Old Bramley apple tree (2), underplanting to reduce vigour

[fishpond]

As it appears I have to underplant an old Bramley apple tree to reduce the trees vigour.
Any suggestions
besides:-
Weeds--no
Grass ---no
Bluebells---no
Mint is an idea but I would need to contain it.
I have poached egg flowers, also Boking 14 Comfrey(even though it now appears to self seed) in enough quantity to fill it by 2016.
Ideally something to remove excess nitrogen whilst being useful. (natural/organic)
How about a wild seed mix?--just a thought.
I actually like the thought of a wild seed mix.
Any suggestions?

[FB.]

What will grow well can depend on your soil type and the subsoil type. Some plants prefer acid soil, some neutral and some alkaline. Go with something that likes your soil type.

[fishpond]

chalk clay, alkaline.