When the guide gets it wrong

After moving to West Cornwall in 2012, we were given a thoughtful housewarming gift: the 1997 edition of “Dirr’s Hardy Trees and Shrubs.” We were told the encyclopedic volume was the definitive gardener’s reference guide — a fact I already knew, having purchased one several months earlier at the recommendation of a gardener I admire.

At the time, we were in the thick of winter invasive removal, and I enjoyed reading and dreaming about the trees and shrubs I could plant to fill in the bare spots where the bittersweet, barberry, multiflora rose and other invasive plants had been.Years later, I purchased the 2011 edition, updated and inclusive of plants for warm climates.

On the cover of the new edition, a quote from Adrian Higgins of The Washington Post boasts, “Michael Dirr is the oracle of ornamental horticulture. I trust his judgements implicitly.”I heartily disagree with Mr. Higgins:I blame this book — and my poor use of it — for some of my worst tree and shrub choices.

I realize some readers might find this declaration inflammatory. The book still occupies a place of high regard among experienced and novice gardeners alike, so please allow me to explain.

In addition to giving the reader his opinion on the aesthetic worthiness of the woody plants included in the book, Mr. Dirr makes good on the book’s title with a review of each species’ hardiness. What makes a tree hardy?It thrives in its intended site, resisting disease with leaves and bark not readily eaten by insects and other critters.

Non-native plants make up the majority of the recommended hardy plants in the book.And here is why:Native trees and shrubs are, by evolution’s design, food source and host to our native fauna — critters large and small. There is no substitute equal to the fauna’s co-evolved flora.A native caterpillar cannot eat a kousa dogwood leaf, as it has not evolved to digest it.Non-native plants seemingly have the advantage if the lens we look through values pristine, uneaten leaves.

In the days when there were sufficient thriving ecosystems to maintain local habitats, a non-native specimen tree here and there was just fine.But where we live in Northwest Connecticut, our woods, meadows, marshes and other natural areas have, for a couple of decades, been severely compromised by invasives that have almost entirely removed the food sources for native insects. It is up to us — now — to plant native plants to save the food chain.Without insects, not only will native animals die, but human food sources will also be at risk.

The security of our food pipeline seems a worthy exchange for some caterpillar-eaten leaves — and to be clear, we’re not talking about non-native infestations such as spongy moth, but rather native caterpillars, which are the singular food source for nesting birds.

My issue is that, in being a trusted source for plant selection, Dirr’s book should give equal — if not prioritized — space to information on ecological impact.For example, it would be good to know when selecting a tree, that a native oak provides food and other ecosystem services to more than 400 native animal species, while a native tulip poplar supports fewer than 30 — though that includes the Eastern tiger swallowtail. Including information on the birds and insects attracted to a given plant would enable reader to weigh these factors in choosing what to grow.But this information is not mentioned at all.

Dirr makes no mention of the role some of these plants have played in the degradation of our natural areas — an omission that is highly relevant, as many of the plants featured in his book are, in fact, invasive culprits. Plants like barberry, porcelain berry and tree of heaven are showcased for consideration alongside native plants without recognition of the devastating infestations they can manifest. Tree of Heaven is now responsible for hosting the spotted lanternfly, which is devastating crops.

Similarly Euonymous alatus (winged euonymous) and Actinidia arguta (hardy kiwi) — two highly invasive plants touted in the book — have been banned or are close to being banned for sale from nurseries in the state of Massachusetts. To his credit, Dirr does point out the invasive nature of Ligustrum sinense (Chinese privet), calling it “a terrible and devastating escapee that terrorizes floodplains, fencerows and even open fields, reducing native vegetation to rubble.” Yet Japanese honeysuckle gets an understated warning, with Dirr describing this massively invasive shrub as “bullying their way into understory and open areas.”

The latest edition of Dirr’s book devotes seven pages of copy and photos to various Berberis species, about which Dirr waxes poetic. He notes the addition of “30 new cultivars” in the latest revision and complains that “this species is under assault for its aggressive invasive nature.” He refers to Berberis thunbergii — Japanese barberry, the most invasive of them all — as “the species of major importance in garden commerce.” This plant has already been outlawed for sale in New York, Pennsylvania, New Hamphsire and Maine.A few weeks ago, a bill was passed in Connecticut recognizing the harm of a broad group of invasive plants. Under this new legislation, barberry will be phased out from sale or transport by October 2028.

In understating the invasive nature of many non-natives and de-prioritizing the importance of native species, Dirr’s widely used reference may be partly responsible for many a devastated woodland, forest, meadow and marsh in New England — if not across the U.S.Certainly, the evolution of species, and scientific knowledge about the environment, is changing faster than new editions of books can be printed. I can only hope that if a new edition of Mr. Dirr’s reference book is in the works that it will account for this criteria we now know to be vital in plant selection.

Which brings me back to that quote on the cover from The Washington Post and the larger issue it suggests:Should “ornamental horticulture” get a pass when it comes to ecological survival?I think we can agree — it should not.The consequences are simply too destructive.

Dee Salomon ‘ungardens’ in Litchfield County.