Food for thought

 This week my garden club got together on Zoom to watch a video of a talk by Doug Tallamy. I’ve been a Tallamy follower since 2011, when his book Bringing Nature Home changed my gardening philosophy. Since then I’ve been working on growing more native plants.

 

Insectary bed: flowers and foliage for pollinators and other insects

    In 2019 Tallamy published a new, more prescriptive book, Nature’s Best Hope. He’s marshaling us to rescue biodiversity by creating Homegrown National Park, a combined nationwide habitat for native insects in our backyards. 

    Tallamy demonstrates how we can get the most environmental bang from our native plantings by prioritizing caterpillar host plants. Caterpillars provide the lion’s share of the biomass birds need to feed their young. 

Hummingbirds feeding insects to their chick

Tallamy has identified more than 800 caterpillar species on his 10-acre property in Pennsylvania as he and his wife have replaced nonnative invasive plants with natives (check out his stunning photos of some of them in the new book).

    A light bulb went on for me when Tallamy mentioned that people occasionally write him to point out that an imported species—the Asian ginkgo tree (Ginkgo biloba), for example--used to be native to North America in a previous geological era and claim it’s therefore entitled to native plant status. That doesn’t matter, he said. What’s important is whether a plant supports any native insects. Ginkgoes don’t.

 

Ginkgo is a beautiful tree from Asia-photo Sunroofguy

 
    We’ve come a long way since concern about supporting insects first surfaced. I remember reading in the 1980s about designing a butterfly garden. That was when we planted butterfly bush (Buddleia spp.), which is now out of favor because it’s nonnative and potentially invasive. Butterfly bush and many other flowering imports do serve a purpose, though, providing nectar for bees, butterflies and moths in their adult forms.

Eastern Tiger swallowtail nectaring on a butterfly bush

    As I became more aware of my garden as an ecosystem, I started planting native flowers that provide the right pollen for native pollinators, especially bees. Now we can take the next step and choose to plant for native caterpillars. These aren’t looking for flowers. While adult pollinators can afford to sip nectar unselectively, caterpillars need to eat the right leaves to be able to grow and pupate.

      Because of decades of research in this area, we now know that about 90 percent of insects specialize: they’ve evolved to live off a limited group of plants. Most caterpillars aren’t equipped to deal with the chemical defenses mounted by plant species outside their small group of native host plants.

 

Monarch caterpillars need to eat milkweed

 
    So with data accumulating about which plants host which native insects, we can stop fighting about whether gardeners should plant only native plants. Instead, we can ask which plants host the most insects. Tallamy proposes that we prioritize keystone plants, such as native oaks, willows, cherries, and goldenrods, that provide food for hundreds of species of native caterpillars. How to find these? You can go to the websites of the National Wildlife Federation and the Audubon Society and get lists for your zip code. Somehow this prescription seems much more doable.

 

Native red oak supports more insects than any other local tree-photo Jason Hollinger

 

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.