My smokebush turns brilliant colors each fall before the leaves drop. Bart Ziegler/The Wall Street Journal

Autumn can be disappointing for those of us who garden—it's the end of the growing season in much of the country—but there are compensations. For many of us it's the spectacle of leaves turning brilliant colors in our yards.

My viburnums already have started to transform into shades of red and purple. Soon, the big, hand-shaped leaves on the oakleaf hydrangeas will turn a deep reddish plum, while those on my smokebush will become bright red. But most of all it is the giant sugar-maple trees that light up my upstate New York yard in October as their leaves transition to orange and yellow.

This fall, instead of simply admiring this annual trick of nature I'm going to help scientists by cataloging when the leaves lose their green.

The leaves of my 'doublefile' viburnum trees already are becoming shades of red and purple. Bart Ziegler/The Wall Street Journal

By using a simple app on my smartphone I am gathering details such as when the first leaves turn color, what percentage of leaves has turned by what date, and when they fall to the ground. The app sends the information to a group called the USA National Phenology Network.

Phenology refers to the timing of seasonal changes in plants and animals, such as when flowers bloom or chipmunks hibernate. Phenology experts study how such annual changes are connected to weather and climate, among other factors.

With the warming climate a growing concern, scientists are seeking even more data on such changes and have turned to the public for help. Some call it "citizen science" or, to use an online term, "crowdsourcing"—harnessing the collective power of thousands or millions of people to answer a question or perform a task.

Another dependable source of fall color in my yard are my oakleaf hydrangea bushes. Bart Ziegler/The Wall Street Journal

Such information about plants' habits can be used to track allergy seasons, monitor changes in ecosystems and help predict wildfire-prone regions.

The National Phenology Network launched its citizen effort, called Nature's Notebook, in 2009, and released its smartphone apps in 2011. So far, about 2,700 volunteers and professionals have contributed data, according to Alyssa Rosemartin, assistant director of the network, who works at the University of Arizona. In addition to downloading the apps for iPhones and Android phones, people can participate by typing in data at the group's website.

The network coordinates phenology studies that are carried out by many other groups. It is funded or otherwise supported by government agencies, universities and other organizations, including the U.S. Geological Survey, National Science Foundation and National Oceanic and Atmospheric Administration.

Taking notes on when plants bloom, birds migrate and the first frost arrives is hardly new. Henry David Thoreau recorded extensive details about nature's cycles in Concord, Mass., in the 1850s. Botanists, agricultural research stations and ordinary farmers and gardeners have written down similar information for decades if not hundreds of years.

Yet such data have become newly relevant in recent years as scientists try to figure out what effect the earth's warming is having on plants and animals. One worry is that birds, insects and plants are getting out of sync, so that a migrating flock of birds will arrive too late for the insects they eat because the plants that attract the insects are blooming earlier, for example.

That's where the Nature's Notebook and its widespread data collection come in. Already, its findings have helped in several research efforts, including one that predicted species such as the red maple will leaf out 17 days earlier by the end of the century, and another that showed the arrival of spring in 2012 was the earliest ever recorded in North America.

Another way you can help this research network is to grow two specific plants in your yard for observation, one a lilac and the other a dogwood tree.

The lilac is called the USA National Phenology Network Lilac. Each is a clone, or genetically identical to each other. That means the plants shouldn't vary in their response to the seasons, providing a standard basis for observations around the country.

The shrub, which produces sweet-smelling purple-red flowers in spring, has been used for research-observation purposes for over 50 year. (Like all lilacs, it requires a climate with cold winters to thrive.)

You can order the lilac, which carries the scientific name Syringa chinensis 'Red Rothomagensis,' at Jung Quality Garden Seeds, which offers versions of the plant online at $10 each.

The dogwood, a variety called Appalachian Spring, can be purchased from a handful of online sources, including RareFind Nursery and Wayside Gardens, though the latter company was out of stock last I checked. The dogwood is a better choice than the lilac for people who live in Southern and other mild-winter states.

I planted two Appalachian Spring dogwoods in my yard several years ago, before I heard of the phenology network, mainly because they are resistant to a nasty dogwood disease called anthracnose. This fall I will join the research effort by recording when they lose their leaves, while next spring I will report when their leaf buds "burst" and the flowers start to open. After all, it's not very much effort to help further scientific research.

Write to Bart Ziegler at

What annual changes do you notice in the plants in your yard? Join the discussion in the comments section