Last fall, we planted six cuttings from a friend's hibiscus syriacus. They were leafy, and sized from about 7" to 12". They're pictured in this question. Though they lost all foliage, the stems held up nicely through the winter. I'm in Massachusetts, USA, and we usually get down to -5°F or -10°F in the winter. This year was unusually warm, and stayed mostly above 0°F, if that matters.
Over these last few months (June, July, August) we've been experienced severe drought conditions, but we've been babying our little saplings, and they're growing nicely.
Five are leafing out, especially at the base, and not budding. They're each about 16" high and 12" at the widest point. However, one is much taller, about 40", but is pretty much a single stalk, and has 8 buds. (I couldn't get a good picture, but in some I can see the purple flowers beginning to unfold.) The taller plant is gently tied to a stake in a number of areas. It has a protective screen like the others, but I removed it to get a better picture.
The plants are spread out around the yard, and each gets about 8 hours of sun a day, from about 9 am to 5 pm. (The taller has begun getting an hour or so less sun as we're heading into the fall.) We water them equally. We don't amend our soil or treat it with anything. We put fallen leaves around all the stems last fall, but haven't used any real mulch.
The bushier are in dirt that has only ever grown grasses, clover, dandelions. The tall one is in a section that up until last year was completely covered in a juniper ground cover, including everything you see, all the way to the big rock. We had to dig at least 8" down to get all the roots, and then we aerated that soil. So even though we didn't add anything when we planted the hibiscus, the consistency was definitely looser. It makes sense that the soil in that spot might be more conducive to growth. In fact, I would think that a looser dirt and wider area might create a bigger, rounder, plant, but that isn't what's happening.
Is it that five are busy growing leaves, while the other is using its energy to produce flowers? If that is the case, though, is there are a reason it would be that specific one?
I've posted a picture of the skinny one and an example of the others. Click on them to get a closer look.