Q: I am interested in native plants as well as some garden favorites that are not native. Why does there seem to be some controversy about the use of
A: I gave the first part of my answer to this question last week, explaining that, to most gardeners, there is no real controversy. Rather, most of the perceived noise is from a few somewhat shrill voices at the extremes. Those voices are responsible for broad generalizations made in favor of those positions – not all of them factual, or rational. This week, I'll address some of those.
About native plants, it has been said we should use them because they are insect and disease-free. The truth is a bit more nuanced and goes like this: native plants have co-evolved with their native pests and diseases, and because of this long relationship, established over geologic time scales, their populations have typically achieved a dynamic equilibrium. Looked at in the short-term, this means that sometimes the individual plant wins (and withstands those maladies) and sometimes the individual pest wins. At any given moment of human observation, simultaneous evidence can be found in favor of the plants or the pests, or both. Yet in the long-term, over many human generations, each side wins enough that both the plants and their pests continue to produce enough offspring to co-exist within this shifting balance.
This makes sense: if the pests wiped out the plants, the pests – many of which are very host-specific – would themselves die out, having eliminated their hosts. That's not a winning evolutionary strategy. Furthermore, the organisms that feed on (and off of) the pests (other animals in the predatory food web, and parasites) would also die out in the absence of their food source. Robust populations of those organisms help to maintain the dynamic equilibrium and keep populations of plant pests at reasonable levels, but enough pests remain to feed so-called higher-level predators.
All native plants have their native pests, some certainly more than others. But much of the time, the plant predation is temporary, or really "just lunch," and not a potentially fatal infestation requiring human control. And, crucially, those pests add to the biodiversity that the plants support. The tiny caterpillars that feed on the newly-emerging leaves of our native oaks and hickories are essential to the lives of migrating songbirds which follow the northward progression of leaf-out in the spring (obviously keeping the insect populations in check, to underscore the previous point). Without those caterpillars – and the countless other insects that feed on plants, or the insects that eat those insects – there would be nothing for baby birds to eat. We so often think of plants that produce attractive fruit as being bird-attracting, and they are. But fruits mature late in the growing season, and so feed only adult birds. Baby birds are born in the spring and they eat mostly bugs. This is one of the central themes of Doug Tallamy's book "Bringing Nature Home," and it's an eye-opener. If you want birds, you need baby birds; and they need bugs.
Many leaf-munching caterpillars will metamorphose into butterflies, charismatic insects which most people like. If you want butterflies, you need caterpillars, which sometimes eat the leaves of the same plants that produce the flowers that the adults sip from. That's why every butterfly garden needs host plants for the larvae – the caterpillars – to eat, as well as nectar-producers for the adults. Biodiversity is based on organisms eating other organisms (or parasitizing them) and since plants (here including blue-green algae) are the only organisms that can harvest energy directly from the sun, the next step in energy transfer is organisms feeding off plants.
A counterpunch from the "natives-are-not-so-terrific-after-all" camp goes like this: if native plants were so perfect, how do you explain the demise of the American chestnut (once the dominant forest tree throughout most of eastern
Not all exotic organisms cause ecosystem meltdown. It is estimated that less than 1/100th of a percent of introduced plants have escaped to become truly invasive, a term that must be strictly reserved for the worst of the worst. Kudzu (Pueraria lobata) is the poster child for invasives in the Southeast because it spreads most readily in disturbed areas, like along highways and at the edges of other kinds of human development, exactly the places people see the most. Agricultural and aquatic weeds are far more economically significant, and weeds of turfgrass (non-native itself, and a vicious weed to some) are among our most familiar. Ecologically, invasive plants that establish themselves in undisturbed ecosystems are of far greater concern. Chinese privet (Ligustrum sinense) and Japanese stilt grass (Microstegium vimineum) are two examples of invasive exotic plants that are devastating natural areas (creating lower biodiversity) wherever they are found.
The vast majority of exotic plants are not invasive at all. Hostas and hydrangeas are not taking over our valleys and ridgetops; crape myrtles and camellias are not infesting our fields and parkland. Dr. Paul Cappiello, author and Director of Yew Dell Botanical Garden in
Cappiello and I also agree on what are the best reasons to plant native and exotic plants, which I'll address in the last part of my answer next week.
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