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Plants with square stems fall mainly into the mint family (Lamiaceae). A huge family of plants that includes herbs, flowers and small shrubs, the mint family is characterized by square stems, opposite leaves, tiny flowers and strong scent. A handful of other plants have square stems, too, but that shape is largely a hallmark of the mints.

Herbs Square-stemmed herbs include familiar plants such as oregano (Origanum spp.), a perennial herb that grows in U.S. Department of Agriculture plant hardiness zones 5 through 9. Basil (Ocimum basilicum) is an annual herb in most parts of the country but is a perennial in zones 10 and 11. Rosemary (Rosmarinus officinalis), which grows into a small shrub, is hardy in zones 8 through 10.

Flowering Perennials Perennial verbenas (Verbena spp.) produce clusters of small purple flowers and are hardy in zones 8 through 11. Bee balm (Monarda didyma), a spreading perennial with deep pink to purple blooms, is hardy in zones 4 through 9. Russian sage (Perovskia atriplicifolia) lifts its square stems to 3 feet tall and is hardy in zones 4 through 9.

Ground Covers Dead nettle (Lamium maculatum) is a low-growing, spreading plant with white or pink blooms used as a ground cover in zones 2 through 9. It is evergreen in zones 8 and 9. Bugleweed (Ajuga reptans) produces square-stemmed flower stalks above deep purple foliage and is hardy in zones 3 through 9. Lamb’s ears (Stachys bizantina), hardy in zones 4 through 9, has fuzzy gray leaves and square flower stems topped with magenta blooms.

Annuals Coleus (Solenostemon scutellarioides), which is grown for its colorful, patterned leaves, is hardy only in zone 11. Plectranthus, an annual that prefers bright shade, also boasts colorful leaves and is commonly used in summer containers. It is hardy in zones 9 through 11. Sweet peas (Lathyrus latifolius) also have square stems and are hardy in zones 9 through 11.

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