A widespread, productive, and handsome mast tree, Butternuts produce edible nuts that are prized by humans and animals for their sweet taste and for the high oil content containing omega-3 fatty acids. However, the story of the Butternut is an all too familiar one in our forests, as their continued existence is threatened by an alien disease to which they have little tolerance.
Butternuts in Ontario are a Species at Risk (Endangered) due to Butternut Canker, which is caused by a fungus (Ophiognomonia clavigignenti-juglandacearum) that is non-native to North America. As the fungus takes hold of a tree, canopy branches become sparse, the trunk bark changes from grey-brown to “high contrast” grey and black highlighting the braided bark structure.
In Eastern Ontario, the Butternut Recovery Program run by the Rideau Valley Conservation Authority collects seeds from trees that appear resilient to the canker, grows them at Ferguson Forest Centre, and offers them to landowners who must protect the trees and report on their survival rate. The program’s goal is to repopulate our area with canker-tolerant trees that will hand down their hardy genes to future Butternut populations.
Butternuts have a long tap root and need deep, moist, well-drained soil, preferably with alkaline conditions (e.g. over marble). It is rarely found in the acidic Canadian Shield, except in lowland areas near streams and wetlands. Butternuts – dead and alive – are found throughout the Ottawa area wherever there are mesic, deciduous woods, growing with Sugar Maple (Acer saccharum), Bitternut Hickory (Carya cordiformis), and Beech (Fagus grandifolia). Butternuts are not long-lived although some few trees live beyond 100 years.
Butternut trees are fairly distinctive and easy to identify. However, the related Black Walnut (J. nigra), a native tree of the Carolinian forest, has been moving into Eastern Ontario and can be mistaken for Butternut.
- On bare twigs, Butternuts have a hairy ridge above the 3-lobe leaf scar with its characteristic “monkey face”. The hairy ridge is absent in Black Walnuts.
- The Butternut seed is oblong, yellow-green-brown, in 4 to 6 sections, tapering to a point. The Black Walnut seed case is round and light yellow-green when ripe, smelling of lime and bergamot especially when rubbed.
- Both trees have alternate, pinnate, compound leaves with 11 to 17 leaflets (Butternut), 8 to 23 leaflets (Black Walnut), and toothed margins. Butternuts usually retain their terminal leaflet, but it often falls off prematurely on Black Walnuts. Leaflets on Butternuts tend to be broader (up to 6 cm) than on Black Walnut (2 to 4 cm).
- Most, but not all Butternuts, have some sign of the canker on bark and twigs. Black walnuts are not affected by the canker.
- Mature Butternut bark is greyish-brown interlaced with raised ridges. Mature Black Walnut bark is similar, but more yellow-brown and the ridges are narrower and often sharper.
- Butternuts like moisture, but need good drainage. Black Walnuts can tolerate much wetter soils. As native trees of long standing in Eastern Ontario, mature Butternuts are found deep in mature woodlands where once there was an opening that allowed them to grow in sunlight. More recently arrived from the south, Black Walnuts are found on the margins of woodlands and along ditches.
- Butternuts also make allelopathic juglone, but not to the same extent as Black Walnut.
Selected photographs with location coordinates in iNaturalist: