Walnuts and murder and chemistry, oh my.
by Ray Burks
In honor of World Book Night
**********
A rose bush planted beside a headstone looked in need of watering, its leaves droopy and yellowing. Pelletier followed Gamache’s gaze.
“Thought that might happen soon. Tried to warn the family when they planted it.”
“Roses don’t grow well here?” Gamache asked.
“Not now. Nothing’ll grow now. It’s twenty-five years, you know.”
Beauvoir wondered whether decades of snorting cement dust hadn’t done something to this man’s brain.
“What is?” asked Gamache.
“This tree. It’s a black walnut.” The sculptor dragged his hammer hand over the furrowed bark. “It’s twenty-five years old.”
“So?” asked Beauvoir, hoping to get to the point.
“Well, nothing grows around a black walnut once it gets that old.”
Gamache reached out and touched the tree too. “Why not?”
“Dunno. Something poisonous drops from its leaves or bark or something. But it’s fine until it’s twenty-five. Only kills things after that.”
Gamache removed his hand from the grayish trunk and returned his gaze to the cemetery, the sun dappling through the leaves of the killer tree.
~A Rule Against Murder, by Louise Penny
Two-hundred-sixty-three pages into book 4 of Louise Penny‘s Chief Inspector Gamache mystery series and the reader reaches the same conclusion as Gamache. His team of Sûreté du Québec officers will have to look beyond the various present suspects and recent wrongs to solve a murder. Investigators will have to unearth every deep, dark secret – no matter how long buried.
Over a hundred pages after Gamache learns of the black walnut tree’s toxicity, readers learn an old wrong-doing is at the root of the murder. While the killer took decades to strike in A Rule Against Murder, the “killer tree” can be lethal in less time.
The black walnut tree (Juglans nigra L.) is one of the many allelopathic trees in the plant kingdom. ‘Allelopathy’ is plant chemical warfare, with the black walnut being perhaps most famous and well-studied deciduous warlord. The black walnut tree’s weapon of choice is juglone, which circulates within a living black walnut tree as harmless hydrojuglone – a chemical found in nearly every part of the tree. Once exposed to the outside world, hydrojuglone is easily oxidized to the harmful juglone. This chemical turns the soil surrounding a the black walnut into dangerous ground for a wide variety of plants. In sensitive plants, juglone disables respiratory and energy-producing functions, leading to “walnut wilt” – the wilting and stunting Gamache noticed with the headstone’s rose bush.
As with humans, it’s the dose that makes the poison. It can take a black walnut tree 25 years to introduce enough juglone into the surrounding soil to cause “walnut wilt” or death in sensitive plants – but less than 10 years can do the trick in some cases. This wide window of lethality results from many factors, such as rate of tree growth, soil and drainage conditions, and diversity of soil microorganisms.
Plants sensitive to juglone include tomatoes, peppers, asparagus, potatoes, peony, rhubard, blueberry, hydrangea, lilac, white birch, white and red pine… the list goes on and on. Juglone’s long list of targets often results in a gardening tip: avoid planting a garden with 50-80 feet a black walnut tree! While the list of juglone victims is long, so is the list of juglone-tolerant plants, including some types of roses.
The black walnut tree is lethal to some, but not to others. It may strike-out quickly or take years to kill. This “killer tree” can stand-in for both secrets and suspects, making it a perfect murderous metaphor.
[A Rule Against Murder book cover image from Louise Penny.com. Structures of juglone and hydrojuglone from Chemspider.com. Thumbnail and front page image of black walnut tree by Jean-Pol Grandmont, via Wikimedia Commons, Creative Commons Attribution-Share Alike 3.0 license.]