Shagbark hickory (Carya ovata) develops over time – a very exciting strain that really captures attention.
Henrik Sjöman and Andrew Hirons
The hickory genus includes about 17 species – 11 native to the United States and north-eastern Mexico, while other species are found in eastern and south-eastern Asia. The species we have the most knowledge and experience of all come from the eastern United States.
Hickory is closely related to walnut (Juglans spp.) and wingnut (Pterocarya spp.). In the eastern United States, many hickory species are associated with the so-called oak-hickory forests, which are found in poorer and drier places, usually on slopes. Hickory have developed a distinct strategy to deal with these environments: they invest in a deep root system in order to increase the chances of finding water and nutrients. Under optimal soil conditions, hickory species grow a coarse taproot that anchors the tree firmly as well as providing access to those deeper soil resources. As a result, in the eastern United States, many species are considered to be very wind-resistant.
However, the strategy of investing in a few dominant roots has contributed to hickory trees being very difficult to handle within traditional nursery production. When hickory are transplanted, these ‘cables’ are usually cut, which usually results in the plant stopping growing and sometimes never getting started again. This problem has meant that hickory trees are rarely available in the trade, which in turn has contributed to them almost never occurring in public parks, despite the fact that many species have the potential for very fine long-term development in large parks and gardens. For best results, use young air-pot grown material and nurture to maturity on the planting site.
In the eastern United States, several species are extremely important for biodiversity because many animals feed on hickory nuts. For us humans, it is mainly the pecan (Carya illinoensis) that we appreciate in everything from pies to ice-cream.
In forestry, several species of hickory are highly sought after because they have a very fine and hard wood, which was previously used for golf clubs, among other things. The wood is also used to smoke meat.
In horticulture, hickory has had a relatively modest position; the closely related walnuts trees have been given a much larger space. Even in North America, home to many species, hickory is considered a forest tree that is rarely planted as an ornamental– with a certain exception for shagbark hickory (Carya ovata) which is considered exciting because of its flaky bark (see opposite). This attitude is regrettable as many species offer fine seasonal qualities. In winter, they have an airy crown that lets through a lot of sunlight, which makes them useful in places that are also used in winter and where you do not want the cold shade of a tree that is too dense. Leaf emergence in the spring is relatively late and very dramatic when the large buds burst and the compound leaves develop. In summer, they provide cool shade, and in autumn yield a fantastic golden yellow.
Leaf sprouts of bitternut hickory (Carya cordiformis).
A solitary large-leaved hickory (Carya tomentosa) in autumn colours.
Autumn colouring of large-leaved hickory (Carya tomentosa).
Shagbark hickory
Of the species grown as ornamental trees, the shagbark hickory (Carya ovata) is probably the most common because of its beautiful autumn colour and dramatic, flaky bark. In older trees, the grey-brown bark develops stiff strips up to a metre long which loosen at both ends and bend slightly outwards, like skis. The whole trunk of older trees gets a very special texture, which is what leads to the common name.
The growth pattern is initially pyramidal to conical. As it matures it transforms into an almost pillar-like habit where the top of the crown is as wide as the base. In nature, it can grow up to 30m high, while in cultivation it usually reaches 20–25m and 15–17m wide. For best development, it requires rich and moist soil conditions. The species is very sensitive to compact and poorly drained soils.
Shagbark hickory is an exciting park tree that over time becomes increasingly strong in character. In the UK, there are several examples, many of which have become large and beautiful individuals.
Dr Henrik Sjöman is a Lecturer at the Swedish University of Agriculture Sciences and a Scientific Curator at Gothenburg Botanic Garden.
Dr Andrew Hirons is a Senior Lecturer in Arboriculture and Urban Forestry at University Centre Myerscough.
This article was taken from Issue 196 Spring 2022 of the ARB Magazine, which is available to view free to members by simply logging in to the website and viewing your profile area.