We have a deciduous tree with thorns and palmate leaves which volunteered in our yard a number of years ago. A neighbor who is an arborist saw me trying to dig it up when it was small and said "Keep it, it's a Hercules Club." But it is definitely not Aralia spinosa. The leaves are palmate, with 9 lobes, are very large, and have very long stems. They grow alternately, often, but not always, in clusters from a single node, and appear on the trunk starting at the ground, as well as on the branches. The few branches off from the trunk arise from a common height on the trunk. The thorns grow in rows up the trunk and stem look like rose thorns. This tree has grown very fast - in a few years it has reached 15-20 feet. I have never seen it grow flowers or fruit, nor shoots. It is currently in the diminishing shade of a dying hemlock tree.
Is it possibly Devil's Club (Oplopanax horridus, Araliaceae; syn. Echinopanax horridus, Fatsia horrida)
It's not a shrub. It's definitely a tree. The thorns are like a domestic rose - the pictures I saw of Devil's Club look more like rosa rugosa.
Kalopanax septemlobus (syn. K. pictus). Tree to 25m tall, starting to show up on invasive species lists in your part of the world.
Thanks! I found some great on-line pictures of young castor aralia at <http://www.weedalogue.com/castoraralia/> and it looks just like my tree. Evidently its growth habit is quite different as it matures.
