Phenology and Flowers: Researching the Growing Cycles of Plants

In spring, flowers begin to bloom, birds fly north to reproduce, insects come out of the ground, reptiles and amphibians finish their brumation (hibernation) process, and mammals begin to come out of their overwintering spots. In the fall, flowers wilt, birds migrate south to forage, and small mammals retreat to burrows. It’s all part of a cycle of events that happens every year as seasons come and go. As a scientific field of study, it’s called phenology.

On Phenology

“Phenology, the study of biological life cycles throughout the seasons, can be used to engage the public, predict bloom times, and anticipate crop yields,” says Lincoln Park Zoo’s Manager of Plant Records and Horticulture Programs Emily Conn, explaining that bird migration and fish-spawning times are good examples of phenological processes that people might already understand.

Phenology includes elements of ecology, meteorology, and other disciplines, and helps scientists understand how humans, animals and plants respond to environmental cues. These can include temperature, soil properties, rainfall levels, the length of the day, the abundance of nutrients available, and the amount of light. (It’s not to be confused with “phrenology,” a pseudo-science that focuses on how the shape of the skull determines mental traits).

The timing of natural events is important, and any change can have cascading effects. For example, warmer weather may cause flowers to bloom earlier. If that happens, insects or other animals that pollinate the flowers or disperse their seeds don’t emerge at the right time. Birds, which time migration to coincide with peak food availability, may miss their mark, causing reduced breeding success.

Thus, when it comes to conservation and building resiliency in the face of climate change, phenology is a vital area of research. That’s why some of the work done at Lincoln Park Zoo involves phenology. The Horticulture department recently completed its fifth season of an ongoing project studying the growing cycles of plants in the native herbaceous perennial hibiscus collection.

Image courtesy of Ellen Neely

The project has helped the Horticulture department document the blooming cycle of hibiscus plants within the collection—a result of the complex hybridization in four native North American hardy hibiscus species: Hibiscus moscheutos, Hibiscus coccineus, Hibiscus laevis, and Hibiscus grandiflorus. These beautifully dramatic flowers, which are easy-to-grow plants for local gardeners, can act as indicators denoting the effects of climate change based upon their varied bloom cycles over time.

Phenology and the Zoo’s Hibiscus Collection

The zoo’s perennial herbaceous hibiscus collection, which includes 86 different taxa and more than 500 individual plants, is accredited by the American Public Gardens Association Plant Collection Network. It includes some cultivars that are no longer available commercially, such as the historically valuable Hibiscus ‘Lou Emmons,’ so phenology information is important in understanding the future prospects of these rare plants as Lincoln Park Zoo continues to build its living library.

In their phenology study, zoo researchers looked at several different stages of hibiscus growth, including periods when it was in bud, in peak bloom, or experiencing a secondary bloom. Hibiscuses generally have their peak bloom in mid-August to mid-September, but many variables exist that affect when it happens, and there are differences between cultivars as well. Some cultivars—such as the ‘Pino Grigio’ and ‘Purple Heart’ hibiscuses—have long blooming periods that last from July through November, while other varieties have much shorter ones.

Horticulture experts at the zoo marked the study’s time period in various stages. The “bloom calendar” below shows different phenophases in the hibiscuses’ life, color-coded by phase. Light green indicates that the flower is in bud, darker green shows it in peak bloom, and blue denotes a secondary bloom.

This data visualization shows seven different phenophases of individual hibiscus plants on zoo grounds in 2024. As you can see, some flowers continued to bloom into November, but “Peak Flower” for most plants took place between mid-July and the end of August.

The Last Five Years

Over the past half decade, the zoo has been able to standardize reporting by defining the phases and measurements for each of eight separate phenophases in the life cycle of a plant. Now, each observation must meet clear metrics, and photo records are taken with every data point. This will help ensure that data collection is as accurate as possible going forward.

As Horticulture staff have introduced new cultivars to the collection, including a recently available one called ‘Watermelon Ruffles,’ they have also begun collaborating with a group of horticulturists, climate scientists, and software engineers working to improve phenology data collection in botanical institutions. Lincoln Park Zoo is currently the only U.S. institution represented in this phenology working group.

Zoo researchers continue to hone their data collection and visualization processes. This includes developing more precise ways to observe and record bloom times, working to incorporate community science and volunteer efforts in gathering data, and connecting with partners regarding regional climate datasets.

“I think of these variables like layers on a map; think of a topographical map,” Conn says. “The more information you have stacked on top of it, the clearer the picture becomes. So the more information you have about environmental data and bloom period, the more comprehensive our view of how these plants are responding to climate change will become.”

The Future of Phenology at the Zoo

In the next five years, zoo horticulturists will continue to improve the accuracy of the phenology data sets. Their strategy includes standardizing data collection procedures to minimize the possibility of human error, continuing to build the living library, increasing interpretation to the public, and making research available to others in the field who might benefit from the knowledge. They also hope to look at more variables, such as frequency of irrigation and nutrient levels in the soil, and sample more often.

Future phenology work at Lincoln Park Zoo also includes a pilot program that will take what researchers have learned so far to start collecting data for spring flowering trees: dogwoods, lilacs, redbuds, and magnolias. The zoo will be able to contribute to the U.S. National Phenology Network’s ongoing community science phenology programs, such as the long-running Cloned and Common Lilacs Campaign and The Dogwood Genome Project, which uses this data to forecast spring blooms each year.

Hibiscuses are often given to zoo animals as browse once the growing season is over. Image courtesy of Lead Keeper, Primate House & Macaque Forest Shannon McElmeel

“NPN uses historical information about when trees like lilacs have leafed out and bloomed in the past, and what the climate was like preceding these events,” Conn says. “They then use this information to develop models called the Extended Spring Indices that can tell us, based on weather forecasts, when lilacs are likely to bloom in a particular year. These have been used as an indicator of climate change by both the Environmental Protection Agency and the National Climate Assessment.”

To take a self-guided tour of the hardy hibiscus collection the next time you’re at the zoo, visit lpzoo.gardenexplorer.org.