The Black Walnut

Sophie Nitoslawski

2020-09-07

Your first thought when hearing the word “walnut” might be an expensive piece of furniture, or that upgrade on the kitchen countertop you splurged on during home renovations. The wood of the black walnut tree has been used for many purposes, including furniture, cabinetry, veneer, paneling, and gun stocks. Hard and dense, it is in high demand due to its resilience and rich brown colouring. Interestingly, black walnut is known to woodworkers as a sensitizer; the more often one is exposed to the fine particles as the wood is worked on, the more one is sensitive to allergic reactions and irritations.

Thankfully, this problem does not arise during a walnut’s life as an urban tree. Black walnuts (Juglans nigra L.), native to southwestern Ontario, are widely cultivated ornamental trees and are planted in the streets of Halifax. Similar to its cousin, the butternut (Juglans cinerea), the walnut’s leaves are pinnately compound, containing anywhere from 14-22 leaflets. The walnut is part of the larger plant family Juglandaceae, which includes all walnut and hickory trees. If you’re wondering how to identify a walnut, look for dark thickly ridged bark, smooth round fruit, and a smaller terminal leaflet compared to the rest. The flesh of the fruit, when removed from its hard exterior, bears a stunning resemblance to the brain. The walnut is known for its role in the theory of the “doctrine of signatures”, promoted by famous physicians of ancient Greece such as Dioscorides and Galen. According to the doctrine, herbs and fruit resembling certain body parts are useful for treating ailments of those body parts. Although this theory is now understood to be pseudoscience, as it turns out, walnuts may actually offer important benefits for brain health – studies have found that English walnuts (Juglans regia L.) are rich in antioxidants and “healthy” fats.

Compared to tree populations that have been severely affected by pests and diseases in parts of Canada, such as ash (emerald ash borer), beech (beech bark disease) and elm (Dutch elm disease), the black walnut has persisted relatively unscathed. However, thousand cankers disease, caused by an invasive fungus carried by the walnut twig beetle (Pityophthorus juglandis) has been found in New England, which could threaten native black walnut populations as well as nursery stock for city plantings. Thus far, Canadian walnut populations have been spared.

One wouldn’t want to be a tree competing for space close to a black walnut! They are allelopathic, which means that they release a chemical known as juglone from their roots to prevent other plants and trees from growing there and stealing their precious resources. Similar to the well-known red and silver maples, black walnut is a pioneer species and is considered a common weed in some areas of its native range. In addition to its trusty juglone, it is fast-growing and enlists the help of squirrels and other rodents for seed dispersal – you may have noticed red squirrels burying (and maybe even forgetting?) nuts in the ground for a quick snack at a later time. As a planted ornamental species along streets, walnuts provide much-needed canopy and shade during sunnier, warmer months, with a round crown shape that can spread quite wide if given the space. Black walnut may find itself right at home in Halifax, where familiar trees from its native range are also planted in the streets, such as tulip tree (Liriodendron tulipifera), basswood (Tilia americana), sugar maple (Acer saccharum) and oak (Quercus spp.).

Although it is mainly cultivated for aesthetic and ornamental purposes these days, the rich ecological and cultural history of the black walnut lives on in our cities. Next time you pass by a walnut tree (which you could do at the corner of Henry St. and University Ave., as per the photo below; the tree was planted in September 2013) , think of its incredible defense mechanism, the strange yet delicious brain-fruit, the squirrels burying (and not always finding) its seeds, and its resilience in the face of ever-changing landscapes.

Photo 1: https://www.forestag.com/products/black-walnut-juglans-nigra?variant=11891268868

Photo 2: https://www.britishhardwoods.co.uk/planed-timber-american-black-walnut-wood.html

Photo 3: https://www.starkbros.com/products/nut-trees/walnut-trees/stark-black-gem-walnut

Photo 4: https://www.cbc.ca/news/canada/new-brunswick/black-walnut-fredericton-1.5010260 (Arielle Demerchant)