Are your plants sick, wilting, what? Are they turning brown or yellow or just drooping and dying? How often do you water? Are there pieces of the plant that have been eaten? If this is happening in a line or radiating area it may be some kind of critter. Go out at night and examine your plants, pull up the dead or dying ones and look at the roots.
Here are some suggestions that might help you:
If you have a fungus type infection, you can work cornmeal into the soil around them, water it in and you should get a positive reaction in 24 hours. If that's not the problem, you might have contamination from an herbicide. Lots of mulch and compost are being found to have herbicides in them and will knock the plants out in no time flat. Impatiens have a really hard time when it's this hot, so that could be the problem right there. Pineapple sage is usually fine in the heat but can falter if there is too little water and die in two days. Periwinkles & veronica are very heat tolerant but susceptible to soil fungus. Cornmeal is an incredible healer of these types of problems. Horticultural cornmeal is the best but you can actually buy the cheapest dollar-store cornmeal (NOT cornmeal mix) and it will work like a charm. Use about a cup for every foot the plant is tall; for a 1 foot plant use 1 cup of cornmeal, 2 foot plant use 2 cups of cornmeal. The most I have ever used is 4 cups for a ten foot red tip photinia. It's cheap and it does wonders so no matter what the problem is you'll benefit, whether you end up planting new plants or cure these.
If you suspect strongly or find you have an insect problem, identify the critter if you can and find an organic solution. A broad spectrum treatment that might help is a Neem product (Lowe's carries one called 3 in 1 that comes in a concentrate, which is also helpful with fungus and some other issues, or a product called Bioganic that attacks a nerve receptor chemical found only in insects. I hope this is helpful and saves your yard!