It was here when we moved in 15 years ago and I assume is full grown – it’s as high as our 2 storey house – has thorns, green berries that ripen to purple and drop off – no one has ever been able to figure out what type of tree it is – thanks for your help!
It appears that what you and your neighbour share on your fence might be Common buckthorn, Rhamus cathactica. This woody shrub can also grow into a 6-metre (20′) tree. For further ID, the bark becomes rough textured with corky projections, and the inner bark is yellow — it’s difficult to see this on your photo, but you might want to test the inner bark. The twigs on common buckthorn end in pointed, stout thorns that become fiercely sharp on dead twigs.
Buckthorn was first introduced in the 1880’s from Europe and Asia, primarily as an ornamental shrub. Its green berries, which turn an exotic dark-purple-black when mature, are NOT safe for human consumption, but are tolerated by birds and animals. In fact, the laxative effect of the fruit on birds helps distribute the seeds. Because of this, its seeds have been widely scattered, with bushes and trees commonly found along fence lines, woodlands and abandoned farmyards throughout southern Ontario.
Today, Common buckthorn resides on Ontario’s list of invasive species. Buckthorn’s popularity for use as hedges resulted in their taking over of many natural woodland areas. They propagate from seed readily, spread very quickly and create dense covers that shade out native plant growth. Buckthorn roots also exude chemicals that can harm nearby plant and animal species. Hence, once established, natural areas can become almost exclusively buckthorn. But what really landed this species on Ontario’s noxious weeds list, is the fact that it can carry the fungus that causes damaging rust on the leaves and crowns of oat crops. More recent concerns, again, in the agricultural community, are soybean aphids, which lay their eggs on buckthorn and use it as an overwintering host plant.
It doesn’t appear that your solitary backyard buckthorn poses an immediate threat to nearby agricultural crops, and you and your neighbours likely appreciate that your tree offers safe haven and food for the birds. Yet Toronto Master Gardeners do take our provincial recommendations seriously and can make alternate suggestions in situations such as this, should you wish to replace the tree.
Any one or more of these suggested flowering/fruiting shrubs, that are native to Ontario, would be a super addition to any garden:
1. Viburnums (V. nadum cassinoides, V. opulus americanum, V. lopulus V. lantanoides )
2. Dogwoods (C.amomum, C.racemosa, C.rugosa)
3. Serviceberry (Amelanchier laevis )
These native replacements would still provide a valuable food source for birds, and would definitely add beauty to your yard when they flower. Here is a TMG guide that might be very helpful.
Also, for further reading, here’s the link to Ontario’s Ministry of Agriculture on the subject: https://www.omafra.gov.on.ca/english/crops/facts/ontweeds/european_buckthorn.htm