On 9 Jan, 12:22, Eddy wrote:
I've been mooching about in Google this morning, and until you suggested
Gunneras it seemed that ornamental grass is the only safe option. *Stuff
I've been reading this morning has made me start to worry about the
roots of three large ornamental cherries on the edge of my
leach--plain/soakaway.

If they are within 6 metres from your soakaway you are safe - that is
the maximum planting guidance for council planting near soakaway. I'm
glad you don't take the rhodos ideas. Perhaps dwarf varieties and
azaleas could be an option, but the grasses would be so much nicer to
the eye, for the lightness they provide, elegance and the wildlife
they'd attract with their long lasting seed heads (not to mention the
low maintenance). The choices are endless, from the deschampsia,
sporobolus, corynephorus, myscanthus and the actae (imagine that one
covered of frost in winter ...) etc... to reed grass, the red one and
you could mix in echinaceas or monardas for some colour too. These
will self seeds and will replace their parents. Nice clumps will
eventually forms, like with the panicum and molinia - this could give
you the 'bulk' of planting you are after. A curvaceous bed of these in
your lawn would be lovely. HTH