Planting above rainwater Soakaway

Hi
First post and I am pretty new to gardening so any help would be appreciated 

We are having to have a new soakaway dug and to cut a long story short the easiest place to put one is in this circular bed in our front patio.

The current plant (I think its a castor oil or false castor oil plant?) will need to be removed. Our builder says anything with a shallow root system will be fine to plant on top…so any suggestions!?
Id like a large shrub/brush and for it to be a least 1m tall (pref around 1.5m tall) and flowering, it MUST have a shallow root system…I was thinking maybe a Hydrangea (if so suggestions on variety) but would welcome any other suggestions.
The bed is quite big…around 2.5m across and I'd like little or low maintenance if possible?
Thanks

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Posts
i am not sure how deep it will be, I think he mentioned it will be covered in some kind of material....
i wanted shallow roots roots to avoid any future problems...
I wanted shallow roots as I didn’t want to have a problem in 10/15 years time.
Also I wouldn't take advice on plants from any builder I have met, although you may have found an exception!!