Keystone Species: 4 Plants Every Garden Should Have
What they are, what to get, and how to use them.
When you look at a Roman stone arch, you can see that the stones stack on each other, held together by what is called the keystone which ensures the stability of the structure. When the keystone is removed, the whole archway crumbles. Why are we talking about Roman architecture in a native plant blog?
Well, in ecology, the concept of keystone species is built on the idea that there are certain plant species that when present in the environment can support hundreds of other species. These keystone plants have a “disproportionally large effect on the abundance and diversity of other species, like insects, in an ecosystem.” -- Jody Walthall. When these plants aren’t present, the quality of the local ecosystem can crumble just like removing a keystone from a Roman archway. Insects are the primary focus for keystone plant species because in the food web, disrupting the primary consumers (insects) leads to greater disruption up the chain.
“The research of entomologist, Dr. Doug Tallamy, and his team at the University of Delaware have identified 14% of native plants (the keystones) support 90% of butterfly and moth lepidoptera species. The research of horticulturist Jarrod Fowler has shown that 15% to 60% of North American native bee species are pollen specialists who only eat pollen from 40% of native plants.” -- The National Wildlife Federation
Identifying species that support a wide variety of butterflies, moths, and specialist bees can require a lot of research. But, fear not! We’ve done the work for you. Focusing on flowering herbaceous perennials, there are four genera of plants that will help support your local ecosystem if you can squeeze them into your garden.
The best part? These native plants are beautiful. There are species from these keystone genera that can work in sunny or shady locations, meaning there is a plant for everyone. Even if you have a small yard, or a place where you can grow a few potted plants, you can help support native pollinators!
The A-Team
Solidago - goldenrod
I’m getting on my soapbox. Goldenrods get a bad rap but we’re going to clear the air. Everyone thinks they cause seasonal allergies. Even when you’re at the allergist's office, they tell you it’s the goldenrods that are making you sneeze in the fall. However, the pollen of goldenrods is too sticky to fly through the air - what’s making you sneeze is the wind-pollinated ragweed blooming at the same time. Ragweed blooms green so people don’t notice it compared to the bright beacon of yellow goldenrods that you can spot a mile away. Goldenrods take the fall for ragweed just because they dare to be brilliant? Not on my watch.
Now that we have cleared goldenrods’ good name, Solidago species are the heavyweight champions for supporting primary consumers in the food web (aka insects). With over 100 species of caterpillars using them as host plants and over 40 species using the pollen, finding room for goldenrods in your garden should be a top priority. While North America has more than 80 native species of Solidago, I’ve pulled examples of some great ones to put in your garden.
Photo courtesy of Mt. Cuba Center
For shady gardens: Solidago caesia (bluestem goldenrod). Tolerant of shady conditions, this goldenrod has a lovely arching habit growing 1-3’ tall with sunny blooms along the stem in fall. Just because you have a shady garden doesn’t mean you can’t have flower power.
For small gardens: Solidago sphacelata 'Golden Fleece' (autumn goldenrod). Great for a small garden or a potted container. Don’t let lack of space stop you from supporting your local ecosystem! This bright yellow goldenrod is very well-behaved (unlike some of its relatives) with a clump-forming habit that grows 1-2’ tall.
For sunny gardens: my goodness, it is hard to choose just one. Do you want a very early blooming goldenrod that is adaptable, grows about 4’ tall and spreads into colonies? Try Euthamia graminifolia (flat-topped goldenrod). In case you’re wondering why this is not a Solidago, let’s just say that it used to be but got the ax during a recent name change. Another early-bloomer, but one that stays in place, is Solidago odora (anisescented goldenrod). Need something that can take intermittent flooding, drought and salt from roads or the sea? Solidago sempervirens (seaside goldenrod) is your friend. Have a hillside that needs help controlling soil erosion? Try Solidago nemoralis (gray goldenrod). Whatever your needs, there’s probably a goldenrod that can help.
Symphyotrichum - aster
Who doesn’t love a daisy-flowered aster blooming brightly on a fall day? Asters are an easy addition to the garden because we humans already love them and, conveniently, so do the pollinators! More than 100 species of caterpillars use asters as host plants and over 30 species of specialist pollinators prefer their pollen. Asters are great for every style of garden and are adaptable to a wide range of conditions.
For shady gardens: Symphyotrichum cordifolium (common blue wood aster). Clouds of fluffy, blue, small daisy-like flowers cover this aster in late September through October. Growing 2-3’ tall with heart-shaped leaves, it will happily self-sow when in a good spot. Not only is this plant adaptable, its lush foliage can hide the tattered leaves of early-season bloomers, giving a fresh breath of life to your woodland garden in fall.
For small gardens: Symphyotrichum laeve ‘Bluebird’ (smooth aster). I’ll be honest, this aster is pretty much a non-negotiable option for me. I’ve never seen so many insects on one plant at one time. The brilliant blue flowers in September, and attractive foliage growing 3-4’ tall, make this a perfect addition to a small home garden. It’s very well-behaved, powdery mildew resistant, adaptable to a wide range of soils, and it was one of the top choices in Mt. Cuba’s Aster trial. Need I say more?
For sunny gardens: Symphyotrichum novae-angliae (New England aster). Why reinvent the wheel? It’s popular because it’s good. Widespread and adaptable, with bright purple flowers, growing up to 6’ tall, this aster is commonly found along roadsides. It can take poor soils, moist soils, salty soils, and thrives on neglect. It will self-sow when happy, that is if the local native songbirds haven’t picked the tufted seed clean first.
Helianthus - sunflower
This isn’t the sunflower you planted as a kid. North America is home to over 50 species of native Helianthus that support over 66 species of caterpillars that use them as host plants, and 50 specialist pollinators that use their pollen. Sunflowers aren’t just the bright, cheery yellow annual crops grown in fields. There is a wide range of shapes and forms of this beloved plant. If attracting insects wasn’t enough, the seeds are high in the fats and proteins needed to feed migrating birds. A number of the perennial species are quite tall and can spread by rhizomes, so they perform best in areas where they can freely roam.
For shady gardens: Helianthus divaricatus (woodland sunflower). A great plant for dappled shade beneath trees that are fine-leaved, or along a woodland edge. It will self-sow happily under the right conditions. This species blooms in July, providing a burst of much-needed color to the mid-summer shade garden. Depending on the conditions (wet/dry, very shady/more sun), plants can vary in height from 2-6’.
For small gardens: Helianthus salicifolius ‘First Light’ (willowleaf sunflower). Covered in flowers in late fall and beloved by insects, this selection of the species Helianthus salicifolius has greater flower coverage and maintains a tidy size of 3-4’ tall. A town near me used this cultivar for its roadside plantings and it stopped traffic when it was blooming last year.
For sunny gardens: Helianthus maximiliani (Maximilian sunflower). If you’ve got the room then maximize it! This powerhouse of a sunflower thrives in poor soils and grows anywhere from 3-8’ tall. It is simply covered in 2” flowers and happily blooms for weeks in late summer.
For food: Helianthus tuberosus (Jerusalem artichoke). Ok, we’re sneaking this category in. Did you know that you can eat the roots (tubers) of a native perennial sunflower like a potato? This plant does it all: covered in flowers, it supports pollinators and wildlife, and is a great perennial food source for you! Just beware, it’s a rampant spreader in the garden so plant it where you can give it room to grow. But that just means more tubers to enjoy in the winter!
Rudbeckia - coneflower
This cheery early to late summer staple is a beloved wildflower across North America. The common black-eyed Susan (Rudbeckia hirta) brightens up our roadsides and is present in 48 states. But with 47 species of Rudbeckia endemic to North America, why stop with just one? As a genus, Rudbeckia supports 20 species of caterpillars and 29 species of specialist pollinators.
For shady gardens: Rudbeckia laciniata (cutleaf coneflower). Growing up to 10’ tall, this lemon-yellow coneflower isn’t a supporting role, it’s the main character. In reliably moist conditions, plants will spread steadily, creating a colony that blooms for weeks on end during the summer.
For small gardens: Rudbeckia fulgida var. fulgida (orange coneflower). Forming tidy clumps of basal leaves, and with bright sunny flowers, this species of coneflower spreads steadily but easily. Growing 2-3’ tall and blooming in mid-summer, the flowers are a favorite for pollinators and birds alike.
For sunny gardens: Rudbeckia maxima (great coneflower) It’s in the name. A large swathe of this plant backlit by the warm golden afternoon sunlight in summer is one of life’s pure joys. Growing 5-7’ tall, great coneflower has large blue-gray basal leaves, and erupts with yellow flowers in July. Adding a strong vertical element to the garden, there are no substitutes for this plant.
Izel Plants Notes:
Why are all these plants so large? Well, it really just comes down to math, or geometry to be precise. Keystone species are hosts to a lot of herbivorous insects. The more insects feeding at your buffet, the larger the buffet you need! So, the square footage of foliage available to munch on is one of the determining factors of keystone species.
Doug Tallamy, as professor and chair for entomology and wildlife ecology, has done extensive research on the value of plants as insect hosts and his conclusion? Bigger is better! I’m not sure he’s ever said it quite this way, but his research has shown that oaks are the most valuable host plants in North America, supporting 897 caterpillar species in the United States alone.
Another good example of why size matters when it comes to host plants, and one that gardeners are more familiar with, is milkweeds. The genus, Asclepias, is not only the poster child for saving monarch butterflies, it has taught countless gardeners to look for feeding caterpillars. Who hasn’t avidly followed the progression of monarchs from egg to caterpillar with the same intensity as ‘Ozark’? Also, who hasn’t gone from elation to despair when they spotted a young caterpillar on their Asclepias verticillata (whorled milkweed) only to realize that all the leaves were gone in two days? In comes Asclepias syriaca (common milkweed) to the rescue with 4 -5’ tall plants loaded with leaves that are up to 7” long and 4” across.
Photo: © Erika Szabo
Why did you include cultivars on your list? Aren’t they contrary to the concept of keystone species? Well, not all cultivars are bad, and not all gardens are large. We included compact cultivars because growing large plants is not always an option. These cultivars are not engineered Frankenplants, but naturally occurring variations of the straight species that were discovered by keen-eyed horticulturists scouring the landscape. So, although these cultivars are scaled-down versions of their straight counterparts, they offer the same ecological services.
All photographs are © Izel Native Plants unless otherwise annotated. Opening photo of Colias philodice (clouded sulphur) on Symphyotrichum laeve (smooth blue aster) © Uli Lorimer.
Chelsea Ruiz is a horticulturalist and garden writer based out of Virginia. She received her bachelors from University of Wisconsin Madison and cut her teeth at Chicago Botanic Garden and Longwood Gardens in research and plant trials. She trialed native plants for a wholesale native plant nursery using her research background to communicate truth over tropes to customers on how their plants grow in a wide variety of challenging settings. The goal of her garden writing is to provide the facts and hard-won experience to empower gardeners to make informed decisions in their landscapes.
