Choosing between cultivars

Despite my recent interest in planting straight native plants, I’ve got lots of cultivars in my garden and expect to bring home more in future. Cultivars are improved versions of a species, selected or bred for desirable characteristics such as bigger flowers or new flower colors. Cultivars of native species are sometimes called “nativars.”

This coneflower, Echinacea 'Sundown', is a nativar-photo Mike Peel

    I’m used to thinking that cultivars are selections culled from nursery beds or native plant populations by sharp-eyed growers. But this is only one of the paths to a market-worthy cultivar, I’ve learned. Other cultivars are the products of intentional breeding programs. Once they’ve been found or developed, cultivars will be propagated asexually, through cuttings or divisions, so that the offspring will be exactly like the parent. That means they won’t add genetic diversity to your landscape, as native species will.

    At a recent garden club meeting, a member asked me whether her white-flowered coneflower cultivar counts as a native for the purpose of attracting desirable insects. It’s an important question and an area of active research. Recording native pollinator visits to flowering phlox plants, both straight natives and cultivars, Keith Nevison, a researcher at Mt. Cuba Center in Delaware, was surprised to find that a cultivar, Phlox paniculata ‘Jeana’, attracted the most pollinators. He hypothesized that this might be because ‘Jeana’ blooms so generously and has small, shallow flowers that are easy for pollinators to use.

Tiger swallowtail on Phlox 'Jeana'-photo Michele Dorsey Walfred

    Similarly, the Nativars Research Project of the Chicago Botanic Garden used citizen scientist observations through project Budburst to determine that cultivars vary widely in their attractiveness to pollinators. Among beardtongues (Penstemon digitalis), for example, popular cultivars ‘Husker Red’ and ‘Blackbeard’, loved for their deep purple foliage, drew far fewer insects than ‘Pocahontas’. 

Penstemon 'Pocahontas' at Bluestone Perennials

Scientists suggest that the difference is in the origin of the plants. ‘Pocahontas’ was discovered in the wild. That’s something it has in common with Phlox ‘Jeana’, which was found growing wild outside Nashville. Apparently when plants change in the wild, the new genetic mixes that survive are the ones that cooperate successfully with local insects.

    Some early guidelines seem to be developing for gardeners who want to pick plants to benefit pollinators. Cultivar flowers that resemble the native’s most closely are more likely to attract insects than blooms that have been radically altered in the breeding process.

Echinacea 'Razzmatazz' is probably too different from its straight native ancestor

And now we can surmise that wild-selected cultivars are also more likely to be popular with pollinators than cultivars produced by human breeding efforts. Much more research will be needed before we can be sure which cultivars to plant for pollinators.

    So what about that white-flowered coneflower? Where did it come from? I see that Echinacea ‘White Swan’ may have been introduced by Piet Oudolf, the legendary Dutch garden designer. Did he find it growing in his garden? I can’t tell.

Piet Oudolf border, Royal Horticultural Society Garden Wisley, with white coneflower bottom right-photo Esther Westerveld

    Growers introduce lots of new cultivars of native plants every year. Right now, I think the only way we can determine which ones are most popular with pollinators is to plant them and observe.

Pollinator-friendly container plantings

Every spring I fill 10 or 12 large containers with flowers and foliage plants for summer enjoyment. At this time of year, there’s a pause in the garden action. Foliage is looking shopworn and dull. A few flowering plants whose growth shut down during the hottest days are coming back to life, preparing for an autumn round of bloom. 

Phlox and a few zinnias are blooming in the late summer lull

Buds are swelling on the New England asters (Symphyotrichum novae-angliae), but few have opened yet. This seems like a good time to evaluate how the container plantings have turned out.

    My favorite pot this year was inspired by advice from reader Patricia McGinnis. Two years ago, Patricia pointed out that instead of neonicotinoid-treated annuals from the garden center, I could fill my pots with divisions of perennials from the garden. What a breakthrough!

    Last fall I’d potted some unneeded perennials with thoughts of passing them on to students at my spring sustainable gardening course (If you’d like to attend the fall course in my garden on Saturdays October 26 and November 2, you can register here through Newton Community Education). Some didn’t find a new home, so I used them to design a pot for a prominent position in the front yard.

Perennial divisions fill out a front yard container

    A purple-leaved heuchera, a nativar, makes a solid mass in this arrangement. Nonnatives provide contrasting foliage: a hosta with chartreuse leaves banded with blue-green and a Japanese forest grass (Hakonechloa macra) with similar yellow-gold coloring. I bought an ostrich fern (Matteuccia struthiopteris) to add some height and added white-flowered wax begonias to bloom nonstop through the season. The begonias' shiny round leaves add another texture to the composition. I knew the fern and begonias were neonic-free, because I’d gotten them at a local farm and garden center that promises untreated plants.

    My next favorite container plantings this summer feature nonnative combinations I’ve been repeating over the years. The “thriller” in these pots is Canna ‘Bengal Tiger’. 

Yellow-striped leaves make Canna 'Bengal Tiger' stand out

I store the roots in the basement through the winter, so I don’t have to worry about neonic-treated replacements. I do the same with the giant tuberous roots of elephant ears (Colcasia esculenta). 

Like cannas, elephant ears need their roots stored indoors in winter

The canna has orange blooms, but I usually cut off the flower stalks so top-heavy plants won’t tip over in a heavy wind. To fill in around the canna stems in a pot that’s located in shade, I added white-flowered impatiens. In two pots on the deck that get afternoon sun, I combined the cannas with cobalt-blue lobelia (Lobelia erinus). 

    This year I’ve had success with another conventional flower choice: a big pot full of apple-blossom pink geraniums. 

Taking a break on a geranium flower. Is it a cricket?

I’d bought three of these plants at a pesticide-free garden center two years ago. I’ve kept subsequent generations alive by taking cuttings in fall. This way I don’t have to search every spring for neonic-free geraniums.

    These are the adaptations that have allowed me to keep neonics out of my pollinator-friendly garden. I’m also happy that my peat-free potting mix is succeeding in providing sustenance for the container plantings.

Coleus thriving in potting mix made from compost and coconut fiber

Do 'nativars' count as native plants?

For the purpose of supporting biodiversity in general and native insects in particular, do we have to grow only the straight species or “wild-type” of plants that grew in our region before European settlement? What about improved versions of those plants, referred to as cultivars or “nativars”? This turns out to be a complicated question.

    Unless you’re shopping at a dedicated native plant nursery, you’re likely to find few unimproved natives at the garden center. Most plants offered as natives will be cultivars that have been selected by horticulturists for desirable traits such as bigger, brighter flowers or unusual foliage color. You can spot a cultivar by a name in single quotes: for example, Echinacea purpurea ‘Sundown’ is a purple coneflower cultivar.

'Sundown' was selected for its petals' orange tint-photo Mike Peel

    Sometimes a spontaneous mutation occurs in a plant population. An observant nursery may notice the plant’s changed appearance and decide it has market value. Other cultivars are purposely created through many generations of breeding or even through genetic modification. Some are hybrids between two species. 

     Are nativars useful in pollinator gardens? It depends. Selection for horticultural use may change flower size, color, time of bloom, or foliage color. Any of these could confuse insects seeking pollen and nectar or could mean that bloom doesn’t occur when the insects need it. 

Pollinators have evolved to synchronize their life cycle with bloom times of flowers they need

Double flowers are often sterile, because in the breeding process, pollen-carrying stamens have been replaced by extra petals. These won’t offer visiting insects any reward at all.

Echinacea 'Razzmatazz' won't help pollinators

    In general, it seems that the more closely a flower resembles the straight native’s, the more popular it will be with insects. It’s an area of active research. Doug Tallamy and Emily Baisden have found that woody plants with purple, blue or red leaves are not useful to leaf-eating insects, probably because of the anthocyanins that give leaves their pleasing colors.

Heuchera 'Amethyst Mist' is pretty, but not attractive to leaf-eaters

      Annie White intensively studied pollinator visits to test gardens of native perennials and cultivars of the same species at the University of Vermont. Delaware's Mt. Cuba Center ran another perennial trial. 

Doug Tallamy and Emily Baisden vacuum up insects for study at Mt. Cuba Center

     In general, the unimproved native flowers attracted more insects, but some cultivars were also winners. A culvers root (Veronicastrum virginicum ‘Lavendelturm’ and a garden phlox (Phlox paniculata ‘Jeana’) were visited by more insects than their straight native counterparts. Researchers speculate that this is because the culvers root bloomed for longer than the straight species and the phlox had shallower flowers that made nectar easier to reach. Hybridization made flowers less useful to insects when it radically changed flower color or reduced production of pollen and nectar.

Small, shallow flowers of Phlox 'Jeana' make nectar access easy

    For gardeners who’d like to participate, three universities are running a citizen science project called Budburst that invites us to plant some flowers from a list and contribute scheduled, structured observations of their insect visitors.

    What can we take from the preliminary research? Generally straight natives seem to do the most for insects, but many nativars are useful too. We can encourage our garden centers to stock the unimproved natives. And plant breeders can help out by selecting and marketing flowers that benefit pollinators.

What's native enough?

Because of the persuasive writing of entomologist Doug Tallamy, among others, I’ve changed my plant choices to prioritize native plants. What’s the evidence for the argument they’re needed to foster native insects? Why can’t insects just live off whatever flowers and plants they encounter?

Won't any leaf do? photo Toby Hudson

     Key to Tallamy’s call to plant natives is the assertion that most native insects are specialists that can only live off a small number of native plants. They’ve evolved over thousands of generations to be able to overcome the physiologic defenses of those plants, such as toxins that make leaves less appealing and digestible. They’ve synchronized their life cycles with those plants' and adapted to be able to sense them in their surroundings. 

     Is this true? The evolving evidence is that it’s more complicated. Some native insects really need particular native plants. We all know how monarch butterflies are suffering from decreased populations of native milkweeds, which their caterpillars need for food. 

Monarch caterpillar chewing a milkweed leaf-photo Judy Gallagher

But it turns out that some insects can cope with a broader selection of food plants, and some are already evolving to be able to live off nonnative plants. With short lifetimes, insects are able to evolve more quickly than longer-lived species such as humans.

     It’s relevant to distinguish between leaf-eating insects and pollinators. An insect that chews leaves for food may truly need its chosen plant species to survive. An insect that collects pollen and nectar may choose from a variety of plants, including some nonnatives. 

Eastern tiger swallowtail nectaring on nonnative butterfly bush

Pollinators can deal with cultivars selected from native populations for new characteristics such as different flower colors or variegated foliage.

A bee doesn't object to this black-eyed Susan's ornamental petal colors

     Tallamy and his team are refining their theory with ongoing research on which plants are attractive to insects. At the Mt. Cuba Center, a Delaware botanic garden, they’re counting insects on a list of native trees and comparing them to counts on “nativars,” selected varieties of the same species that differ from the straight natives in characteristics such as growth habit, disease resistance, foliage color, and berry size. So far they’ve found that blueberry bushes with larger berries actually attracted more insects, disease-resistant American elms drew as many insects as the unimproved species that succumbed to Dutch elm disease, and trees with ornamental purple leaves were less popular with insects than trees with green leaves.

Disease-resistant elm selections are OK for native insects-photo Msact

     This kind of painstaking species-by-species research is what will provide gardeners with meaningful planting guidelines. There are other choices to make. Do we need plants from our local ecoregion, or will other North American natives do just as well? Will we choose organic or conventionally grown plants? Locally sourced or not? Will we use clones, plants that are produced asexually through cuttings or tissue culture?

     So far I’m not feeling cramped by prioritizing native plants. There are plenty of appealing native shrubs and perennials I haven’t grown yet. I do prefer insecticide-free plants because of my concern about harm done by neonicotinoids. I’m looking forward to more information on “nativars” to guide future shopping trips.

Native purple coneflowers enliven a curb strip