The tall shrimp plant in the photo was grown as a "standard". It's a form of topiary. To achieve that shape, a stake was driven into the container and the plant was tied closely to the stake. You can see this in the photo. Then all side shoots except at the top of the plant were pruned away. Once a year in the spring, the top of the standard is pruned to keep its shape. It takes years to grow a plant that large. I am working on standards for rosemary and scented geranium. I have been growing them for about three years and they are still less than 3 feet tall. The woody stem is not unusual. Most perennials develop woody stems after a few years. They support tall plants better than soft, green herbaceous stems.