Nothing... you need clear soil around your trees.

Ive sown and planted taget's for some 20 years now,
they look good help keep away verious insects and after they are removed at the end of the summer you'll have enough seed heads to never have to ever buy a pkt of these seeds ever again.

Ive cut circles around the base of all my fruit trees (34 in all) and removed the grass so im left with soil only

You now have the means for apply manure to this area over the winter along with mulch, during the summer flowering season as you feed the flowers your also feeding the fruit tree,
Plus by keeping grass/weeds away from the base of the tree your helping to stop insects nesting in both the grass "you've removed and the trees trunk".

Many garden centrs have bulbs planted at the base of fruit trees to add that wow factor.

I myself have a steel rim from an old cart horse wheel to use to get a good circle shape,
But if you have fully grown high trees you can use a piece of hose pipe cut to the size of a circle you want and by putting a piece of cane to hold the two ends together you can still make a round shape for cutting out the grass etc.