What type of flower or flowering bush/shrub can I plant in order to block access to a retaining wall?

Our insurance company told us we need to block access to a 10ft high retaining wall, so that people on our property can't get near the edge & fall off. We need something that is substantial enough to meet their requirements, but we only have a 7 ft long by 4 ft wide space to grow in. We live in growing zone 5. We have the option of putting a fence there, but I would rather have a natural barrier.