Few gardeners and designers have a full understanding of how plants are selected and produced for their landscapes. Plant selection is a core component of both food production and environmentally-friendly gardening, conversations around it are often politically and emotionally charged. Bring on the trigger words. Native. Hybrid. Heirloom. Enough to make the prudent run screaming in the opposite direction.
We’ve all seen the colorful catalogues and message board discussions claiming that hybridization and plant selection are the devil, a detriment to the environment and deeply disrespectful to god. After all, somebody’s to blame for the squat purple-leaf shrubs we see in every box store parking lot and the horrid mounds of annuals you see out front of golf courses. Plus the yearly round of pimped-out double echinaceas and weirdly colored heucheras.
Besides, we’ve all met the crazed hobbyist hybridizer. The bearded iris fiend, or rose fanatic, or daylily fancier. The numbers are staggering. The American Iris Society listed 30,000 registered iris cultivars in 2010. (that's Caroline Dormon's Louisiana Iris collection at Briarwood in the photo above, not quite 30,000 cultivars) 33,000 rose cultivars registered with the American Rose Society. 83,543 registered cultivars of daylilies on the American Hemerocallis Society Database. Nobody needs more than 83 daylilies. Much less 83,000. At least we’ll have a variety of daylily buds to eat in an apocalypse. (Plus the half-bag of dried daylily buds languishing at the back of my cupboard - romantically labelled “golden needles”)
So how do we actually go about defining these thousands upon thousands of plant selections. Most of us know genus and species. That’s the taxonomists’ area - and then once we’ve got used to a name, they change it up again. But plants are differentiated with varying degrees of specificity. It’s within the broad characteristics of genus and species that the real fun begins.
At the broadest level of selection, seeds can be collected from representative populations of a species - maybe wild-collected, maybe cultivated (photo above is of a white Rhododendron canescens that Caroline Dormon selected from a wild white population). Ecological restoration projects, and large-scale naturalistic landscape designs, often use open-pollinated seeds from regional populations in order to establish plantings that are both genetically diverse and physically adapted to regional conditions. You get a diversity of physical characteristics this way, depending on the species - variable heights, bloom times, colors.
Designers and gardeners can use the variability found within open-pollinated plants to their advantage. In From Art to Landscape, W. Gary Smith talks about using a color-variable seed strain of flame azaleas (Rhododendron austrinum) at Peirce’s Wood in Longwood Gardens. Since the plants were grown out as large specimens, Gary was able to selecting seedlings of varying shades and arrange them to create a gradient of color through the yellow to deep orange - offering a sensational effect of enhanced depth. By contrast, at the R.W. Norton gallery in Shreveport, Louisiana, an even wider array of flame azaleas are completely mixed up in giant random swaths with different colors, plant habits, and bloom times (photo above). This results in a constantly-shifting, almost kaleidoscopic effect, as plants of different colors come into and out of bloom.
At a more focused level of detail, you can collect seeds over several generations to establish seed strains that exhibit specific traits - usually height or flower color - and get a more stable population. As an example, consider the seed strains of butterfly milkweed (Asclepias tuberosa) “Hello Yellow” or tropical butterflyweed (Asclepias curassavica) “Silky Deep Red” and “Silky Gold”. With these populations, you can plant a mass - all yellow, all red - but the plants aren’t genetically identical. These changes in flower color don’t make any difference to the the monarch caterpillars that feed on the leaves. But if a gardener or garden designer wants to have a planting in a specific color range, they can get uniformity.
However, if you want to eliminate any variation in your plant selection - go asexual. Plants have this amazing ability to generate entirely new individuals from one bit of an original plant. You can cut a sprig of mint off the original plant and shove it in a pot. A few weeks later, you’ll have a brand new plant. For trickier (and more numerous) multiplication, we can do tissue culture and micropropagation. Every plant is genetically the same as the original plant (for example, if you wanted to propagate more of this very fine specimen of Rhododendron canescens var alba).
This is the method of plant selection that is most often maligned by garden naysayers. Asexual propagation is associated with so many trigger words: Cloning. Micropropagation. True Dolly the Sheep stuff. People say it’s “unnatural”. But plants disregard our human ethical concerns. Most employ their own aesexual propagation methods. Monardas send out runners beneath the soil. Honeysuckles root along their stems. Kalanchoe grow little plantlets along their leaf edges. Lilies make bulbils at each leaf junction along their stems. Plants aren’t binary - they propagate themselves any way they can, both sexually and asexually.
Too much aesexual propagation can cause problems. The Irish potato famine is the classic example. All the potatoes cultivated in Europe were derived from a very narrow gene pool susceptible to a virus. If the entire cultivated population is comprised of just a few cultivars - or even a single cultivar - the lack of genetic diversity in the population makes it highly susceptible to disease and pest stress. However, if you curate a wide diversity of plant varieties - open-pollinated seeds, seed strains, and cultivars - from different ecotypes, you’re helping maintain a wider gene base in your own garden. Studies at Sheffield University in the series Urban Domestic Gardens have offered a thorough exploration of urban garden flora and its role in supporting pollinator and other wildlife populations.
In addition to growing a wider mixture of plant selection types, gardeners can also begin to demand that breeders select for different characteristics. Right now, consumer demand has led to breeders choosing plants with bigger flowers, brighter colors and squatter habits than their parent species. Selecting for these qualities can - but doesn’t necessarily - affect a plant’s viability as food and habitat for pollinators and other species. A seedless sunflower offers no food for goldfinches. But a variety with differently colored flowers - dark red or pale yellow - may not affect the plant’s seeds at all. Through agriculture, humans have developed edible crops that offer enhanced nutritional value, are more flavorful, and are easier to grow - think of the difference between a wild carrot (Daucus carota) and those high-nutrient flavorful varieties (‘Purple Dragon’) we can grow in gardens today.
We could do something similar for “ornamentals”. Start selecting for different characteristics. No more double flowers, virulent neon colors, or blobby plants. Instead start selecting cultivars with characteristics that are beneficial to other species - berries with more abundant nutrients for birds, nectar-heavy flowers for butterflies, flower color that’s most attractive for hummingbirds. Phlox trials at Mount Cuba Center in Delaware revealed that certain cultivars were more attractive and had higher nutrient levels for pollinators than straight Phlox paniculata.
Despite the excesses which we see on box store shelves and sold out of cardboard boxes at local plant sales, don’t blame the plant breeders for those characteristics. It’s not the act of cultivation and selection itself that’s problematic. It’s that consumers buy short plants with large double brightly colored flowers. Stop buying those double rose pink impatiens and the breeders will turn their attention in other directions.