Can the parts of plants we can see help predict the parts of them we can’t?

There’s a lot we can tell about plants by looking at them. We can see their leaves, stems and overall structure. If it’s a flowering plant, we can see them, too. But we can’t see what the roots look like under the ground, like root depth or structure. Our research team decided to study if above-ground characteristics could predict below-ground characteristics.

In the United States, we often plant “native plants” to help protect the environment. This can be in private yards or in city landscaping. Native plantings might be designed to help rain go into the ground after storms, to make the soil healthy, or to provide beauty through colorful leaves and flowers.

Prairie plants are commonly grown in the Midwest because these species can have deep, strong root systems that create healthy soils.

Common milkweed is a mid-season bloomer (right) and has an excellent root system (left). Credit: Marie Johnston

The simple question that began this research was: We can tell that prairie plants are very different above the ground, but how different are prairie plants below the ground? We also wanted to know if plants that bloomed at the same time in the season (flowering phenology) would have similar characteristics, and if plants with a similar shape (crown morphology) would share characteristics. Information about the species when grouped this way might be useful in picking one plant or another for a city planting.

We inventoried which prairie plants were used in city plantings. There is a core list of dozens of species usually selected for their color, height, bloom time, and other factors. From this list, we chose 16 species of prairie plants and grew each species separately for research purposes. All plants were potted and grown outdoors. Our research team regularly weeded, watered, and monitored for size, flowering time, and leaf traits.

When species reached the end of their flowering phase, the plant is usually done growing new leaves. So, when this happened, we cut down each individual specie, and removed the roots from the soil. It took a team of us several weeks to (1) measure, cut, bag, dry, and weigh all the leaves, stems, and flowers; and (2) tip the pot over, cut open the side, and hand-sort the roots from the soil. We then bagged, dried, and weighed the roots. From these weights, we used statistics to test for differences when they were grouped by phenology and morphology.

After the plants in the research plots had flowered, the aboveground parts were cut off for analysis. The roots were also collected, separated from the soil, dried, and weighed. Credit: Marie Johnston

Our research confirmed some things we already knew about prairie plants (for example, that grasses can have very large root systems), but also some new information about how these prairie species might differ.

Of the 16 common species we used, we found that the shorter, early-flowering plants had large leaves for their size, and that some of the mid-season or late-season flowering plants (or, forbs) can develop robust root systems that are as massive as the roots of grasses. These differences in traits were consistent with what we know about how plants share space and resources like water and sunlight when growing together as a community.

One take-home message was that we might use aboveground traits like bloom time and height to predict what the species looks like below the ground.

Sixteen species of prairie plants were grown in separate pots for evaluation. The research findings may inform city landscape planners on the best plant selection for their needs. Credit: Marie Johnston

We hope that studies like ours will encourage others to ask good questions and not be afraid of some hands-on work to reach new answers. In this project, the hardest part, clearly, was the physical and time-consuming effort to sort out roots, but the work added to what we know about prairie plants.

Answered by Marie Johnston, UW-Madison Arboretum

This blog is based on a paper originally published in Agrosystems, Geosciences & Environment Journal (DOI: 10.1002/agg2.20059), a publication of the American Society of Agronomy.

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