Dahlias have so many variables. Yes, some types simply bloom much more than others. In general, the smaller blooming ones are the most prolific and the giant dahlias make the least flowers, but there are still some at any size that are more or less vigorous than the others.
I am growing both Spartacus and Vassio Meggos this year. I believe one is a seed parent of the other. The flowers are very similar is size and form, but my Spartacus has twice as many laterals and has been blooming constantly since mid-July, dozens of blooms, while the Vassio Meggos has maybe put out 8 blooms all summer.
I have had some dahlias that have only put out 3-4 blooms in a season, but that is usually because they are badly placed, (really shaded out by the others) or some other environmental fluke. I have had some that have never bloomed very well for me, but know that in other peoples gardens they do really well. I don't know whether I had bad stock, or if some dahlias just are not able to thrive in my slightly shady yard. I have had some that really vary in productivity from year to year too.
Yes I do fertilize, but not a huge amount compared to what some people feed their dahlias. They get some composted manure and bonemeal when planted, and this year I put in a tablespoon of some fert specifically for bulbs, because I had a bunch left over from planting new lilies. twice this season I gave them all some Dramatik K fish/seaweed fertilizer. and mid July I mulched the beds with more composted manure. That is my usual routine.
This year for the first time I also foliar sprayed them with Jacks "bloom buster" fertilizer twice (late June & late July) I prefer to stick with organics, but read that light foliar spraying doesn't leave the salt build-ups that in ground chemical fertilizing does, so thought I would try some. I have no idea if it made the difference. I did have more blooms this year, but we also had the most rain we have ever had in a sumer, and that may have been the difference.
I have read that late fertilizing, especially high nitrogen fertilizer, can badly effect the tubers, making them more likely to rot in storage, so I don't fertilize at all once August rolls around.
