Nicotina? Night scented niccotina grows 3' and doesn't need staking and it's white... tubular flowers... pretty... volunteers pop up year after year...

Rocket series snapdragons can get to four feet and are sturdy, but lately I've had a hard time finding packs of plants in the spring in solid colors. If you're ordering seed, that shouldn't be an issue.

My favorite is Centaurea americana, Basket Flower. It is 3-4 feet tall, blooms are 4-5 inches across. Very easy to grow; birds love the fat seeds, too.
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What kind of sun is this area?
I second the idea of white rocket snapdragons and I love nicotiana sylvestris,this is the woodland nicotiana and grows to 5 feet.
A few groups of'Casa Blanca' lilies among the other annuals might be nice for August. This is a tall lily(5ft) after a year or so. ( I know this isn't an annual )
White gladiolas(bulbs)?
Acidentheras(bulbs) are nice...they are white with a dark magenta/maroon center.
I know you said annuals, but I see a nice white clematis growing up that fence...maybe in two or three places.

Not an annual, but perennial phlox 'David' is gorgeous! I added only one this year to try and am so impressed with the pure white flowers and pretty green foliage. It seems to be very resistant to mildew.

I just sowed some white zinnia seeds, Benary's Giant White, and hope they perform as well as the other Benary's Giant colors. They seem to practically germinate overnight with the rain that we've been having.

Cameron

Thanks for all of the excellent suggestions. The area is a high sun area with heat and reflectivity from the driveway, so I am a bit concerned about the nicotiana sylvestris performing well there. But..if my only expense is a seed packet, I may take a chance. I have grown it before and was impressed with its size and fragrance.

Keep your suggestions coming! Thank you!

Smaller front of the border flowers might be white petunias and/or vincas.