Cold-Hardy Flowering Perennials

Yellow Giant Hyssop

• The yellow giant hyssop (Agastache nepetoides) is a cold-hardy plant, but also withstands exposure to summer heat and humidity -- growing from USDA zone 2 through 8. Yellow giant hyssop stands between 4 and 6 feet high and is a member of the mint family that produces 5-inch spikes of green-yellow flowers in the summer. Hyssop grows in shade or sun and is a candidate for mass plantings in perennial borders and woodland gardens. The flowers lure butterflies and appeal to bees as well.

Thimbleweed

• The size of the perennial is not responsible for thimbleweed's (Anemone virginiana) name, but rather the shape of the center of the flower and its resulting seeds are. Thimbleweed grows in the eastern and central states, with an ability to tolerate the cold of USDA zone 2. Thimbleweed grows from 1 to 2 feet tall, with erect stems developing over its foliage and producing flowers in April and May. The white flowers have a raised yellow middle. The seed heads resemble thimbles and stay on the perennial well into winter. Use thimbleweed in shady parts of a woodland garden or wildflower gardens, advises the Missouri Botanical Garden.

Creeping Phlox Sherwood Purple

• The spreading characteristic of creeping phlox (Phlox stolonifera) makes it appropriate as ground cover, since it expands by means of underground shoots called stolons. Creeping phlox hybrids such as Sherwood Purple grow to 6 inches, with tube-like purple flowers. Sherwood Purple is very tolerant of cold; the hardy cultivar grows best in average soils with solid draining qualities. Sherwood Purple works well in a rock garden or when planted in wide open woodlands.

Glossy-Leaved Aster

• The height of the glossy-leaved aster (Symphyotrichum puniceum) makes it suitable for planting in the rear of perennial borders. Some reach 8 feet high, and these cold-hardy perennials feature shiny leaves shaped like spear points. The flowers do not bloom until late in the growing season, emerging in August and September. Even when the first freeze of the year occurs in northern regions, this aster still looks good. The centers are yellow, and the surrounding rays are a combination of blue and purple. Glossy-leaved aster is an option for damp areas; in the wild,it thrives in wet meadows.