Plato wrote:

we're getting conflicting responses from two landscapers that we are
talking with about redoing the front of our house. We live in
northern virginia.

"A" says that they don't plant in July-August due to the heat (90's,
high humidity in this area)

"B" says they plant all summer long, if we plant in July-August we
just need to water more often.

Who should we believe?

Theoretically speaking, B is correct. As long as you keep your plants
well watered, you should be able to plant them. However, just because
you can doesn't mean you should. It is considerably more stressful on
the plants if they go in the ground when the environment is "hostile",
and they may take longer to get established. It is more work for you,
and the probability that something will go wrong is much higher.
Landscaper B most definitely needs to have a pretty solid plant
guarantee if they are going to plant during the hotter months.

I live in NoVA as well, and generally, I do a vast majority of my
planting in Spring (starting Mid-April), reserving the Fall for putting
in bulbs. If you choose to wait till fall, it will give you the
opportunity to do a little more research on what you want to see in your
garden.

Suja