How to turn area under fruit trees into a nice meadow.

How would you go about turning the area under some mature and some brand new fruit trees into a wild flower meadow? the trees are planted on a 5m grid so there is plenty of light now that we have cleared all the self seeded plums and sycamore.

It's in zone 7b heat zone 2 (important that bit) alkaline sandy soil (8.5) the soil is quite rich being an old vegetable garden so anything planted gets big. right now it's a mixture of grasses, wood avons, wild parsnip, cow parsley, horsetail, dandilion, chives, broadleafed plantain, milk thistle, sow thistle, groundsel, pineapple weed, mugwort, tansey, field poppy, yarrow, thrift and strawberries, now none of them are an issue, but we also have nettles and creeping thistle which are not welcome since we have to walk through this area many times every day.

So my question is how do I get rid of the nettles and thistles without killing everything there? the area is about 1/2 acre in total. right now we're mowing it every couple of weeks and while mowing more often might kill the nettles/thistles it's also going to kill all the rest of the mix as well, and make something more like a traditional lawn, (though not one I would want to play croquet on)  if the area is un-mown as it was last year it comes up as a thick carpet of cow parsley, that gets to shoulder height and as that dies down the nettles come up over my head interspersed with thistles, and then some goosegrass comes along and ties the whole thing together, making a totally impassable barrier.

Good question. I've got some areas with thistle issues (Someone suggested it was "Canada Thistle" but I really don't know other than it spreads with underground runners.) What I found helped in one area was to let the grass grow tall and then pull out the thistle leaving it too shady for the thistle to compete. Unfortunately, that's a *lot* of work due to the number of areas affected, so I'd love to hear other ideas.