Rebuilding 'veggie filter'

About 2 1/2 years ago we purchased a home with koi pond in the Phoenix
area. The pond is about 800 gallons, with a few growing fish. After
rebuilding the main filter (using a variation of the 'Skippy filter'
the water has stayed clear year round (other than a problem with
string algae in the wither). Assisting this was an area at one end of
the pond containing water hyacinths, and another along part of one
side with water irises. This area is the problem.

The area with the water irises appears to be a shelf about a foot wide
and 10" below the surface of the water. There is a wall of loose set
2" x 4" x 12" cement 'bricks' on the edge of the shelf and separating
it from the main body of the pond. Apparently the area enclosed by the
wall was filled with soil and the water irises planted, or possibly
the soil was in pots weighted down so they wouldn't float.

Of course, the plants have multiplied and are pushing the wall over.
By looking at them I would say they are overcrowded and need to be
thinned. I would like to remove enough material to permit rebuilding
the wall, then replant as many of the irises as needed. Obviously, I
would like to do this without seriously disturbing the balance of the
pond. Any suggestions are welcome.

PlainBill