Question
I live in Washington State. 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? My annuals in other areas are doing very well.  

Answer
Susanne,
Ususally, leaves turn purple either because they are suddenly exposed to strong light (after being raised in a greenhouse) or because of a nutrient deficiency.  If the pH of the soil is off, that could also cause signs of a nutrient deficiency because the acid or alkaline conditions prevent the plants from absorbing the nutrients that are there.  If I were you I'd pop the plants out of the soil and replace it with new potting soil.  Put the plants back in and water well.  Once the plants have started to grow again, begin to fertilize with a liquid fertilizer.

I hope this helps,
C.L.