Lychnis typically does well in poor, sandy soils and will do just fine in hot temperatures. Did you over water or over fertilize?

Rhizo - thanks for your info. Did not realize orange zwerg and lychnis are one and the same. Actually I know nothing about either. The plant I purchased was in a typical potting soil - certainly not a sandy content - I planted it into a larger container with potting soil and placed in the sun. It wasn't overwatered. I'm afraid I've lost it - only a small green stem - maybe an inch high - with 2 leaves . Any help you can give would be appreciated.
Thanks-