Form over function: the debate over nativars and cultivars

The lure of the ‘tweaked’ plant. The one that we like because it is bigger, brighter and showier than the original. These ‘improvements’ showcase our home and impress our neighbours, but do they work for all the others with whom we share our outdoors space? Bees, butterflies, birds and all creatures which have an ever-shrinking choice of places to call home and make a living in. And whose survival becomes ever more precarious as they try to cope with the stress of pollution, the vagaries of weather and decisions that take no account of their needs.

For some, the effort to find a host plant or enough food to feed their young has just been too hard. In Ontario, native bee species that were widespread are now a rarity. Native wildflowers that once brought colour to rural landscapes are gone as even the hedgerows that were their refuge are being grubbed up. The swallows that would line up on the hydro wires at this time of year in anticipation of migration are seldom seen.

Gardeners to the rescue! Be the refuge. A space, no matter how small, if planted to at least 70 per cent native plants, can become a community and contribute ecosystem benefits, and may even provide a vital link to ensure a species’ survival.

So much depends on the choices we make. Gardeners are recognizing this and native plants are newly fashionable. The trend has prompted growers to make the natives that much more alluring by selecting and breeding for the traits that will attract buyers, like variegated foliage, disease resistance or compactness. The result is a cultivar - or “nativar,” as they’ve been called. Do these plants provide the same services (hosting, nutrient content, accessibility of pollen and nectar)as the original “straight species” plant? There isn't a complete answer but in most cases, they don't.

Let’s first remember why we grow native plants. In short, it is to support insect populations that are the foundation of our garden ecosystems. Insects tend to be specialists, feeding only on “host” plants with which they share an evolutionary history. That’s because plants erect defences (toxins, repellants and more) that over millennia insects learn to digest or overcome. Introduced species seldom support native insect populations.

Going a step up the tree of life to birds, even those we know to be seed-eaters feed their young with insect larvae, the easiest and most protein-rich of which are caterpillars. Native moths and butterflies have specific native host plants - and if those plants have been replaced on the landscape by trees, shrubs and perennials introduced from other continents, there will be no caterpillars and nestlings will starve.

So the question for native plant enthusiasts is, will these nativars support our insects? The tweaking has been done to make the plant look attractive to us, not to make it easier for our bees to collect lots of pollen, or so caterpillars can get more of the nutrients they need. Often, our pollinators will tell us in no uncertain terms which plants have been tweaked too far - ignoring the flashy fashion icon and congregating around the quietly beguiling native.

Summerina® Brown Echibeckia™ is a hybrid cross between Purple Coneflower and Black-eyed Susan, featuring the “fast growth of Rudbeckia, plus the vigour and disease tolerance of Echinacea,” according to one promotional blurb. A friend who planted Echibeckia, says it looked fabulous - to her. Not to the pollinators that stayed away in droves. So she banished the plant for not doing its job.

I think the key question is how far the breeding has taken the plant from its origins. Cultivars can occur in nature, and one such is the Phlox paniculata cultivar ‘Jeana’ that was found naturally occurring in Tennessee, and proved to be more resistant to downy mildew than the straight species.

Keith Nevison of the University of Delaware studied insect attraction to different phloxes and their cultivars and was surprised to find that ‘Jeana’ was the most attractive plant in the trial. In an article for Fine Gardening, Nevison’s advice is that wild-derived cultivars may be ok, but gardeners should plant the straight species as well as the cultivar. “With a straight species, you can be certain that pollinators will respond to it. A cultivar, on the other hand, might or might not be better, but more research needs to be done.”

Many are uncompromising on the issue, among them Prairie Moon Nursery of Minnesota. In an article entitled Our Stand on Cultivars, Prairie Moon states: “Changes in blossom size and colour can confuse or deprive nectaring and pollinating insects. Many cultivars are sterile, depriving wildlife of winter seed sources. Vegetative propagation produces identical clones, depriving the plant community of the genetic diversity and flexibility that should be its strength.”

Justin Wheeler of the Xerces Society makes the same points in a blog on cultivars: “Many of these cultivars are sterile and have no benefit to pollinators. Others have flower structures so complex a pollinator couldn’t find its way to the centre with a map, a compass, and a native guide.”

As vital as the pollination issue is what happens when a plant’s leaf is bred to a new colour, because the leaf is where the all-important task of growing the next generation of moths and butterflies takes place.

In the interest of supporting the natural life cycle, the diligent gardener chooses a native shrub like Ninebark (Physocarpus spp). In the interest of enhancing the spectacle, a cultivar with interesting coloured foliage is selected. Unfortunately, the Xerces blog points out, dark red foliage is high in anthocyanin, a compound which may be toxic to herbivorous insects. The bottom line, Wheeler concludes, is that “when you can get your hands on the straight species, you’re always going to have the best possible plant for pollinators.”

Last word goes to Doug Tallamy, University of Delaware entomologist and champion of the planet-saving importanceof insects. In his latest book, Nature’s Best Hope, he cites two reasons for shunning cultivars. The first is that most cultivars are propagated in the nursery industry by cloning, not by seed. “In the age of climate change and highly variable weather, loading our landscape with plants that do not have the evolutionary mechanism to adapt (genetic variability) makes little sense.”

His second objection is that “offering only cultivars for sale perpetuates the notion that plants are simply decorations and how they interact with other species is irrelevant. I would love to see straight species sold alongside their cultivars so that people who value function over aesthetics have the option to buy these plants."

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