Four Tips for Pest Control on Green Roofs

03/14/2019 | By Jennie Morton

Your green roof is an incredible engineering feat that provides insulation and stormwater management, but could its lush greenery be putting you at risk for an invasion of pests?

Vegetated roofs are often touted as a way to encourage biodiversity, but that isn’t limited to the plant selection. Whether your roof has low grass, flowers, shrubs, or food gardens, insects and animals will inevitably be attracted to the foliage.

Integrated pest management can help you stay ahead of pest activity and ensure any infestations are treated in an environmentally sensitive manner. Follow these tips to ensure your roof will thrive without becoming overrun with wildlife.

Rooftop pests for vegetated roofing

1. Understand which pests frequent vegetated roofs.

Animals need three things to survive – food, water, and shelter – so it’s no surprise that a manicured green roof can provide these in abundance.

“Green roofs are living architecture,” explains Pat Copps, a board certified entomologist and technical services manager for Orkin. “As soon as you start putting plants up there, pests will start moving in.”

Pests run the gamut, from those that are attracted to ornamental plants to ones that thrive in urban areas:

  • Insects such as ants (fire or carpenter), spiders, hornets and wasps, and silverfish are typically the biggest group of unwanted guests.
     
  • Birds, mice and rats, and even nocturnal creatures such as raccoons and possums may become an issue.
     
  • Mosquitoes can create a problem, particularly if there is ponding water for them to lay eggs in.
     
  • Large cockroach species often thrive in heavily mulched areas,” notes Judy Black, a board certified entomologist for the pest management firm Steritech. “Occasional invaders such as earwigs may also be brought in on potting soil, and ground beetles may be initially attracted to lights at night and then take up residence in soil and mulch.”
     
  • Termites are less common but not unheard of, adds Paul Curtis, a board certified entomologist and entomology and regulatory services with Terminix Commercial.

Because your roof is designed to be a habitat, keep in mind that one pest problem usually encourages another. For example, birds may become more attracted to the area because there are better nesting opportunities and a greater amount of insects to eat. In turn, feathered friends may be carrying mites that will then establish a population on your roof. It’s the circle of life, but it can be managed so it doesn’t get out of hand.

[Related: 4 Tips for Preventing Springtime Pests]

2. Recognize the risks to your building and occupants.

A pest infestation on your roof is a three-fold problem that can cause structural issues, spread to other areas of your building, and scare off occupants.

“One of the main concerns is that the pests will make their way from outside and into workspaces below,” says Black.

Not only could this be problematic as you try to keep pests from expanding into new areas, but it can also impact your business operations. Imagine the issue of having insects invade healthcare facilities or laboratory and manufacturing settings with clean rooms that must be absolutely sterile, notes Copps, much less the general cause for alarm when someone sees an ant or cockroach.

Mice and rats not only leave behind contaminated droppings, but they can chew and burrow straight into your building as they create nests, Copps says. They may gnaw through wiring, irrigation systems, the roof membrane, and other structural components, adds Curtis.

Spider webs and bird droppings can certainly decrease the aesthetics of a rooftop space, but a far greater concern is the possibility of insect bites or stings, says Curtis. Particularly for those who have allergies, these nibbles could cause a serious reaction that requires medical attention.

3. Look for signs of activity.

If you already have a pest inspection routine for your grounds, simply add your roof to the rotation. The same methods you use to look for insects and animals among landscaping can be applied to your vegetated roof.

“Look for obvious signs like droppings, nests, burrowing, footprints, and damage to plants,” advises Curtis. You can also ask users to reports any issues they see whenever they visit the roof, Black adds.

Because some roofs are intended to be self-sustaining once established, they often have a minimal maintenance schedule, notes Curtis. To avoid missing signs of pest activity, make it a habit to visit the roof more regularly than what is required to keep the vegetation in good health.

Inspection needs can vary depending on the type and size of your vegetated roof and climate factors. A roof with a lawn-like covering may only need to be inspected once a month, whereas one that’s landscaped like a park should be examined every two weeks, Copps says: “The more complex the roof is, the greater frequency you should have for inspections.”

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“If the issue has already pushed down into the building, often a pest management professional is the best option,” Black recommends. “They can utilize baits in a targeted way to reduce the populations to levels that can then be managed with non-chemical techniques. The important piece to keep in mind is to inspect regularly so that issues do not get so high that they need chemical intervention.”

4. Use integrated pest management.

Integrated pest management (IPM), which prioritizes prevention strategies over reactive ones, favors low-impact methods to address pests. This includes no-kill traps, sealing cracks in the envelope, anti-roosting devices, and the like. Consider that one of the benefits of a green roof is its ability to filter pollutants from rainwater – you don’t want to be adding more chemicals, stresses Curtis.

Pesticides can be used in limited applications when other methods haven’t addressed the issue.

[On topic: 5 Things to Consider When Turning Your Rooftop into Usable Space]

Beyond a good inspection program, be proactive about the landscaping, says Copps. Work with your roof provider to ensure plant selection is appropriate for your region and minimize species that attract common pests.

“Inspects plants and potting soil for pests before they are planted there,” Black recommends. “Keep mulch levels fresh and to a minimum.”

Curtis recalls a project where he happened to notice a colony of ants in the soil that was destined for the roof. He was able to head off the issue and new mulch was ordered, but the story goes to show that humans may be responsible for inducing a pest to a site.
“Keep the plants healthy,” advises Black. “If they become infested with aphids, for example, then ants will be especially attracted and establish colonies.” This includes typical strategies like trimming and pruning. Because vegetated roofs are typically modular, you may simply be able to replace the soil medium in an affected area if the issue is contained, adds Curtis.

You should also be mindful of overwatering. Ponding water is an ideal place for mosquitoes to lay eggs and overly moist soil can be attractive to a variety of insects. Resolving this issue is as simple as keeping irrigation levels in check.

If your roof is accessible to the public, make sure to offer trash bins with liners and empty them on a daily basis, suggests Copps. You should minimize the use of lights at night as well, Black says, a move that will also reduce energy costs.

There’s no reason you need to abandon plans for a vegetated roof just because there’s a possibility a few pests might make their home there. Simply be proactive about inspections and low-impact treatments so your green roof can provide years of eco-friendly service. 

Jennie Morton was senior editor of BUILDINGS. This article was originally published on April 22, 2015. It was optimized and republished on March 14, 2019.

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