Stringy Dianthus

So I'm doing something wrong :( Looks like my dianthus are growing really stringy and weak.  My straw flowers are doing the same thing.  They were on the top shelf of my greenhouse, and upon googling why this is happening, temperature and sunlight seem to be the culprits.  I don't think they were getting enough direct light and possibly getting to hot. I've moved them to a lower shelf and I think I might have to open more of the greenhouse during the day.  Hopefully this will solve the problem, I will keep you posted.