Dave, as colonies grow in size they tend to become more defensive, but that doesn't mean they will become a problem, but you never know.
Some colonies let you mow right next to them, others don't like it, or weed eaters, can't tell you what they're gonna do for sure, as hives have different personalities.
Seal the colonies before you spray, I would say at night or before daylight and do the spraying in the morning on a sunny day, so the sun can dry the product you use, after the sun has done its thing, I would then open the hives and let them out.
Bare in mind that bees are very sensitive to chemicals, so please bare this in mind when you choose one, as environmentally friendly for the bees and you of course. And read the label carefully.
And start with two hives, this way you can have resources from a strong hive to bolster the strength of a weak one. Who knows, you may wind up with two strong ones but it sure is nice to have the resources if you need them.
Glad you stopped by, and welcome to the world of beekeeping!!