On planting a Yellowwood tree

As an inveterate collector of all possibly winter hardy East coast native shrubs and trees, I take a rather expansive view of the term “native”; anything goes as long as it grows along the East coast. After I killed those impenetrable thickets of Asiatic invasive shrubs and vines which surrounded our property, I suddenly found myself with plenty of open planting space.

That’s when, a few years ago, I also planted a Yellowwood tree, (Cladastris kentukea). It is a rare, medium-sized tree in the legume family—spectacular when in bloom and golden yellow in fall. In the wild, it has a very disjointed distribution in southeastern states, yet a large specimen, obviously once part of a long-gone garden, has now become part of the woods bordering Route 4 on its highest point between Sharon and Cornwall.

It has always intrigued me that so many species, whose native ranges today are much further south, grow so well here. Besides Yellowwood, that includes, for example, the Bald Cypress (Taxodium distichum) of southern swamps whose natural range reaches into southern New Jersey. However, it also grows in Litchfield hills gardens and, incongruously, even as a street tree in New York. Among others are PawPaw (Asimia triloba), Carolina silverbell (Halesia carolina), American snowbell (Styrax americanus), Redbud (Cercis canadensis), and Sourwood (Oxydendron arboreum), a good-sized tree with white flowering racemes and outstanding brilliant, deep red fall foliage. Besides our local swamp and Pinxter azaleas, others that are very hardy here include the early-flowering Pinkshell azalea (Rhododendron vaseyi) of the North Carolina mountains and from the mountains of Georgia, the rare-in-the-wild, July-flowering Plumleaf azalea, (R. prunifolium). Robust and larger in habit and most stunning of all is the Flame azalea (R. calendulaceum), which historically grew as far north as southeast New York, where it is now extirpated.

After some research, I learned that Cladastris kentukea might have grown over a much wider and more northern area, but during the last glaciation episode, it survived in the wild only in those isolated southern locations. That it survived at all so close to the glaciers edge is in itself an indication of cold hardiness. It is estimated, based on fossil plant and pollen records, that with the retreat of the icecap around 16,000 years ago, trees migrated again northward at a rate of approximately 50 km per century. This may apply to prolific sprouters such as maples, ashes and oaks, but it could be much slower, I suspect, for the many rarer plants which don’t get around by wind, squirrels, and birds, or are—unlike our common trees—finnicky to sprout from seeds. I can also imagine how many of the rarer trees and shrubs, for example Fringetree (Chionanthus virginicus), became collateral damage during the 19th century clearcutting for charcoal and farming. In any case, Yellowwood didn’t manage to recolonize its former range, and a similar scenario may apply to those other woody plants with a more southerly distribution today but that are perfectly hardy up here.

Climate change, which used to occur over millennia, now seems to happen within half a century. Birds, like the Carolina wren, can adapt quickly and become abundant after just a few warm winters—but plants are stuck. They need help to keep up with such rapid change. In the case of trees and shrubs, human gardeners can assist them in expanding their ranges into areas that are now warm enough for them to live thrive again.

I bought a sapling Water tupelo (Nyssa aquatica), native to southern swamps, from a Missouri nursery. It has survived two winters so far—albeit quite mild ones—in a woody swamp here at 1100ft elevation. One hundred years from now, who knows—ugar maples, which thrive in cold, snowy winters, might be replaced with sweetgum (Liquidambar styraciflua), a more southern hardwood.

I found space for trees by clearing thickets of invasives. The modern trend of transforming excessively large lawns into natural meadows also provides an opportunity to plant native trees with an understory of shrubs and ferns, whether as a stand-alone copse or a forest edge. This is not meant to replace flowers and grasses but to complement them, based on the understanding that, in our region, the primary ecosystem is forest—an exceptionally species-rich one to boot, with over a half dozen oaks alone.

Compared to watching grass grow, watching a tree—any tree—grow is almost exciting, given a bit of patience. Early on, protection against deer is necessary, but later, maintenance is relatively minimal; the leaves can simply fall and stay where they may.

Fritz Mueller lives in Sharon.