It’s popped up in my garden recently, and the roots are strong enough that I was unable to pull out one that was roughly 12 cm tall. The main stalk and all the branches have hard thorns. At a glance, the leaves look like Virginia creeper, but they’re definitely different. In an area of about 2 sq metres, there are now 5 of these plants of varying sizes. The largest is about half a metre tall, and the smallest is 12 cm.
Thank you for writing to the Toronto Master Gardeners. It appears that your shrub is a Fiveleaf Aralia (Eleutherococcus sieboldianus syn. Acanthopanax sieboldianus), a tough Japanese shrub that grows where little else would. For more information check out this University of Connecticut College of Agriculture, Health and Resources factsheet on Fiveleaf Aralia.