Keystones in the Garden (Full Version)

By Leah Brooks
As featured in our 2023 DE Sierra Magazine

People are becoming increasingly aware of the importance of including native plants in their landscapes to support wildlife. But how to decide which plants provide the most bang for your buck, especially when space is limited? The answer is simple. “Plant keystones!” In architecture, a keystone is the large stone at the top of an arch. The pressure from this large stone keeps the other stones in place.  If the keystone is removed, the entire arch collapses. It’s the same concept with keystone plant or animal species. Remove them from ecosystems, and they collapse.

Keystone plant species have a disproportionately large positive effect on their natural environment relative to their abundance. In other words, planting a single keystone tree benefits an ecosystem much more than planting multiple non-keystone trees! This is because keystone plants host (feed) more species of caterpillar than other plants do. These insects are an essential component of food webs. They are a critical food source for nesting birds, who need copious quantities to raise a brood of babies to fledging.

To give just one example, common sneezeweed (Helenium autumnale), a popular native plant for home gardeners, supports less than 10 caterpillars, while swamp sunflower (Helianthus angustifolius), a lovely native with similar growing requirements, supports more than 50! Such knowledge is a boon to gardeners who wish to select native plants for maximum impact, particularly in a small landscape.

In Delaware and the larger Eastern Temperate Forest ecoregion, keystone genera include trees like oak (Quercus), maple (Acer), wild cherry (Prunus), birch (Betula), and hickory (Carya), shrubs like blueberry (Vaccinium), willow (Salix), and wildflowers like American aster (Symphyotrichum), and goldenrod (Solidago). Within these genera, there are many species with different habitat requirements and varying growth habits. Although this plethora of possibilities may seem overwhelming, it means that there is a keystone plant that will thrive in nearly any landscaping scenario!

Gardeners should also double-check the native range of plants before they purchase, if their goal is supporting wildlife. While many nurseries sell plants labeled as “native,” tags don’t always specify what geographic area those plants are native to. They may not even be native to Delaware! For example, the very popular purple coneflower (Echinacea purpurea) is often sold locally as a “native plant,” but its range is actually further west and includes Missouri and Arkansas. Another example is Hubricht’s bluestar (Amsonia hubrichtii). This popular perennial, known for its billowing golden autumn foliage, is commonly marketed as “native.” Yet its native range is concentrated in Oklahoma and Arkansas.

Both Hubright’s bluestar and purple coneflower are lovely plants, bringing joy to gardeners and adding seasonal interest to the home landscape. However, the native wildlife in our area did not evolve alongside these plants. They did not form the specialized relationships with coneflower (Echinacea) and bluestar (Amsonia) that exist with oak (Quercus), goldenrod (Solidago), and other critical genera. Thus, plants in genera that are novel to our local ecosystem are best used alongside - rather than in place of - keystone species to maximize benefit to wildlife!

After figuring out whether a plant is native to your area, it is a good next step to check if they are cultivars. A cultivar is a plant within a species that exhibits a particular appearance or characteristic. Some cultivars are created by human manipulation and repeated crossing/breeding in greenhouses. Some are the result of propagating a unique, naturally occurring specimen. Most cultivars are propagated clonally, meaning that all plants of a certain cultivar name are genetically identical to each other. While clonal propagation ensures the specimens present the desired characteristics of the cultivar, these specimens are not “true to seed.” This means that when they reproduce by seed, their offspring have a range of different appearances and may not look like the parent plant.

If gardening solely for landscape value, cultivars are frequently chosen because they are uniform and there is little to no variety between specimens. This is beneficial if you want to repeat plants of the same height or know what to expect in terms of habit. For certain tree species, like flowering dogwood and green hawthorn, disease-resistant cultivars are a must-have if you are planting in a landscape, as these species are prone to disease. Yet studies show that cultivars with certain characteristics are less attractive to insects and other wildlife. Cultivars should be evaluated on a case-by-case basis, but there are several trends among the data that gardeners should be aware of. For example, cultivars in which leaf color has been changed from green to red/purple/blue are less attractive to leaf-eating insects (Tenczar and Krischik 2007 ; Basiden et al. 2018) as are those with heavily variegated leaves (Campitellu, Stehlik, and Stinchcombe 2008 ; Soltau, Dotterl and Lied-Schumann 2008). Pollinators find double-headed Echinacea and mophead hydrangeas less attractive due to flower characteristics (Mt. Cuba Center Trial Reports), and hybrid cultivars less appealing due to decreased nectar and pollen production. (White 2016).

So what’s a wildlife-loving gardener to do? Educate yourself about the keystone species for your area and their relationships to insects and other wildlife. Maximize their impact, especially when limited by space. Learn more about how to support wildlife in your garden by reading “Nature’s Best Hope” by UD professor and entomologist Dr. Doug Tallamy. Most importantly, spread the word to other gardeners!