By Ken Mammarella
Contributing Writer

The trial garden at the Mt. Cuba Center in Hockessin is an unheralded example of how the Delaware Valley is a national leader in horticulture and gardening.

“If you’re interested in visiting terrific in-ground trials, check out the Chicago Botanic Garden in Illinois and the Mt. Cuba Center in Delaware,” Fine Gardening magazine wrote in 2022. 

Mt. Cuba devotes several years to plan for each trial; cares for trial participants the first year so they can become established; lets them survive on their own thereafter; monitors them weekly for the best in flowers, foliage, form and other factors; and finally issues a report on the results. “Our research team evaluates native plants and related cultivars for horticultural and ecological value, highlighting the ecosystem services native plants provide,” Mt. Cuba explains on the landing page for the program, mtcubacenter.org/research/trial-garden.

Mt. Cuba was first gardened by Lammot and Pamela du Pont Copeland in 1935 and became a nonprofit center after her death in 2001. In 2003, it began its first trials, and their design is modeled after the Chicago Botanic Garden. Most trials are for herbaceous perennial flowers and run for three years, but in 2015, it began a rhematically related 100-year test of trees and shrubs, aimed on recommending the best way to bring back the region’s forests.

It focuses on two types of plants for trials, said Sam Hoadley, its manager of horticultural research. 

One is plants that can easily be purchased but lack solid, comparative information. Such as the 70 varieties of coneflowers Mt. Cuba could buy. Or “there is a great deal of confusion among gardeners because some perennial cultivars do not perform as well as advertised,” the trial about coreopsis noted.

The other is plants “that we feel have great ecological value,” he said, but are not as well known. Such as goldenrod and ironweed. Or, as the report on baptisia concluded: It “is a fantastic group of plants that deserves greater garden use. Not only does the floral display rival the beauty of any other spring bloom, but the plants are deer-resistant and require almost no maintenance.”

“Many readers of our reports are increasingly interested in supporting pollinators, said Laura Reilly, the trial garden assistant. That includes hummingbirds, bees, wasps, butterflies and other bugs. “Anything that feeds on the flower,” she said.

A tough-love approach

The trial garden is a short walk from the main house. The 15,000-square-foot plot features clay-loam soil with an average pH of 6.5. It is surrounded by a 4-foot fence to reduce damage by rabbits and groundhogs, and taller fences far in the distance keep out deer. The shade panels covering the shade garden are adjusted seasonally to imitate the local tree canopy. 

During a midsummer visit, Hoadley showed off dozens of plants being trialed, including 62 ferns, 26 oakleaf hydrangeas (in the sun and shade), 25 milkweeds (ascelpias), 18 foamflowers (tiarella) and 14 obedient-plants (physotegia).

Signs also highlight an upcoming trial on pycnanthemum (mountain-mint), and more signs point out plants that did the best in earlier trials. One last sign marks Mt. Cuba introductions, referring to “underappreciated or underused native plants” that it’s evaluating.

In the first year of a trial, the “plants get in their groove,” Hoadley said, and aren’t rated. After that, “we’re the worst gardeners at Mt. Cuba,” he said. “We’re so hands off.” No fertilizing, no watering, no herbicides, no fungicides, no insecticides. Two exceptions: Mulch and pesticides if there “is a serious threat to the entire trial’s survivability,” the site said.

That tough-love approach is intended to see which varieties do best on their own and hence would be likely to repeat that success for homeowners who might not have much time or training. 

He and Reilly devote three to five minutes to each plant for weekly ratings, May to September. They grade on a 1-to-5 scale, with most plants, predictably, in the middle. 

In addition, “dedicated citizen scientists … observe and tally insect visitation to help determine ecological value of plants in our trials.” “We hope that they show up,” Hoadley added.

Some plants don’t survive and are marked NR (for not rated) or “dead” in the final reports.

A few stars among hundreds of subjects

Mt. Cuba expects in early 2025 to publish a report on veronia (ironweed), followed by solidago (goldenrod) in 2026.

This fall, Mt. Cuba is planting big and little bluestems, the first of two trials on grasses.

The landing page for the trial garden lists multiple completed trials. Many plants, sometimes confusingly for gardening newbies, have multiple names, including one or more common names, plus botanical names. Trial subjects have included amsonia (aka bluestar), aster, baptisia (wild or false indigo), carex, coreopsis (tickseed), echinacea (coneflower), helenium (sneezeweed), heuchera (coral bells or alumroot) hydrangea, monarda (bee balm) and phlox.

All of Mt. Cuba’s latest completed trials are labeled “for the mid-Atlantic region,” a logical distinction for a garden that emphasizes local plants.

Of the 600 plants listed in the published reports, only two scored a perfect 5.0. 

One was Baptisia sphaerocarpa ‘Screamin’ Yellow,’ one of 46 selections in that trial of wild and false indigo. It scored so well for producing a lot of flowers – as many as 350 inflorescences on a single plant – amid “ beautiful, bluish foliage that creates a rounded, medium-sized habit and remains dense and sturdy all season long.”

The other was Echinacea purpurea ‘Pica Bella,’ “a compact and floriferous form” of coneflower that fits into a broad range of garden designs. Plus it “was also a favorite among pollinators that flocked to its prominent orange cones.” “A great plant,” Reilly said.

Two phloxes scored just 1.1 and 1.0. Phlox × arendsii ‘Susanne’ “was plagued by powdery mildew, and four out of five plants completely failed to bloom.” Only one of the Phlox stolonifera ‘Variegata’ plants survived the first winter, and “it barely grew during the rest of the trial. Variegated plants are typically less vigorous than their fully green-leaved counterparts, but this cultivar is significantly worse than normal.” Ouch.

“There are always surprises,” Hoadley said of the trials. “Carex was thought to be a shade-loving plant, and they did well in the sun. There are always plants that break the rules.”

A century-long trial of reforestation

Mt. Cuba Center in 2015 began a test on reforestation that’s planned to run 100 years.

Four times – in 2015, 2018, 2021 and 2024 – the center planted 2,700 trees and shrubs on 3½-acre plots that had been previously farmed. The first goal was to return them to forest, and the second was to study the best way to do so, said Nate Shampine, Mt. Cuba’s natural lands manager.

The plantings, all natives and all sourced locally, includes big trees for the canopy (like oak and hickory), midstory trees (like ironwood and witch hazel) and shrubs (like sumac and viburnum). 

The study began in an era when “the accepted practice for reforestation was a simplistic one, low in diversity, mainly canopy trees, planted 10 to 12 feet apart like an orchard,” he said. “We were questioning if there was a better way.”

Mt. Cuba, working with West Chester University and University of Delaware, is testing six ways:

1. Sparsely planted trees, at 10-foot intervals.

2. Sparsely planted trees and shrubs, at 10-foot intervals.

3. Densely planted trees, at 5-foot intervals.

4. Densely planted trees and shrubs, at 5-foot intervals

5. Natural succession, with Mother Nature allowed to run its course.

6. Control, with sparsely planted trees and a different maintenance schedule.

To keep meadow voles out, plots 1-4 are being mowed for three to five years after planting, while the control plot will be mowed for 10 to 15 years. To keep out deer, all plots have 10-foot fences. All the plots are in areas of Mt. Cuba far from the main house.

“The densest plantings are filling in the quickest, are much more diverse and a requiring a lot less maintenance,” Shampine told TownSquareDelaware.com. “But the tradeoff is that they were more expensive and labor-intensive.”