Description and habitat

Butternut (Juglans cinerea) is a deciduous tree widely distributed across southern Ontario. They prefer loam or sandy-loam soils, and especially areas with a limestone-based substrate. Though it is widely distributed, large populations are concentrated in certain areas; along the Niagara Escarpment (especially Grey County), the Kawartha Lakes Region, Simcoe County, and the Long Point Basin along Lake Erie. It also occurs southwards into Tennessee and Alabama. Butternuts are named for the oily, rich texture of the nutmeat when ingested, which resembles butter. These trees can grow up to 35 m in height, although they are often smaller.

Also termed White Walnut, Butternut is one of Ontario’s two species of native walnuts – the other is the more widely observed Black Walnut (Juglans nigra). Leaves are alternate and pinnately compound, totalling 25-40 cm in length. Around 20 years of age, Butternut trees with begin bearing fruit each autumn, readily consumed by small mammals and frugivorous birds. The flowers of butternuts are mainly wind pollinated. Similarly to many oaks, hickories and beeches, seed production is highly uneven across years – more butternuts are produced in years with favourable conditions, termed “mast years”. In other years, few or no butternuts are produced, even on mature trees. Like other walnuts, the foliage will turn yellow in the autumn. More than 100 species of lepidopterans rely on the foliage of walnuts as a larval food source, including the Bride Sphinx Moth (Catocala neogama), Walnut Sphinx Moth (Cressonia juglandis) the Distinct Quaker Moth (Achatia distincta), Sleeping Baileya (Baileya dormitans), Sad Underwing (Catocala tristis), Three-spotted Sallow (Eupsilia tristigmata), and Walnut Caterpillar Moths (Datana integerrima and D. angusii).

Identification features

There are several important features distinguishing the two walnut species apart. Butternut has oval nuts which 5 cm long and 3 cm broad (as opposed to spherical, 5 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.

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.

Butternut canker

The Butternut Tree is threatened by a non-native fungal pathogen, Butternut Canker (Ophiognomonia clavigignenti-juglandacearum). It was introduced to North America in the 1950s, most likely from northeast Asia, considering the resistance of Manchurian Walnut (J. mandshurica) and Japanese Walnut (J. ailantifolia) to the canker. Butternut Canker causes black outgrowths on the bark, which slowly weaken and kill most trees. Occasionally, a tree is resistant to the Butternut Canker, displaying no symptoms. Canker resistance rates to the butternut canker typically range between 1-15 %, typically around 5%. Resistant trees are of great scientific interest to the conservation scientists as a potential seed source for propagation, which may eventually replenish populations of healthy butternuts across eastern North America. Orchards have been established in several municipalities across southwestern Ontario, attempting to cultivate disease-resistant butternuts by concentrating genes conferring disease resistance.

Fun facts

• Butternut trees produce a sweet sap that can be tapped and made into syrup! Butternut syrup has a different, nuttier flavor compared to maple syrup.
• The presence of Butternut trees can sometimes indicate specific soil conditions, particularly those rich in calcium and with a neutral to alkaline pH, which is often associated with limestone bedrock.
• Like Black Walnut, Butternut roots and other parts of the tree contain juglone, a natural herbicide. While less potent than Black Walnut, it can still influence the growth of many garden plants, such as tomatoes.
• Butternuts are sometimes called “Oil Nuts” due to their high oil content, highlighting just how buttery these nuts are – a delicacy for both humans and wildlife!

 

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

All photos are provided by Matthew Palarchio

 

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