Blackthorn, Prunus spinosa, is a member of the rose family (Rosaceae).
Blackthorn is a small, deciduous shrubby tree.
It grows best in moist, well drained soils and full sun.
It is native to most of Europe, the UK and Western Asia.
Blackthorn was likely introduced as an ornamental, to harvest its berries, or to harvest the wood.
Blackthorn is often referred to as “sloe”.
It has a native lookalike on PEI, Hawthorn (Crataegus spp.).
Blackthorn spines have buds growing along their length while hawthorn does not.
Blackthorn also starts producing flowers before its leaves are out, and hawthorn does not.
Here are some key features that may help to positively identify Blackthorn:
Can grow 6-7 m tall
Densely branched and spiny
Bark is dark brown, smooth
Twigs are black-purple and develop into spines
Leaves are small, narrow, oval, slightly wrinkled with a toothed edge
Leaves are pointed at the tip and tapered at the base
Flowers are white, on short stalks, and bloom singularly or in pairs
Flowers appear in March and April, before the leaves are out
Flowers mature into blue-black fruits, 1 mm across
What it does in the ecosystem
Blackthorn is considered a “pioneer species”, meaning that it has the ability to be the first colonizer of a newly disturbed area.
It can spread vegetatively, by root suckering, and by seed dispersal.
These growth properties allow the plant to spread quickly over an area, outcompeting native species for valuable resources.
The berries are eaten by birds and other wildlife, offering a means of long-distance spread.
Blackthorn is considered a large shrub or small tree.
It is distinguished by large, long thorns growing all over the plant.
These thorns are extremely sharp and can pose a hazard for humans or animals that might bump into them.
Gradually, a blackthorn infestation will expand in a circular pattern outward, creating an impenetrable hedge with a clearing in the center as older trees die off.
This can drastically alter an area’s accessibility to wildlife and cause habitat degradation for certain wildlife, although this may have a positive effect on local birds that use the thicket for cover.
Before selecting a control method, consider the size of the infestation, your available resources, and the amount of effort you are willing to expend.
Often, multiple control methods are used simultaneously. Consider the local ecosystem and what other organisms or ecological processes may be affected by management.
Management should be undertaken in the fall to protect nesting birds that may call the blackthorn home.
Due to the sharp thorns covering the plant, appropriate personal protective equipment such as safety glasses, a thick jacket and pants (to protect your limbs), and thick gloves should be worn.
Buckthorn is a heavy plant with deep roots and will require considerable labour to remove.
Cutting the tree down with a saw will remove the bulk of the plant, but will not solve the problem. Cutting and digging should be used in combination.
After cutting away the bulk of aboveground growth, carefully dig out the roots. If the roots are not removed, they will sucker and resprout.
This means digging out and removing the roots will be necessary to completely eradicate the plant.
The site will need to be monitored in subsequent seasons for regrowth after initial control to treat any regrowth from roots or seeds.
Although this remains unstudied, an extractigator may be an option for uprooting smaller plants.
Invasive species with similar biology can be controlled by girdling.
Girdling is the removal of a two-inch-wide ring of bark from the base of the trunk. This will prevent nutrient and water circulation, gradually killing the plant.
There are yet to be any available sources describing the efficacy of girdling this species.
Girdling in similar plant species sometimes causes sprouting so below the girdle, so monitoring for regrowth is important.
The PEIISC does not offer advice on chemical control methods at this time.
If using chemical controls, it is imperative that all local legislation and manufacturer’s instructions be followed during application.
After any control effort, repeated visits to the outbreak site after initial management will be necessary to kill any new sprouts and regrowth.
If management occurs before berries are formed, or if the plants have not yet matured to produce berries, the most important thing is that the roots of the plant are left in such a way that they cannot root themselves back in the soil.
The PEIISC often hangs buckthorn plants upside down by their roots from nearby trees.
Each time you manage the site, collect, bag, mark, and dispose of all berried branched properly.
Bag the branches in clear plastic bags.
Mark the bags boldly with a permanent marker. Write “INVASIVE PLANT” or “BLACKTHORN” on the bags.
If the bags are thin, double bag them to prevent accidental dispersal later on.
Tie the bags tightly and place them in your usual residential waste collection (THE BLACK BIN)
Never place invasive plants into the compost. This may allow them to become established once they reach the heap.
If you like the look of your invasive Blackthorn trees, try replanting the area with native hawthorn, which looks very similar to Blackthorn.
Blackthorn removal can even be a profitable enterprise.
Blackthorn sticks are a favourite material in the production of traditional Irish walking sticks, which are frequently sold online.
The fruits, called sloes, can be collected and used to produce a beverage known as sloe gin or autumnal sloe.
Having something to look forward to at the end of your efforts may make the work a little lighter!
Sloes also contain antioxidant and wound-healing compounds, and their extract is being studied for use in cancer treatment. Unfortunately, they are best not eaten raw as they contain trace amounts of hydrogen cyanide, which is a poison and gives them a foul flavour.