Description

Black Walnut (Juglans nigra) is a large deciduous tree widely distributed across southern Ontario. It can attain 35 m or more, and is a long-lived tree despite being shade intolerant. Walnuts prefer clay or loam soils, though they also thrive in sandy areas when other preferred growing conditions are optimal. Natural populations of Black Walnut exist in Canada from Grand Bend to Hamilton and all areas southward. Walnuts are especially abundant on Pelee Island, Point Pelee National Park, Rondeau Provincial Park, Skunk’s Misery (southwest Middlesex), the St. Clair River Watershed, the Thames River Valley, and the Long Point Basin. However, Black Walnuts have spread northward, aided by the clearing of the dense forests that once covered much of Southern Ontario. Once deforested, farmers planted Black Walnut as a shade tree or windbreak. The tree naturally spread into open meadows and pastures from these walnut plantations. Across North America, Black Walnut extends westward to the plains of Nebraska and Texas, south to the Florida Panhandle, east to the Atlantic Ocean.

Black Walnut is one of Ontario’s two species of native walnuts – the other is Butternut or White Walnut (Juglans cinerea). With the exception of the Kawartha Lakes Region, Grey County, and on limestone substrates of the Niagara Escarpment, Black Walnut is the more commonly observed of Ontario’s walnuts. Leaves are alternate and pinnately compound, totalling 30-60 cm in length. Each leaflet is elliptical, measuring 5-10 cm in length, and 3-5 cm in width. The largest leaflets tend to be in the middle of the leaf (unlike Butternut, where the largest leaflets are typically near the tip). The terminal leaflet in Black Walnut is often absent or very small – an important identification characteristic in the summer months. While the leaves are covered in fine hairs, they tend to be less noticeable then pubescent Butternut leaves. In the autumn, the leaves will turn a brilliant yellow, but they do not last long on the tree. Black Walnut usually defoliates earlier than other trees – in late September to mid October. The leaves do not grow back fully until June. Around 15 years of age, Black Walnut trees begin bearing fruit each autumn, readily consumed by small mammals, especially Grey Squirrels (Sciurus carolinensis). The flowers are wind pollinated, like Butternut, oaks, hickories, and beeches. Optimal fruit production begins at around 30 years old, and continues for decades, though large fluctuations occur between years, with most walnuts bourne during heavy-yielding “mast years”. Black Walnut fruits are spherical (not oval as in Butternut), measuring 4-7 cm in diameter, containing 1 hard seed. They are our largest native fruit bourne on a large tree. The bark of Black Walnut is brown or dark grey, developing deep interlacing ridges and furrows. Old Black Walnuts can reach diameters of over 1 m in diameter.

ID Features

There are several important identification features distinguishing the two walnut species apart. Butternut has oval nuts which 5 cm long and 3 cm broad (as opposed to spherical, ~6 cm walnuts for Black Walnut). Its leaves are also quite different – Butternut has 11-17 leaflets (fewer than Black Walnut), but the leaflets of Butternut are substantially larger than Black Walnut, at 5-10 cm long and 3-5 cm broad. They are also readily distinguished by their leaf scar – black walnut leaf scars are generally notched at their upper edges, whereas butternut leaf scars lack such notches. The bark of mature Butternut trees consists of grey, scaly ridges divided by large black furrows. Black Walnut has dark brown to grayish-black bark, deeply furrowed with narrow interlacing ridges.

Habitat 

Ontario’s two walnut species differ substantially in preferred habitat. Butternut is often observed in moist upland forests, often with Sugar Maple (Acer saccharum), Red Oak (Quercus rubra), White Ash (Fraxinus americana) and Basswood (Tilia americana), including limestone karst areas, such as the Niagara Escarpment. This is in wide contrast with Black Walnut – though Black Walnut is also found in moist upland forests, it is generally more abundant in riparian floodplain areas with deep, fertile soils. Black Walnut usually grows with Hackberry (Celtis occidentalis), White Elm (Ulmus americana), Slippery Elm (Ulmus rubra), Green Ash (Fraxinus pennsylvanica), Burr Oak (Quercus macrocarpa). On slighter drier riparian sites, Black Walnut is found with Black Maple (Acer nigrum), Basswood, Blue Ash (Fraxinus quadrangulata), White Ash (Fraxinus americana), Red Oak, Bitternut Hickory (Carya cordiformis), and Shagbark Hickory (Carya ovata). Understories of Black Walnut forests are diverse, but often contain a variety of shade-tolerant shrubs and wildflowers, such as Bladdernut (Staphylea trifolia), Blue-beech (Carpinus caroliniana), Pagoda Dogwood (Cornus alternifolia), American Hazel (Corylus americana), and in more sunny areas, Black Elderberry (Sambucus canadensis) and Ninebark (Physocarpus opulifolius). They typically contain many lianas as well such as Virginia Creeper (Parthenocissus quinquefolia), Riverbank Grape (Vitis riparia), American Bittersweet (Celastrus scandens), and Virgin’s Bower (Clematis virginiana).

Fun facts

• One of the most valuable hardwoods, valued for fine furniture, cabinetry, and veneer. 
• Black Walnuts produce an allelopathic chemical called juglone which is toxic to many plants, most notably tomatoes (Solanum lycopersicum). These chemicals allow walnuts to compete more effectively against other plants. Most native plants have evolved resistance to allelochemicals produced by Black Walnut as a product of thousands of years of coevolution.
• Black Walnuts are the hardest native seed to crack. This makes these trees less utilized as a commercial walnut than introduced species, such as the Persian Walnut (Juglans regia).
• The thick, green outer husks of black walnuts contain a strong, dark brown to black dye.

 

Written by Matthew Palarchio, HBSc in Environmental Sciences, Western University.

All photos are provided by Matthew Palarchio

 

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