Blogs Dec 26, 2019

Greenhouse Design: Top 4 Lighting Considerations

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How is it that my greenhouse design only allows 50-65% of sunlight to make it to the plant? How come my greenhouse manufacturer said I’d be getting much more? This was a question posed to us by a recent greenhouse grower, and he has a good point:

They asked, “What could I have done differently?  Using as much sunlight as possible is the whole reason I built a greenhouse in the first place.  WHAT SHOULD I DO NOW?”

The optimistic answer is that there’s actually a lot you can do!  This article is our response to this unfortunately common problem.

Wouldn’t it be great to use as much free sunlight as possible?  In greenhouse growing, this should really just be called “getting the job done right”, and we’re here to help with the heavy-lifting.  In this post we’ll look at the top 4 lighting considerations that can have a huge effect when designing or retrofitting your greenhouse:

  • Site Selection
  • Glazing
  • Form Function
  • Supplemental Lighting

Let’s get started.

Select a site for the highest year-round light levels

Choosing the wrong site for your greenhouse can drop your available sunlight significantly.  The ideal site for your greenhouse will ensure consistent exposure to light year-round, most critically in winter when the angle of the sun is lower.

That means orienting your greenhouse appropriately for your latitude while minimizing shading from surrounding features.

Quick Guide to Selecting Your Greenhouse Design Site

  1. Gutter-connected greenhouses should always be oriented north/south, regardless of latitude, to minimize shading from the multitude of these structures’ trusses.
  2. Free-standing greenhouses (structures with a single bay) located above latitude 40°N should be oriented east/west to allow the maximum light available to enter from the sides. This keeps the low-angled winter light from being blocked by the greenhouse structure.
  3. If your free-standing greenhouse will be located below latitude 40°N, a north/south orientation will let the sun travel over the greenhouse and provide more light throughout the year while minimizing shading from trusses.
  4. If possible, orient your greenhouse on a slight southerly slope (1-2%) to further increase light exposure while also providing helpful drainage.
greenhouse-gutter-design
Gutter-connected greenhouses have multiple gable or gothic arches connected together at the gutter-level. This style of greenhouse is best suited for large installations and budgets.
freestanding-greenhouse-design
Free-standing greenhouses, also known as hoop houses are less expensive to build but are more difficult to install utilities and more expensive to heat.

Quick Guide to Minimize Your Shading

Surrounding features can block light to the greenhouse differently throughout the year depending on the angle of the sun.

  1. To avoid shading from trees and topographical features like hillsides, position your greenhouse at a distance 2.5 times the height of any trees or features. Any closer and these features will shade your greenhouse to some degree during in the year.
  2. If you have tall hedges or trees around an existing greenhouse that can’t be removed, keeping them pruned to allow light to enter between and above them into the greenhouse will add to your overall light transmission rates.
  3. Remember that any structure will shade its surroundings, including storage areas, delivery and parking areas that will have a lot of large truck traffic, office buildings and other growing structures like shade and hoop houses, so it’s important to consider how that shade will fall on your greenhouse.
graph-showing-tree-by-greenhouse
Ratio of shadow length and obstruction height for selected solar altitudes. (Walker, J. N., & Duncan, G. A.)

Often there is a compromise between maximizing light with the ideal greenhouse orientation and avoiding infrastructure obstructions. For example, when pre-existing buildings or hillsides limit open areas for greenhouse construction.

You’ll need to weigh the effects of shading and orientation to determine which will allow the highest light transmission possible.

Choose a greenhouse glazing that transmits the best light for your crops

Greenhouse glazings have different light transmission rates, from 91% down to 75%, which determine the amount of light that will enter the greenhouse.

Your greenhouse glazing should allow the appropriate light transmission rate and type for your crop throughout the lifespan of the glazing. These light transmission rates should be readily available for all glazings from the manufacturer.

The type of glazing, whether diffuse or direct, and partial or full spectrum, will determine the type of light that enters the greenhouse. As with site selection and other design elements, there is often a compromise when choosing glazing, since cost, insulating ability, maintenance and lifespan vary among the different types.  There are some general aspects of glazings that will maximize light and minimize shading.

LED Grow Light Scientist
Insulating ability of glazing and R-values

Every glazing type has an R-value that indicates how well it insulates against heat loss. The higher the R-value, the greater the insulating value.

When choosing a glazing, consider the crop you’re growing, your year-round climate and the time of year to decide on the transmission rate and insulating ability you need. There is often a compromise on glazing type to allow maximum light transmission while ensuring adequate insulation.

When insulating value compromises light transmission, supplemental light can be added to ensure ideal levels of light in a greenhouse.

Maximizing light with greenhouse glazing

Producing crops in a greenhouse rather than in the open field means losing some percentage of light to the glazing. Understanding the light levels at your latitude and what light level your crop requires can help you determine the glazing that’s ideal for your crop.

If you’re growing a high-light crop, like tomatoes, you’ll want to prioritize glazings with a higher light transmission rate. If you’re growing orchids, or another low-light crop, you can focus your choices on glazings with lower light transmission rates.

Below are a few of the most common glazing types and their light transmission rates when new.

glazing-transmission
Detail on transmission rates, cost range and lifespan (Ball Redbook Greenhouse and Equipment, 18th Edition)

Minimizing shading with greenhouse glazing

Glazing can transmit direct or diffuse light.  This means that light either passes directly through the glazing and onto the crop below, like with single-pane glass. Or that it passes through the glazing and is diffused, or scattered, in many directions onto the crop below, like with dual-wall polycarbonate, for example.

Direct light is bright, with the majority of its intensity hitting the top of the crop and creating heavy shading on the lower canopy. Direct light also casts dark shadows from greenhouse infrastructure onto the crop below. Imagine that you’re standing outside on a clear sunny day in summer and how intensely bright the light is and how dark your shadow is cast on the ground below.

Diffuse light is still bright, but because the light scatters when it transmits through the multiple walls of glazing, the light is distributed throughout the canopy which results in less shading from crop canopy as well as overhead infrastructure. This is like standing outside on a bright but overcast day in summer. You don’t cast much of a shadow on the ground, but you need to squint because the light is still quite bright.

Regardless of your glazing transmission rate and type, it’s important to keep up with its regular maintenance and replacement to ensure you’re getting the most light possible by minimizing the shading effects of dirt and dust that accumulate over time.

diffusion-graphic
Graphic showing direct light (left) versus diffused light (right)

Keep greenhouse glazing clean

All glazings will need annual cleaning to minimize the shading effect of organic debris like dust, dirt and algae that build up in a short period of time. If you plan to apply shade paint to your greenhouse during high-light periods of the year in order to manage temperatures inside your greenhouse, this shade paint will need to be thoroughly removed before light levels drop in early fall.

Glazings that become etched or yellowed with time, like polycarbonate, will gradually lose light transmission quality unless they are coated with UV protection or replaced regularly. As with all glazings, it’s important to understand what the crop development effects are from any light spectra that are eliminated with the coating.

LED Grow Light Scientist
A little about UV coatings

UV coatings are helpful for preventing yellowing of polycarbonate and other glazing types by preventing the degradation of the material from UV light.

However, UV light is one of the wavebands that triggers anthocyanin production in plants, so without it or other high energy wavebands like blue light, red plant varieties will remain green unless supplemental light of the right light quality is provided.

Consider whether you’ll be growing crops reliant on high energy wavebands for morphological traits like coloration when evaluating UV coatings for your greenhouse. You can use supplemental blue light to trigger coloration in crops if a UV coating is chosen for the greenhouse glazing.]

Use infrastructure that’s functional and streamlined

Greenhouse managers should assess how greenhouse infrastructure, the greenhouse frame and equipment, affect light quality and intensity while providing the right growing environment for the crop.

Building a greenhouse to maximize light

The less infrastructure you have, the more light you’ll get to your crops because you won’t have as many obstacles blocking light to your plants.

One way to maximize light is by choosing a simple structure like a freestanding greenhouse with lightweight glazing like poly film that doesn’t require heavy framing to support.

But what if you need more substantial infrastructure for a commercial production environment?

When designing a gutter-connected greenhouse, you can streamline its substantial framing by choosing a lightweight glazing like polycarbonate that won’t require as much structural support as would heavy glass glazing.

Greenhouses with more substantial framing also allow for the installation of supplemental lighting, which make up for shading resulting from the additional infrastructure.

Building a greenhouse to minimize shading

This is where you limit the amount of overhead infrastructure and surrounding equipment inside your greenhouse to that which is essential to produce your crop efficiently.

Blackout and thermal curtains will shade segments of your crop even when they’re retracted due to the cabling, motors and retracted fabric overhead. Of course, these are valuable systems that perform necessary tasks like cooling, photoperiod control and heat retention, so evaluate the tradeoff and impact of the light loss when choosing curtain systems and focus on streamlined systems that tuck away when not in use.

Irrigation booms and sprinkler systems will also block light to your crop below, so it’s important that they’re regularly maintained and able to be retracted to positions that don’t block light to the crop when not in use. Overhead electrical and irrigation lines can also run above walkways to minimize shading over growing areas.

Add supplemental light to your greenhouse

Greenhouse production has advantages over field production, including the ability to grow year-round regardless of weather, control ambient light conditions, and stabilize plant growth cycles when you effectively manage your lighting strategy.

Adding supplemental light is often the only way to reach consistent light levels year-round.  Still, there are considerations to make when choosing a light fixture to maximize the power of the sun for your greenhouse.

Maximize sunlight with smart lighting

Not all light fixtures will maximize your use of free natural sunlight for your crops.  The benefits of the right smart lighting solution can ensure that you’re using as much sunlight as possible and not spending more than you need to on supplemental.

In the case of the LumiGrow Light Sensor, your LED fixtures are triggered to dim or turn on or off so that your solar and supplemental light combine to hit your crop’s target.

When using the LumiGrow Light Sensor for your LumiGrow fixtures, the daily light integral (DLI) program allows you to add only the amount of light necessary to reach your crop’s DLI needs that ambient light doesn’t supply.  This means you save on energy costs as the lights automatically adjust when the sun is supplying adequate DLI to your crop.

DLI vs. PPFD supplemental light programs

Photosynthetic Photon Flux Density or PPFD (measured in μmol·m−2·s−1), is the most common supplemental lighting control method.  Adjusting your light fixtures to PPFD means that the lights will adjust instantaneously with the current solar conditions in the greenhouse to maintain a specific PPFD.  Although this adjustment method may be desired for research facilities, it is not optimal for commercial production because in many cases you may over-light or under-light your crop.  Also, when PPFD control is paired with HPS lighting, as many growers today do, the long cooling periods of HPS make consistent light targets almost impossible to achieve.

Daily light integral (light accumulation in 24 hours), measured in mol·m−2·d−1, is now possible with smart lighting.  Using the LumiGrow Light Sensor you can achieve consistent amounts of light every day of the year for optimal commercial production at the lowest energy cost.

Both programs are available using smart lighting control with the smartPARTM Wireless Control System.

Minimize shading with supplemental lighting

When adding supplemental lighting or other light management infrastructure like blackout curtains, the key to minimizing shading is to focus on the form factor of the equipment.

Streamlined light fixtures that attach to existing or new trusses will result in a negligible amount of additional shading. Fixtures that include large ballasts and bulky reflectors will shade the crops below. The ideal light fixture will have a form factor that follows the trusses of the greenhouse so that it doesn’t contribute additional shading.

When making choices for greenhouse design or retrofit, keep in mind how light levels will be affected in the growing areas and how that can affect your production. Determine the best options to ensure that your structure can produce healthy crops and a successful business.

Contact LumiGrow for a Lighting Quote Today!
TopLightTM
Designed for Greenhouse & High Bay Applications
BarLightTM
Designed for High Performance Vertical Applications
smartPARTM
Wireless Grow Light Control