Have you tried the white Easy Wave Petunias, they are very prolific in my experience?

Or you can try dwarf Nicotiana, Geranium, Daisy or Dianthus.

White periwinkles might be worth considering.

hi
although we are not in the same zone, i think dwarf zinnias might do the trick- i had them in white,hot pink, yellow and orange. they also reseed beautifully and are bushy and gorgeous for a border.i haven't planted them for years save for the white (with yellow center) which i added here and there for accent.
if u choose them- pinch off the first bud so they expand!!!
clusters of allisium mite be a possiblity.
good luck,
dory
ps. if they don't have to be white, how about powder blue ageratum -lovely tufts of flowers... they also reseed profusely!

mylab- forget to mention that the reason i haven't planted them (the dwarf zinnias) for years is that they come back yearly in beautiful clumps so there is one border- all done! :))
dory

I can grow most things but one of the types of plants I don't do well with for some reason are petunias. So I substitute 4 O'Clocks (Mirabelis) for them.

Jenna

Vinca may fit the bill. There are pure white varieties as well as pure white with a red eye. They are *pure* white, just sparkling. :)

Allysum, white Pansies, Garlic Chives, German Camomile, white Cosmos, Baby's Breathe, Datura all from seed in my all-white garden bed. There are more things in my white garden, perennials like Lilies, Catnip, Daisy, Lily of the Valley, Leucojum.