Planting in rocky soil
I recently moved into an area that is a former riverbed. Although the lawn is planted and has an existing sprinkler system, the flower beds are a mix of river rock and soil. Can I plant trees, ferns and hostas in this type of soil?
You'll want to research the types of trees you would want to plant to make sure they'll grow well in your soil type, but ferns, hostas and even ornamental grasses should do fine (I have a very similar issue on my property and I have huge hostas. The ferns, depending on the type, require a little bit better soil but it's still do-able).
These articles will give you some ideas about flowers and ornamental grass that do well in rocky soil: