Question
I planted an array of annuals (wave petunias, lobelia, bicopa, zinnias, etc.) in a large cedar planter using Miraclegro potting soil. They looked happy for about a week and started to grow. At the end of the second week, the stems on most of the plants turned very stiff and purplish black and everything stopped growing. Soon the leaves on the bicopa and lobelia turned purplish black and the zinnias developed purplish veins in their leaves. What on earth is going on?

Answer
did you use all new potting soil??? it sounds like it could be a nutrient difficiency. get a granular fertilizer(flowertone by espoma, or other) and sprinkle some on top of the soil.
are there drainage holes in the container???make sure you are watering properly. water by weight. . water and let the plant sit in the excess water for 5-10 minutes. after that time pour off any excess water in the saucer. pick up or lean the pot. it should feel heavy. don't water again until it feels considerably lighter in weight. outdoors check the plant everyday. indoors check it every 3-10 days. watering frequency depends on many factors(sun exposure, type of plant, wind, day temps, etc.).
hope this helped
rick