I hadn't realized these plants were in pots when I read this the other day. If you are able to wipe each leaf, as described with a toothpick, a cotton ball moistened with rubbing alcohol might be more effective. The bottom of the leaves and the crotches made where leaves join stem are where they like to be. When the plants have time to dry before the sun hits them would be best.
For long-term blooms, especially in pots, temperate perennials really aren't the ideal plants for the job. If you can find Angelonia (with the annuals where you are,) they may have both colors, comes in purple, white, (and pink.) Loves to be dry, doesn't mind being in a pot, will bloom until it gets cold w/o deadheading.
