Many years ago a grad student and I embarked on a strip mine reclamation project near Oskaloosa, Iowa. One of our experiments was to learn what it took to grow prairie for erosion control, and we were delighted to find a seed company in Nebraska who sold a tallgrass prairie mix that contained several species of grasses and forbs (doesn’t sound like much now, but back then it was innovative, as most seed companies were clueless about prairie). The first few years I was pleased at how strong the plants came on, but then began to realize they were somewhat different from the local natives along the nearby railroad tracks. So I visited Nebraska and learned that their version of prairie had been selectively bred for decades to maximize grazing and hay, with rainfall more skimpy than our Iowa rainfall. And they had already heard from cattlemen in Iowa and Illinois that their seed was really productive where more rain was available. But I realized that even though the species were the same, this wasn’t really our native prairie.
Since then, in the course of many dozens of reclamation/restoration projects, plus numerous little test plantings at home, I’ve tried to pay attention to cultivars, selections, varieties, hybrids, etc. And have had no qualms about murdering the misfits. With the understanding that planting only natives in their appropriate communities is our goal, here is my spin on what is and is not acceptable practice for native planting.
Keep in mind that unintentional and intentional selection is common. (Suppose you hand-harvested Indiangrass seed from a genuine sand dune prairie in early October during a drought year with the intention of planting it next spring on your land across the road. You have just selected for late seeding ecotypes that are very drought tolerant, and if you are a tall person you probably stripped seed heads within easy reach and didn’t stoop over to gather much from the short ones, thereby also inadvertently selecting for tall type. Or suppose you are in a grove of pin oaks gathering acorns to plant for wildlife habitat, and the ground under one tree has an abundance of acorns while the other trees have few, you are likely to gather much of your seed from that one tree, thinking that the next generation might be better mast producers.) The real issue here is what their progeny will be after their seeds have escaped around the neighborhood. Casually selected seed, intentional or unintentional, carry as broad a mix from the ancestral gene pool as individuals not selected by you. I do not see this as a problem because every year “nature” is also selecting for and against a variety of traits, and you are a rather minor player in the game, which also includes drought, flood, frost, caterpillars, mice, wind, fire, snow cover, birds, etc.
However, plants subjected to repeated selection for the same traits over multiple generations, especially if grown in garden conditions, increasingly deviate from their ancestral gene pool. During the last 20 years, this has noticeably happened to the genus Echinacea, and garden catalogs today carry a wide selection of unusual coneflower varieties, which you can purchase and grow from seed, and this means that the next generation is likely to be able to add it’s weird gene combinations to a nearby native prairie gene pool via cross pollination. Fortunately, natural selection in a native prairie is likely to cull these garden traits back out again over subsequent generations, but it is unwise to introduce them in the first place. As an example, field corn germinated from spilled grain a year after harvest is a cultivated plant that vanishes within a few generations, even without intentionally killing it, because natural selection becomes fatal within a couple of generations. For a genus like the Echinacea, which has been artificially selected for perhaps 20 years (20 generations), the artificial traits will first become recessive and less noticeable if they reenter the wild gene pool, but may take 5, 10, or 20 generations before natural selection culls them back to the levels that existed in the ancestral population. When Paul Christianson, early leader in prairie restoration, and I were talking at a Prairie Preview in the 1980s, a man approached Paul and proudly mentioned that he had been separately growing orange, red, yellow, and white varieties of butterfly weed. Paul shook his hand, gave him a beaming smile and said, “That’s wonderful. Now stir all your seed together and plant it in with other natives.” They guy wandered off looking confused, not getting Paul’s message that a species’ strength lies in its diversity, which provides the ability to cope with steadily shifting environmental conditions.
The words “variety” and “cultivar” (cultivated variety) don’t mean much in terms of the native question. Some are simple selections of natives, given names for marketing purposes, while others are hybrids with non-natives.
Some of our natives do hybridize naturally with other closely related natives, and planting seed from intermediate all-native hybrids simply duplicates natural hybridization and is not a problem for future generations. But forcing hybridization by intentionally incorporating non-native relatives is a bad idea. For example, one reason that reed canary grass is overly aggressive is that today it is a hybrid of North American and northern European varieties – with “hybrid vigor.” This process also happens accidentally (or negligently, depending upon your perspective), because non-natives are planted within easy pollination reach of most wild areas. Over the next century, I expect that much of our native flora will become lightly contaminated with exotic genes via cross-pollination, a process which is possibly well underway already but not studied enough to evaluate. For many of our natives, cross-pollination might remain at low levels because progeny carrying many foreign genes may get whacked by local natural selection.
But the real problem is going to be alien species obtaining enough local ecotype genes to acquire the hardiness to thrive in our climate. For example, oriental bittersweet had been planted locally for a century without problems, but about twenty-five years ago it rather suddenly became aggressively invasive in northern Johnson and southern Linn County. Based on intermediate leaf shape, it appears that it has crossed with our native bittersweet just enough to become better adapted to our climatic extremes and other selecting forces, while still retaining the aggressive growth form we label oriental.
If you want to keep your plantings as native as possible and don’t have opportunity to search for local seeds and experiment, then make your purchases from upper midwestern suppliers who specialize in natives. My experience with Ion Exchange, Prairie Moon, Possibility Place, Missouri Wildflowers, and Boehm’s Garden Center has been especially good. Good luck and happy planting!