My new house came with this berry plant which a landscaper identified as raspberry. The fruits are ripening black. The taste of ripe fruit is nice, if a bit sour, not quite raspberry nor blackberry.
Which is it? Should I prune the trailing branches?
Here's a wider shot
Based on the pictures at this website and some general knowledge of my own what you have is a blackberry. Blackberries are shiny fruits with large cells and a white core just like the picture you show. By comparison, black raspberries are less shiny with smaller cells and, when fully ripe. come off the plant hollow inside, giving them another common name of 'Blackcap'.
For pruning blackberries there are two stages. The first is done in early spring and is called tip-pruning. When you are tip-pruning you cut the top few inches of the tips of the canes off. This encourages the canes to branch out more, leading to more berries. It's also a good time to clip out any canes that look unhealthy or damaged so the plant focuses it's efforts on stronger canes.
The second stage of pruning is done in summer. Once your blackberries have all been harvested the canes they just came off of are done. They will not fruit (or even grow) again next year, so this is a good time to cut them out, which will encourage the plant to send up more new canes that will eventually bear next years crop.
Heh, Dr. Frank Lang did a little talk on plants of the Pacific Northwest and noted as a joke that getting rid of blackberries in Western Oregon is nigh impossible as there is one single root that runs from Portland to Ashland and grubbing it out is a Sisyphean task.
Blackberries are manageable as a berry vine, don't let the fruit drop so they seed out. Smooth, thornless varieties are available from our local grower's coop.
What you have has been well maintained and kept in trim, enjoy the berries.
As to invasiveness, a Loganberry is pretty impressive as to the sheer growth, it will nearly run you off the property.
