Zonals are weeds from Africa and usually are pretty much care free besides deadheading.I give mine a fair amount of Osmocote early in the season and it's time release really helps the plants bloom well and stay healthy.Another tip is they bloom much better a little pot bound.Yellowing leaves generally means too much water,so I'd definitely check and see if this one is planted deeper than your others.

The beautiful new varieties of zonals are bred more for bloom size than abundance. It's not terribly unusual this early in your zone to have some varieties with only a couple of flowering stems on them at a time. I have switched over to a floribunda type like the Stardom line. There are less florets on each stem, but there are lots and lots of them blooming at once. I've been scratching my head about why they're blooming so close to the foliage. It may be their little quirk, but it might also have something to do with the sun/temperature balance. I know with many other plants, the cooler it is when at the same time having high light levels the shorter the bloom stalk. Lower light levels coupled with higher temps do just the reverse, tend to make the scapes taller. Just a shot in the dark, as I'm not sure if this applies to most plants in general.......it does to plants I force out of season.

Thanks for the tips! I'll definitely watch the watering and planting depth. Also, we did have some cooler temps in the weeks those flowering stems were forming -- maybe that's it. Well, the cooler days are over now, so we'll see how they do! Thanks too for the info on floribundas. I'll have to look into those for next year!