I find that very puzzling about the toonies...

I personally have huge trouble with lobelia. I saw the most lovely lobelia in
Maine when I went there for 2 summers long before my son was born. The
window boxes were beautiful with it. But alas, it dies every time for me!

I think petunias are heavy feeders. What I do now when I do my containers
up there, is to mix in a pellet form fertilizer into the soil mix. I also mix in
SoilMoist granules, which provide more even watering. The toonies in
the picture on Flickr were in 1/2 day sun. All my toonies are mostly 1/2 sun since I have alot of shade. Toonies can take cooler temps, and it is possible they don't do well in heat. We don't have heat up there. They will mildew in
high humidity. I have failed with marigolds up there too. But the toonies
like it.

In general I have learned that container gardening gives more success. That is what I do up there now. (except for the wildflowers I put out from seed).

You know I have had petunias self seed even. The multiflora types with smaller flowers and more of them are easier than the grandiflora ones.
Perhaps you were using granifloras. This year I used a lemon yellow multiflora in my window box planters on the porch. These were really nice..so I don't miss the lobelia much!

It is best to pinch off the seed heads, which come even with the hybrids now.
If you don't pinch the seed heads off they will stop blooming. There was a place next to a gas station here that sold "sale" plants in late June, early July, for very low prices. I found the lemon yellow toonies there for a song!