Late summer is a fantastic time to shop for plant bargains but it's not a good time to plant them. To take advantage of late summer sales and not lose your bargain plants to the heat follow these steps:
Check to be sure the plant is not root bound (the pot is so full of roots there is hardly any soil left). If it is, "bump it up" to the next size larger pot.
Don’t plant yet, hold the potted plants in a semi shaded area--afternoon shade is ideal
Keep them well watered and fertilize if they are showing signs of nutrient deficiency—i.e. yellow leaves
Don’t plant your bargain plants until the weather turns cooler in the fall.
If you’ve just got to get those plants in the ground, at least wait until after a heavy rain shower. There will be plenty of moisture in the soil and the newly planted plants don't have to compete with the established ones.