We'd love to have some marionberries growing along the southern side of our house which gets great sun. I've heard horror stories of blackberries taking over and nearly impossible to get rid of. So, here are my questions: Are marionberries as aggressive? Will they have potential to damage my foundation if grown within a few feet? if unpicked berries drop, is it likely they will become new plants? Will they spread over time or remain pretty compact?
Some blackberry plants or species are indeed invasive. The main one in Oregon is the Himalaya blackberry (Rubus armeniacus) which was introduced into Oregon from Europe in the late 1800s. It will spread via seeds and buds on roots. It is very difficult to eradicate as a result.
The good news is that our commercially cultivated blackberry varieties, including Marion, are not at all invasive. These trailing types of blackberry (see the cultivar publication at Blackberry Cultivars for Oregon and the how to grow publication at Growing Blackberries in Your Home Garden) are not at all invasive because they do not readily produce new plants from buds on the roots (only from the plant base) and do not germinate very easily from seed. They will "tip root" where the tip of a cane left on the ground will form a new plantlet in autumn, but this is relatively easy to prevent by proper training/pruning.
Note that we have grown Marion in Oregon since 1956 and there are no "wild" Marion to be found. This type of blackberry is thus very safely grown near your house/foundation.