Also called energy curtains, heat curtains, or thermal screens, shade cloth can help control temperature, boost humidity, and filter sunlight in a greenhouse.
That’s a lot of impressive work for what is basically a piece of fabric!
By using shade cloth in your greenhouse, you can grow species that might otherwise struggle in your climate and protect tender plants during extreme weather events.
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With the climate becoming more unpredictable, most of us could probably use some shade cloth now and then.
Whether you are experiencing hotter springs that you did in decades prior or you have surprise heat waves during the summer, it’s going to come in handy.
In our guide to greenhouse gardening, we provide all the information you need to get started with this gardening method.
If you’d like to learn more about how to put thermal screens to work in your greenhouse, here’s what you can expect in this guide:
What You’ll Learn
What Is Shade Cloth?
Shade cloth is a woven or knitted fabric used to protect greenhouse plants from excessive sunlight.
The material diffuses light, creating an effect similar to leaves filtering sunlight to the plants below.
Available in various shapes, sizes, and materials, you can choose the fabric that best fits your specific needs.
Most options are made from polyethylene or polypropylene, or blends, though you’ll also find materials like vinyl-coated cotton.
Aluminet reflective metalized high-density polyethylene (HDPE) incorporates aluminum to reflect light and keep plants cool. It’s ideal for species that should remain cool.
Shade cloth comes in multiple colors, including brown, white, black, and green. Black and green absorbs sunlight and filters a small amount through, creating dappled sunlight similar to what you’d find beneath a tree.
White or light-colored cloth reflects some sunlight while allowing more to pass through, helping to create a cooler environment that still promotes healthy plant growth.
Dark fabric might hinder the growth of some sun-loving plants, but is ideal for species that require shade.
It also allows heat to build up, which plants like peppers, grapes, and tomatoes will enjoy – use a low density black cloth, for these crops.
Red HDPE cloth filters out specific light wavelengths and works well for many tropical plants like philodendrons and orchids.
According to Virginia Tech researcher Shawn Appling, the use of red cloth improved the size of basil and parsley plants.
Thermal screens will have a density percentage, ranging from five to 90 percent. This percentage refers to the amount of UV light that is blocked by the material.
Depending on the time of year, temperature, and species you need to protect, you can vary the color and density to accommodate your plants.
For example, blueberries might benefit from a white 35 percent cloth in Maine during the summer, or a black 50 percent cloth in Virginia during the summer.
In a greenhouse, a black or green 70 percent cloth will protect hostas, cast iron plants, and pothos.
As a general guideline, northern climates typically need 30 percent density, most vegetables require around 50 percent, succulents perform best with 60 percent, and shade-loving plants need 70 percent or higher.
Benefits of Thermal Screens
You can use thermal screens with any species, even sun lovers like tomatoes and peppers.
It’s effective for protecting tender seedlings and can cool the area underneath during the hottest periods of the year or surprise heatwaves.
The cloth also helps reduce evaporation for plants that need plenty of moisture, both while starting and after they’re established.
The use of shade cloth also enables you to grow species that might not be suitable for your climate because of high temperatures.
For example, if you’re growing lettuce in your greenhouse during the spring and along comes a heatwave, with temperatures suddenly in the 90s.
Your lettuce will likely bolt, even if you’re running fans within the structure. But the addition of a fabric barrier can help block some of that sun, reducing the heat reaching your lettuce.
Or perhaps you’re propagating philodendron cuttings in the northern section of your greenhouse and beans on the south side. These plants need very different light exposure, so you’ll need thermal screening to protect your philodendrons.
Shade cloth can provide temporary protection during heat waves or extend the cooler growing season for plants like lettuce and peas in spring and fall.
While sunlight increases beneficial compounds like anthocyanins and flavonoids in plants, heat can cause these nutrients to degrade.
Thermal screens help you grow healthier, better-tasting vegetables and can result in stronger plants that produce larger, more abundant fruits.
They can also be used to increase humidity levels. A loose weave allows air circulation while a dense weave helps retain moisture.
Where to Buy
Most nurseries and home or garden supply stores carry some type of shade cloth.
You might find basic fabric sold by the yard, or cloth designed for specific greenhouse brands that comes with fasteners.
You can find products with pre-made fastening holes, or buy the fabric and add grommets yourself.
While this second method requires more work, it lets you place fasteners exactly where you need them and saves money.
For example, you could grab a six- by 15-foot piece of brown, blue, green, or gray polyethylene cloth with a 70 percent density at Shelter Logic.
Then, find some DIY grommets for making holes where you need them.
You can also find these at Shelter Logic.
Finally, you’ll need some ties to feed through the holes and affix the fabric to the greenhouse structure. You guessed it, Shelter Logic carries sturdy black tie wraps.
Or if you’re securing the thermal screen to wood, you can skip the grommets and ties and just use wood fasteners, which you can also find at Shelter Logic.
You hammer these onto the top of the fabric so the claws on the fasteners grip the wood underneath.
I love this latter option because they’re strong, reliable, and you can put the fasteners anywhere you need them.
If you have thin wires or metal bars running through your greenhouse, you can either use the grommet and ties or use Shelter Logic’s snap clips.
These clip around a wire or rope and then clip onto the fabric.
Pre-cut polyester shade cloth with pre-set grommets costs more but is ready to use straight from the package.
The only drawback is if the grommets don’t align where you need to attach the fabric to your greenhouse.
If you bought your greenhouse rather than constructing it yourself, you can often find pre-made thermal screens that are sized specifically for the structure and its support.
There are even automated systems that will retract and extend the thermal screens based on a pre-set schedule.
Using Shade Cloth in the Greenhouse
I know gardeners who drape thermal screens over their small greenhouses, and I’ve done something similar with my “pepper palace” – an arbor covered in heavy plastic to create a greenhouse designed to increase heat around my peppers.
One year, temperatures jumped from the high 80s to 116°F in just a few days, a record for my area.
Even my chilis couldn’t handle that heat, so I draped some fabric over the “pepper palace” to protect the plants.
You can use shade cloth on the exterior of a greenhouse in a pinch, but it will last longer and is less likely to be torn off by strong winds if you install it inside.
Plus, if it is install on the outside, it can interfere with windows or vents.
If you opt to use it on the outside, you can secure the fabric to the ground using ropes and stakes, or attach it to the structure with wood grips.
If the structure has areas that can accommodate grommets, you can also use ropes or zip ties.
The cloth should be fastened about every three feet to keep it secure. Only the roof and down to about five feet above the ground needs to be covered.
Anything lower is below the sun’s angle and won’t receive direct sunlight except during the winter in extreme northern or southern latitudes.
Let the angle of the sun in your area guide you. Of course, if your greenhouse has hip boards or opaque walls, you don’t need to cover these areas.
Inside the greenhouse, you can secure it to the roof or just above the plants you want to protect. There’s no minimum height for shade cloth, but avoid letting it touch your plants.
If it’s too close, it can reduce air circulation around them. If you keep the fabric flat, be aware that water might accumulate on dense fabric. Position the fabric at a slight angle to encourage drainage.
To provide temporary shade while allowing full midday sun to reach plants, use fabric as sidewall curtains.
You can further reduce greenhouse heat by wetting the fabric regularly.
The American Society of Agricultural and Biological Engineers in St. Joseph, Michigan, published a study showing that wetting the fabric in a greenhouse produces a significant temperature reduction compared to leaving it dry.
Throw Some Shade
For a simple piece of fabric, shade cloth offers remarkable versatility.
Depending on how you use it and what density and color you choose, you have numerous options to adjust your greenhouse environment.
Are you using shade cloth in your greenhouse? Let us know in the comments section below how it works for you!
And for more information about greenhouse gardening, check out these guides next:
