On Mixing Native and Non-Native

by Linda Dietrick (l.dietrick@uwinnipeg.ca), Master Gardener

Above photo – Painted lady on Zizia aurea – all photos- Linda Dietrick

When I give presentations about the New Perennialist style in landscape design, I always make a pitch for including our own prairie grasses and flowers in our landscapes. After one such talk, an audience member identified herself as a native plant enthusiast and asked: “Who would ever want to pay $50 for a peony?” I replied: “I would, and I have.” “But why?” she asked. Before I could say anything, another audience member answered for me: “Because it’s beautiful!” Exactly.

I love plants of all kinds, and the main reason I garden is to enjoy their beauty. Of course, I want to do so in way that is environmentally responsible. Indeed, having a garden that is alive with pollinators, birds, and other living things is part of the enjoyment. I believe that native plants – many of which are quite beautiful – have an important role to play in this. Yet the goal of a sustainable, living landscape does not mean we have to remove all of our non-native plants and replace them with natives.

Douglas Tallamy’s Homegrown National Park project and the Butterflyway project of the David Suzuki Foundation have found supporters across North America. Both call on us to create habitat for threatened pollinators by planting native plants. Tallamy, an entomologist at the University of Delaware, has argued that where alien exotic plants have reduced diversity and displaced regional natives, this sharply impacts insect populations, especially lepidoptera. As we know from the example of the monarch butterfly, their larvae or caterpillars can only feed on the foliage of certain genera or families of plants, their hosts. Since they have evolved together, the insects cannot suddenly adapt to the chemical composition of exotic replacement hosts. And since many birds depend on caterpillars to feed their chicks, the result is declining bird populations. Moreover, the flowers of native plants are probably the best sources of nectar and pollen for adult lepidoptera, native bees, and other beneficial insects, again because they have long coexisted with them.

This certainly seems convincing. However, it does not mean that we have to rip out all of the non-native plants in our gardens. Such either-or thinking overlooks the many ecological benefits of non-native plants. For example, if your garden is full of flowers from early spring until frost, your own observations will tell you that many are providing pollinators with nectar and pollen, regardless of their origins. As I will explain, we should definitely add certain native plants to our gardens, but most non-invasive garden cultivars can also provide valuable ecological services and hence do not need to be removed.


Lilac ‘Maiden’s Blush’ and Hover Fly

Tallamy’s research is specific to the mid-Atlantic region of the U.S., or in terms of ecoregions, the eastern hardwood forest. The top lepidoptera host plants there are native trees and shrubs, as they are here. The problem there is the loss of native woodland to development, and people have planted alien exotics, some of them highly invasive, that are not capable of serving as larval hosts. Tallamy’s list of worst offenders includes many plants that are not hardy here, but two of them also threaten woodland habitats in Manitoba: purple loosestrife and buckthorn. Also problematic are certain shrubs from Asia and the vast swaths of suburban lawn, which do almost nothing to support pollinators. He would like people to replace those as much as possible with native shrubs, trees, and perennials.

One thing Tallamy does not talk about is the reason why less than 1% of the original tall-grass prairie is left: agriculture. I don’t think we want to call our food crops alien exotics, but the hundreds of millions of hectares of farmland across the Central Plains of this continent are what have impacted the original prairie habitat most. The monarch butterfly is endangered because agriculture has led to the disappearance of its habitat and hosts. And so it seems plausible that, even if we can’t bring back a world before large-scale farming, gardeners can help by providing milkweed. It also seems likely that, if we know what they are, we can help other species by planting their hosts.


Asclepias incarnata and Fritillary

To find them, we can use an online tool based on Tallamy’s research: the Native Plant Finder (www.nwf.org/NativePlantFinder). Enter a zip code like 58271 for Pembina, ND and you get ranked lists of the top local native host plants for butterflies and moths. The plant genus is given with the number of supported lepidoptera species. I have combined these lists into a table.

Native Plant Hosts for Lepidoptera in Pembina, ND 58271
Adapted from Native Plant Finder (www.nwf.org/NativePlantFinder)

Notice the huge numbers of lepidoptera that use our native trees. The forbs and grasses are hosts for many fewer species. This can be explained by the fact that along with many butterflies, most moths use trees as hosts – even on the prairies. The implication for us may be that the biggest threats to our lepidoptera are things like Dutch elm disease and emerald ash borer. Clearly, we need to preserve our native host trees without using chemicals harmful to larvae, and we need to plant resistant trees that can still be hosts.

Drawing on other sources, including information from Dr. Richard Westwood, the butterfly expert at the University of Winnipeg, I’ve compiled a list of host plants for Manitoba butterflies.

Host Plants for Manitoba Butterflies

Download Host Plants for Butterflies as .PDF >

Note that I’ve put the host plants best suited for gardens, i.e. not trees or common weeds, towards the top of the list. In addition, the butterfly species are limited to the 35 that, according to Westwood, have a reasonable chance of showing up in a garden. As he explains, virtually all of Manitoba’s endangered lepidoptera require highly specialized microhabitats within larger native habitats like the Manitoba Tall Grass Prairie Preserve. Unfortunately, we can’t duplicate those in our gardens.

Nevertheless, the first dozen host plants in the list include many beautiful options for the home landscape. Try to plant more than one, so that the butterflies can find them. Once these must-have hosts for larvae are there, you can add a range of other plants to provide nectar for the adult butterflies and moths. As adults, they are much less specialized feeders, usually glad to take nectar from any number of annual and perennial flowers, not necessarily native ones.

It is a similar situation with bees. Like adult lepidoptera, most bees, including honey bees and our native bumble bees, are not fussy about where they collect nectar and pollen. However, some native bees are specialists that can only feed on certain native flowers. If we are aware of them, we can add those species to our gardens, too. Based on various sources, including input from Dr. Jason Gibbs, the bee expert at the University of Manitoba, here is a table of floral host plants for specialist bees.

Floral Host Plants for Manitoba Specialist Bees

It is not known whether bees with only one floral host will seek out that species in gardens, but it can’t hurt to offer it. Note that bees needing legumes or sunflowers may find food on farms that produce alfalfa and sunflowers. Willows are similarly abundant.

So what are the best host plants to add to our gardens? If we set aside those that are abundant – trees, common weeds, and crops – we end up with the following shortlist of garden-worthy native plants that are larval hosts for butterflies and/or floral hosts for specialist bees: alexanders, alumroot, asters, false indigo or leadplant, goldenrods, harebells, milkweed, purple prairie clover, violets, and the prairie grasses. These support most specialist pollinator needs not supported elsewhere.

Now we come to the potential benefits of non-native plants. These will usually be garden cultivars, i.e. cultivated varieties that have been selected or bred for particular characteristics. They may be natural variants, or the result of selective breeding within the species, or hybrid crosses with related species. There are many reasons to choose a cultivar, such as bigger or brighter flowers, resistance to disease, or – attractiveness to pollinators. Unfortunately, we can’t always find this out from the plant tag or online.

Nonetheless, there are a couple of characteristics that can tell us when a plant will likely not be able to support pollinators:

• Sterility, i.e. the flower produces no fertile pollen or seeds. Example: most hydrangeas, except for Quick Fire, Pinky Winky, and Lime Rickey, which Dr. Philip Ronald says have some pollen-bearing flowers.
• Double flowers. These are often the result of mutations where petals have replaced some or all of the stamens, which of course affects pollen availability.
• Purple foliage. When green leaves have been changed by breeding to purple, the anthocyanin pigments consistently deter insect feeding.
• Significantly earlier or later bloom than the native species.
A cultivar is more likely to be able to feed local pollinators if it
• is a selection of a local native. Examples:
little bluestem (Schizachyrium scoparium) ‘The Blues’
lavender-flowered culver’s root (Veronicastrum virginicum)
• is a hybrid with a prairie native in its parentage. Examples:
Morden monarda ‘Marshall’s Delight’ (parent: Monarda fistulosa)
Morden heuchera ‘Brandon Pink’ (parent: Heuchera richardsonii)
• is an eastern North American native or cultivar of the same genus. Examples:
Echinacea purpurea ‘Magnus’ (our native species is E. angustifolia)
Liatris spicata ‘Kobold’ (native species are L. punctata and L. ligulistylis)
• has plentiful, accessible pollen and nectar. Examples:
lilacs, speedwells, alliums, and many annuals like those listed on the Bee Better Manitoba website.


Echinacea purpurea and Monarch

A UK study called Plants for Bugs has found that “regardless of plant origin (native or non-native), the more flowers a garden can offer throughout the year, the greater the number of bees, hoverflies and other pollinating insects it will attract and support.” Although we have to do our own local research, broadly speaking, my observations support this.

To summarize: if we agree that habitat loss threatens native pollinators, it seems reasonable to try to help them by incorporating certain native plants into our gardens while minimizing plantings that provide little or no food and habitat. High-priority native plants are those that are exclusive larval hosts for regional lepidoptera or floral hosts for regional specialist bees, and that are not otherwise abundant.

What remains are the plants that are not exclusive hosts, but that are also not invasive or sterile exotics. That’s still quite a few. You can see for yourself if there is pollinator activity on them (let me know what you observe!). But beyond this, we can see for ourselves that all plants, including most non-native plants, have useful foliage, flowers, fruits, seeds, roots, and other living tissue that benefit wildlife as well as us plant-loving humans.

References
Allard, Simone Hébert. Manitoba Butterflies: A Field Guide. Winnipeg: Turnstone Press, 2013.
Baisden, Emily C., Douglas Tallamy, et al. “Do Cultivars of Native Plants Support Insect Herbivores?” HortTechnology 28.5 (2018): 596-606.
Bee Better Manitoba. “Non-Native Pollinator-Friendly Plants.” beebettermb.ca/non-native-plants.
David Suzuki Foundation. Butterflyway Ranger Program. davidsuzuki.org/take-action/act-locally/butterflyway/national/.
Gibbs, Jason. Personal email communication. April 27, 2021.
Gibbs, Jason. Series on “Gardening for Specialist Bees.” The Prairie Garden for 2019, 2020, and 2021.
Home Grown National Park. homegrownnationalpark.org.
Mallinger, R. E., and J. R. Prasifka. “Bee visitation rates to cultivated sunflowers increase with the amount and accessibility of nectar sugars.” Journal of Applied Entomology 141 (2017): 561-573.
National Wildlife Federation. Native Plant Finder. Online database. nwf.org/NativePlantFinder.
Narem, Diane M., and Mary Hockenberry Meyer. Gardening with Native Grasses in Cold Climate‪s‬ and A Guide to the Butterflies They Support. Minneapolis: Univ. of Minnesota Libraries Publishing, 2020.‬‬‬‬‬
Nature Manitoba. Naturescape Manitoba. Winnipeg: Manitoba Naturalists Society, 2006.
Royal Horticultural Society. Plants for Bugs. Bulletin 1 – August 2015: Gardens as Habitats for Pollinators. rhs.org.uk/plants4bugs.
Sheffield, C.S., S.D. Frier, and S. Dumesh. “The Bees (Hymenoptera: Apoidea, Apiformes) of the Prairies Ecozone with Comparisons to other Grasslands of Canada.” Chapter 11 of Biological Survey of Canada: Arthropods of Canadian Grasslands. Vol. 4, part 2. 427-467.
Tallamy, Douglas W. Bringing Nature Home. 2nd ed. Portland: Timber Press, 2009.
Westwood, Richard. Personal email communication. May 18, 2021.
White, Annie. “Flower Power: Cultivars vs. Straight Species.” Interview with Nancy Lawson. The Humane Gardener. July 27, 2017. humanegardener.com/flower-power-a-qa-with-annie-white.
Willburn, Marianne. “In Defense of Inclusive Biodiversity.” The American Gardener. March/April 2021. 40-43.