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College of Food, Agricultural, and Environmental Sciences


Global Warming in Your Garden? Common Plants, Bugs Reveal Important Climate Changes

July 17, 2006

WOOSTER, Ohio — Evidence supporting climate change often comes in striking forms, anything from gigantic ozone-layer holes to remote melting glaciers. But signs the planet is getting warmer can be found in familiar places a lot closer to home: say, your flower bed or your tree lawn.

So argues Dan Herms, an Ohio State University entomologist based on the Ohio Agricultural Research and Development Center’s (OARDC) Wooster campus. Herms coordinates a statewide growing-season monitoring system that predicts when tree and shrub pests will first appear at any given location based on the blooming of ornamental plants.

This “biological calendar” helps nursery growers and gardeners make fewer but more effective pesticide applications. But it also collects key data about plant and insect activity that reveals the occurrence of warmer winters and a significant extension of the growing season.

“We are not talking about changes from one year to the next, but long-term changes,” said Herms, also a specialist with Ohio State University Extension. “Winters are getting warmer. Bagworms are a good example to illustrate these changes. These pests of shade trees and woody ornamentals never were a problem north of I-70. But today they are found in northern Ohio as the temperature has gone up during the winter months.”

The growing season is starting earlier and ending later, Herms added. A good indicator of this phenomenon is the black vine weevil, which attacks many species of trees, shrubs, vines and flowers and is one of the most damaging and tough-to-control nursery pests in Ohio. Research conducted by Herms and his predecessor at OARDC, David Nielsen, shows that adult weevils are emerging three weeks earlier than they did back in 1970 — mid- to late May in northern Ohio. The reason? Herms’ biological calendar has found that black vine weevil emergence closely corresponds with the blooming of black locust trees. And black locust trees are blooming earlier, too.

“OARDC records show that in 1884-1885, black locust trees were blooming in Columbus in late May, but in recent years they have been blooming in early May in Columbus, and late May along Lake Erie,” Herms pointed out. “So if the (black vine weevil) adults have always emerged during blooming of locust trees, then they have been emerging earlier in recent years.”

By the time nursery growers began spraying insecticides to control the weevils in mid- to late June (when they were previously thought to emerge), the insects had already been feeding and laying eggs on their precious stock.

Changes in black vine weevil development also correspond to larger-scale growing season changes documented by the study of lilacs and other ornamental plants. In 2004, OARDC and OSU Extension established the Ohio Phenology (the study of plant and animal lifecycles in relation to climate) Garden Network to further expand growing-season monitoring. The largest such network in North America, it consists of 35 gardens located at public areas throughout the state, such as arboreta, schools and OSU Extension offices. Each garden features 16 cultivated varieties with blooming periods that, collectively, span the growing season — including early bloomers like forsythia and star magnolia and late bloomers such as Rose of Sharon and elderberry.

And of course, the aromatic lilac.

“The USA National Phenology Network (USA-NPN) has been looking at lilacs to track global climate change,” Herms explained. “Lilacs have an easily monitored flowering season, good survival rates and large geographical distribution, making them good indicator plants to study growing-season changes over a long period of time and across a wide range. Our Ohio network gardens all have lilacs, and we are reporting data to the national network now.”

Data from the Ohio lilacs, which is collected by OSU Extension master gardener volunteers, contributes even further to the wealth of phenological information amassed by USA-NPN (, an international leader in climate change research as evidenced by plant development.

Lilac development reports took center stage in a study published in the February 2006 issue of Global Change Biology (available at, which shows how spring is starting earlier across the Northern Hemisphere based on daily maximum-minimum temperatures and observation of lilacs from the U.S. network and hundreds of other sites in Canada, Europe and Asia since 1955. The study found spring has arrived 1.2 days earlier each decade on average, with North America and Central Europe warming up the soonest.

“The purpose of our phenology gardens is to provide industry and individuals with a tool for predicting when pests emerge, which is critical for any successful pest-management program,” Herms said. “But we also see them as educational resources to create awareness about the role weather plays in climate change. Having a monitoring system like this becomes even more important as environmental changes are happening so fast.”

OARDC’s biological calendar is available online at Users only need to enter their Ohio zip code and desired date, and the system will automatically provide information about plant development and insects to watch for. The calendar obtains this information by combining lifecycle data from indicator plants and insects and daily growing-degree-day values, which are calculated using daily maximum-minimum temperatures from 12 OARDC and three U.S. Department of Agriculture weather stations throughout Ohio.

OARDC and OSU Extension are part of Ohio State’s College of Food, Agricultural, and Environmental Sciences.


Mauricio Espinoza
Dan Herms