Answer: I'd be really careful about planting shrubs over a septic tank. The larger the shrub, the more extensive the root system. Two plants with relatively small root systems are azaleas and Pieris. Pieris can grow upwards of six feet tall so it might be what you're looking for. Otherwise, I'd erect a trellis or some other support system and grow vines rather than growing tall shrubs with extensive root systems.
Best wishes with your garden!
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