Gardening

There's a small bed in the corner of my landscaping. It's separated from the landscaping coming down the side of the house by a stone drain for the downspout. Behind it is the back porch and it's edged by grass on two sides. I want something in the corner of the bed. Currently there are three English lavender along the curve of the two open sides with a red double knockout rose at the center of the curve. There's a catnip in the center of the bed and catmint filling in around it. The corner closest to the house has orange canna. I want something on the other corner that's out against the yard. I'd like it to be in the five to ten foot range with a spread of no more than three feet. A little taller would be better because I want it to provide some color above the lavender and rose as well as a little privacy for the porch. Along the back of the porch are orange rocket barberries so the plant would be a transition between the barberries and lavender. I'm not overly particular about the color but yellow or orange would stand out from all the purple or red would match the rose. A dwarf tree would be great there but I'm concerned about how close it would be to the concrete porch. My best idea right now is a to put a trellis there with a clematis, there's already a sturdy upright that I could connect it to. I would really like something with a longer bloom season though. It's full sun, zone 6b, with rocky clay soil.