Answer: Since it may be hot in August, it is probably safer to move them in your own car. (Don't leave it parked anywhere or you will cook your plants.) Houseplants seem to travel just fine in a car, but do protect them from intense sun and wind during the trip.
Water the perennials well the day before you dig them, then dig them with as large a rootball as possible. Set the rootball only into a plastic grocery bag (they never seem to fit neatly into pots), leaving the top open to the air. Keep them in a shady location until you can replant, and replant immediately or within a day or two of arrival. You may find a temporary nursery bed handy for this and can plant them in their permanent spots in the fall or next spring.
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