Key factors in choosing lighting include power draw, light intensity, fixture costs, lifespan, and light spectrum.
| Light Type | Intensity | Power Use | Fixture Cost | Lifespan |
|---|---|---|---|---|
| LED | Very High | Low | High | 50,000+ hours |
| High-Pressure Sodium | High | High | Low | 10,000-24,000 hours |
| Fluorescent | Medium | Medium | Medium | 10,000-20,000 hours |
| Metal Halide | Medium-High | High | Low | 6,000-20,000 hours |
LED fixtures provide the most intensity for the least electricity and have the longest service life, making them the most popular choice. High-pressure sodium lights are also quite intense but use more energy. Fluorescents and metal halides offer good mid-range light output.
Different electric lights also have varying spectral outputs, which can stimulate specific plant responses. Blue and red wavelengths are most critical for photosynthesis and other plant processes. Full-spectrum white LEDs or a combination of red and blue LEDs work well for general growth.
Photoperiod and Light Intensity
The hours per day and light intensity of supplemental lighting should match the greenhouse crop’s needs. Most plants grow best under 14-18 hours of light. Light-loving fruiting plants usually require at least 16 hours of light. Intensity should provide 500-1000 μmol/m2/s or 10,000-20,000 lux for most crops.
LED lights can be controlled with timers and dimmers to provide customizable photoperiods and intensity. Running lights at 100% power for 16 hours and at 50% power for 2 hours before and after can extend the day length while saving energy.
Hanging Lights vs Top Lighting
Supplemental greenhouse lights can be mounted above the plants or hung between rows. Top-mounted fixtures provide the most uniform light coverage across the whole growing area. Hanging lights channel light more directionally to the rows beneath and in between them.
Combining top and inter-row lighting helps ensure all plants get adequate illumination, especially helpful for taller crops. The layered lighting also reduces shadows from overhead hanging components like pipes. Top lighting takes care of upper leaves while hanging lights directly light lower leaves.
Automation
Automating supplemental lighting offers several advantages over manual operation. Timer or controller systems can turn lights on and off consistently at set times daily. Light sensors allow automatic adjustments to fixture output based on changing sunlight levels. These features remove the need for daily human intervention.
Automated systems can ramp lights up or down gradually to mimic natural light transitions. Gradual changes prevent stress on plants. centralized, web-based controls enable monitoring and adjustments to the lighting schedule remotely. This automation provides optimal supplemental lighting for maximum plant growth with minimal effort.
Conclusion
While greenhouses allow helpful sunlight penetration, supplemental lighting is usually needed, especially at higher latitudes and during winter. Cool weather crops may grow adequately under only natural light but most plants require added light intensity for best growth and yields. LED fixtures are the most electricity-efficient and long-lasting option that can be controlled for customized lighting recipes per crop. Automating greenhouse grow lights ensures optimal, uniform illumination with minimal labor to boost productivity year-round.