Screens, Screens, Marvelous Things!

A screen around plants

There are few things in life as insidious as plant-eating insects. These little monsters sneak into the greenhouse when you aren’t looking and before you know it, a full scale invasion has begun! You spray and spray, but hordes of them just keep pouring in—what is a greenhouse keeper to do? Although they’re rarely standard equipment on a hobby greenhouse, screens are an invaluable tool in the battle against bug build-up.

During the day, you’re probably ventilating your greenhouse like a good little gardener, running your fan on the hot days and propping every window and door open on cooler days. You’ve got it down to a science, but every year, you’ve got so many bugs that it almost makes you want to give up. Well, that’s because your windows and doors aren’t screened. It may seem too simple, but bugs are everywhere and they often just kind of whiz about randomly—sometimes they get lucky and fly into places where food is abundant, like your greenhouse.

How to choose a screen for your greenhouse

When you get to the hardware store, you’re going to find a remarkable assortment of screens with a range of opening sizes. The same goes for any greenhouse catalog you might dive into—a million screens to choose from, but no explanation of what they do. Obviously, they screen things out, but with a variety of sizes, some screen out more than others.

It’s helpful to think back to the problem insects you’ve had in your greenhouse and garden before you select your screen, since tighter meshes tend to be more expensive. If you’ve never seen one thrip in your garden, there’s no reason to assume they’re coming for dinner any time soon. Another important note while you’re in the screen aisle: adding screens to your greenhouse will cut down on air flow, so be prepared to enlarge your ventilation openings to compensate. How much will depend highly on your greenhouse and the screen you choose, but the tighter the mesh, the more restrictive to air flow.

Common screen sizes for hobby greenhouse owners and the bugs they keep out

40 mesh – This is the absolute biggest mesh that can be of use in your greenhouse. The screen holes measure a bit over one-fifth of an inch (640 microns), perfect for fending off leafminers, moths, wasps and other large insects. The 40 mesh screens will be among the cheapest options, but if you’re concerned about any pest smaller than a fly, keep shopping.

52 mesh – With an opening of just under one-fifth an inch (462 microns), 52 mesh screens are a good option if whiteflies are plaguing your greenhouse or you want to keep beneficials screened inside (more on that later). This is the larger of the middling sizes, and as with 40 mesh, don’t allow yourself to be tempted by a lower price point. If you have to redo your screens because the first round wasn’t tight enough, you’ll end up paying twice as much in the end.

78 mesh – Aphids are wily buggers that can squeeze through a tighter hole. Choose the roughly one-tenth of an inch (340 microns) opening of 78 mesh screen when aphids are multiplying rapidly in your garden. The 78 is a good all-purpose size for most pests, but there are still smaller sizes if you’re dealing with smaller pests.

132 mesh – Practically speaking, this is the smallest size of screen for a home greenhouse. You can get smaller, but the errant bug doesn’t make a lot of impact in a small greenhouse where individual plants are frequently checked for pests and disease. The 132 mesh has an opening well under one-tenth of an inch (192 microns), perfect for keeping flower thrips out of your plants.

What about beneficial insects?

Many growers have taken greenhouse pest control to a whole new level by introducing beneficial insects into the mix. They’re touchy, though, and you have to keep your greenhouse conditions habitable all the time if you want your population to grow naturally. The other thing you absolutely must have are screens. Many beneficials are larger than their prey, but they will fly the coop as soon as pest levels drop if you’ve not got tight screens installed.

Depending on the beneficial you’re keeping, aim for something in the middling range. You can use a much bigger screen if you’ve got a group of beneficials on patrol all the time in your greenhouse, just as long as the mesh is small enough to keep them in. Choose a 52 mesh screen for most beneficial insects—you can go even bigger if you use large predators like mantids, just make sure the screen is keeping most of the other bugs outside.

Other possibilities with screen

There’s one thing I didn’t mention and should, because, let’s face it, sometimes we all have that one plant that’s just really bug eaten and we can’t bear to part with it. You can use your extra screen to build yourself a box that will fit around the plant and let your beneficial loose inside. Don’t leave them in there for long, though, or they’ll eat themselves out of house and home and starve. This is a fairly effective way to quarantine newcomers to the greenhouse, as well.

No matter what type of greenhouse you own or what’s growing inside, screens are an important part of good greenhouse management. After all, you can have the best seeds and medium on the block, but if your plants are terribly bug eaten, none of that will really matter.

Kristi Waterworth
image: greyman
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