For the vegetation for the base of the cannon I ended up using sculpt detail and paint in substance to make it look like there was moss / algea from the ocean on top of the rocks I sculpted for the environment.
Here are a few close ups of the moss detail.
Here are some of the final renders of the cannon in ue4 with the environment attached. I ended up fixing some connection issues in my material and it solved my roughness being to low:
The final images that I chose as reference for my cannon environment were the ones below:
