A second year seed plant in my garden has surprised me with its 40" H by 28" W, and that's before blooming. The information I'd found before siting it was of a plant that generally grows 12 to 24 inches in height. That information holds true for a first year plant, but a singly planted specimen kept over for seed production may require more space, especially if it's been planted in moist, fertile soil. It doesn't have heavy foliage, so shading out its neighbors isn't really an issue, but in an ornamental setting it can appear too large for its space. Give it plenty of room if you're planning on growing it to maturity in your flower beds.