Sunday, November 26, 2023

Native Plants. Part 4. Cultivars

Increasingly you see advocates for planting natives adding "do not buy cultivars." 

Cultivar is defined as "a plant variety that has been produced in cultivation by selective breeding." In the context of native plants, we are talking about wild native plants that have been grown and selected for desirable characteristics, perhaps a more intensely pink flower or resistance to mildew. 

red osier dogwood with variegated leaves
dogwood cultivar with variegated leaves

Breeding changes plants. Of course. A cultivar is not genetically the same as its wild relatives. Offspring are not the same as their parents. The key for using or not using cultivars is whether the changes in the cultivar cause insects that eat leaves or gather pollen and nectar from the wild variety to avoid the cultivar. 

Do native insects avoid cultivars of their host platns? For most cultivars of most species, nobody knows. 

Recognize that this is a radical change of direction, growing plants that you hope that insects will eat. For generations we have protected our cultivated plants from insect feeders. 

goldenrod (Solidago) cultivar and pollinators
goldenrod (Solidago) cultivar and pollinators

I have been searching for publications that address any part of this question. The vast majority are studies of crop plants looking to improve fruit or flower or grain production. Usually they have a crop--a cultivar--that is attacked by insect pests and they find wild relatives that are less susceptible to the pests. So in the vast majority of these publications, cultivars are better insect food than their uncultivated relatives. (Though there is usually no distinction between native and exotic insects.)

In studies directly addressing the question of whether changes in the cultivar discourage native insects, the answer very much depended on the trait of the cultivar. Baisden et al. (see references below) compared native plant cultivars to the "straight" (unmodified) species, using eastern red cedar (Juniperus virginiana), arrowwood viburnum (Viburnum dentatum), winged sumac (Rhus copallinum), flowering dogwood (Cornus florida), sweetgum (Liquidambar styraciflua), redosier dogwood (Cornus sericea), american elm (Ulmus americana), highbush cranberry (Vaccinium corymbosum), winterberry (Ilex verticillata) and red maple(Acer rubrum), for these traits: red/purple colored leaf, variegated leaf, growth habit (dwarfing), disease resistance, fruit size and yield, and enhanced fall color. They compared insect damage to the plants as leaf-area lost, counted all the insects found on the plants, and did a feeding test with a generalist caterpillar (evergreen bagworm, Thyridopteryx ephemeraeformis). The only trait that consistently had lower insect feeding was leaf color: the insects avoided red, blue, or purple leaves. Cultivars with variegated leaves were often, but not always, eaten more by insects. The other four traits had either no difference in consumption between straight species and cultivar or the differences were inconsistent in direction and intensity.   

very purple leaves
very purple leaves

Polythress (see refs) compared plants of wild collected bluestar (Amsonia tabernaemontana), coreopsis (Coreopsis grandiflora), bee balm (Monarda fistulosa), narrow-leaved sundrops (Oenothera fruticosa), and little bluestem (Schizachyrium scoparium) with their cultivars. Plants behaved differently: wild coreopsis had more hemiterans (insects in the Hemiptera, true bugs, such as aphids, leaf  hoppers and shield bugs) while the sundrops cultivar had more. Discussing this difference, he noted that the coreopsis cultivar was quite different from the wild form (in standing more rigidly, forming clumps, and having variegated leaves). The sundrops cultivar seemed very similar to the straight plant. In both cases, though, the cultivars set no seed (were sterile), which would discourage any true bugs that feed on seeds. In another experiment, cultivars of sundrops and little bluestem both had more hemipterans than the straight species. Bee balm was the reverse, with more insects on the straight species than the cultivars.  

coreopsis flower
Coreopsis flower with bee-fly feeding. 
(No coreopsis leaves visible. I don't know how modified this commercially-purchased coreopsis is.)

People are also urging that we grow native wildflowers to provide food for pollinators. Pollinators--bees, butterflies, moths, flies, beetles, hummingbirds--forage for food on flowers. Some gather nectar (sugar water), some collect protein-rich pollen. Plants whose seeds are produced because pollinators carry pollen between flowers have evolved floral structures that facilitate that. They advertise for pollinators with colors that attract the attention of bees or butterflies. They provide landing platforms for bees to light on, and have nectar guides (bright spots that indicate where the nectar is, often colored only if you can see ultraviolet light). These are generally not the traits important to plant breeders creating cultivars that will sell well to customers. Many changes in flowers will discourage pollinators. Many changes will have no effect. It will take a number of studies to identify which traits affect pollinator attraction to flowers and which do not. And, different pollinators will react to different changes. 

Lovely rose. Where are the pollen and nectar?

For the short term, look carefully at the flower, especially in comparison to a straight flower of that species. Is the nectar still visible? Are the stamens holding the pollen still in the same relative position? Has the color changed radically? I suggest that double-petal flowers where the new row of petals cover up the nectaries or displace the stamens are likely to prevent pollinators from feeding or reduce pollination. Drastic changes in flower shape also risk discouraging pollinators. For many wildflowers, natural variation in color ranges from, for example, white to deep red, so a dramatically rust-colored cultivar isn't likely to have any effect. Green or black flowers, very rare in nature, might not be recognized as flowers. Finally, sterile cultivars are likely to fail to support insects (moths and butterflies, beetles) that feed on seeds or developing fruit. 

I conclude that some cultivars are not as good hosts to insect feeders or pollinators as their unmodified relatives, but that most changes will not have a detectable effect. Cultivars are readily purchased and many are aesthetically more pleasing to the human eye. Far better to get cultivars than non native species (a lot of publications find exotics are avoided by native insects) or to grow nothing waiting for a truly unmodified native plant. Look critically at the cultivars and avoid red leaves if the plant ordinarily has green leaves that fade to yellow and flowers where everything is concealed by petals. As more studies are completed, more information will be available to understand what changes affect insect uses of cultivars and which do not. 

hawthorn, Crataegus
hawthorn (Crataegus) cultivar: thornless
Do insect visitors care about thorns? Unknown.

Comments and corrections welcome.

Postscript: many pollinating insects--bumblebees, hummingbirds, butterflies--are opportunists. Growing lots of flowers is often as good or better than growing a few of the "right" flowers. Predictable food is important so provide numerous flowers from early spring to late fall, even exotics. --Won't that be a beautiful yard?!

See previous posts in this series: 

Grow Native Plants. Part 1. Why? link

Grow Native Plants. Part 2. Finding and Growing Native Plants link

Grow Native Plants. Part 3. Finding Native Plants to Grow, Continued. link


Baisden, E. C., D. W. Tallamy, D. L. Narango, and E. Boyle. 2018. Do cultivars of native plants support insect herbivores. HortTechnology. 28(5): 596-606. 

Poythress III, J. C. 2015. Gardening for wildlife: a comparison of native plant cultivars and wild-propagated plants as food sources for herbivorous insects. Masters Thesis. University of Georgia. online pdf 

Salisbury, A., J. Armitage, H. Bostock, J. Perry, M. Tatchett and K. Thompson. 2015 Enhancing gardens as habitats for flower-visiting aerial insects (pollinators): should we plant native or exotic species? Journal of Applied Ecology. 52:1156-1164.

Tallamy, D.W., D. L. Narango and A. B. Mitchell. 2021. Do non-native plants contribute to insect decline? Ecological Entomology. 46: 729-742.

Kathy Keeler, A Wandering Botanist

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