Juliarugula specifically that a much larger percentage than usual tend to consistently blow their centers
Julia, the first thing I would suspect is the shorter day lengths at your latitude compared to the longer days of the northern breeders/vendors.
When I first started growing dahlias it took me a long time to figure out why my dahlias blooms were seldom as deep or show-worthy as the same varieties grown by my father in Montana. It was more of a mystery because it wasn't true of all varieties - some of the dahlias I grew produced blooms as nice as those grown in Montana.
I tried eliminating all possible variables (water, fertilizer, etc.) but it wasn't until I started growing dahlias year-round that I discovered the variable I was looking for was "day length".
Many of the best show/cut varieties I trial do not have centers at my latitude. If you are growing in the South this is something to consider.
The day length necessary for dahlia show/cut quality bloom is something that needs more research. It varies quite a bit from variety to variety.
