Most garden plants have rounded, mounding shapes, so adding contrasting forms such as columnar evergreens or tall grasses helps create greater architectural interest. Tall, thin annuals and perennials can form a visually permeable screen as they blow in the wind, creating a sense of mystery as to what lies behind them. Mix tall, medium and short plants in layers for a natural-looking arrangement.
Narrow conifers are classic tall, thin punctuation points for a garden. Italian cypress, long used for formal hedges, reaches 70 feet tall, but stays about 6 feet wide. Leyland cypress grows quickly to 100-by-25 feet. Smaller cypress varieties, such as Wilma Goldcrest, reach 10-by-2 feet; Tiny Tower grows 30-by-3 feet. Skyrocket Juniper grows to 20-by-3 feet.
Perennials that top single, thin stalks with flowers include Herbstone rudbeckia, which grows to 7 feet; Verbena bonariensis, which has purple blooms to attract butterflies; and Oriental trumpet lilies, which can reach 8 feet. Other tall perennials include Joe Pye weed, delphinium, foxglove, mullein, hollyhock, canna and calla lilies, valerian and thalictrum.
Calamagrostis is a narrow, upright grass with finely cut foliage. The variety Karl Foerster is widely planted and grows to 4 feet tall, but only 18 to 24 inches wide. Panicum varieties Prairie Sky, Northwind and Heavy Metal reach 6 feet. Miscanthus giganteus reaches 15-by-6 feet and the Strictus variety, with bands of gold across its leaves, grows 8 feet tall, but stays about 2 feet wide.
Sunflowers come to mind when considering tall, thin plants for a garden. Others include ornamental corn, which has white, green and pink stripes; broom corn (sorghum) with sprays of bronze seedheads atop 15-foot stalks; and amaranth, which also reaches 15 feet. Cosmos grow to about 4 feet on thin, airy stalks. All are easily started from seed.