Idaho is a combination of USDA winter hardiness zones 4 through 6. The average minimum yearly temperature is -30 degrees F in zone 4, and -10 degrees F in zone 6. Temperature ranges from -60 to 118 degrees F in the course of a year. Excessive hot spells do not usually last more than a week. Lush, green areas receive over 60 inches of rainfall and the southwestern valleys receive less than 10 inches a year. The growing season is the longest around Lewiston, with 200 frost-free days.
Obedient plant (Physostegia virginiana) is also known as false dragonhead. This perennial flower grows 2 to 3 feet tall and spreads 6 to 12 inches wide. The oblong leaves have serrated edges and are 4 to 8 inches long. The flowers form four rows along the floral spike. White and rose-purple blossoms appear from mid-summer to fall. Obedient plants rapidly seed themselves so it may become a weed if let go wild.
Pink buttercup (Oxalis adenophylla) is also called the silver shamrock, Chilean oxalis and pink carpet oxalis. This flower reaches 6 to 8 inches tall. The silvery leaves are pleated looking. The five-petaled blossoms open wide and cover the foliage with flowers. The blossoms are white with pink petal tips and tiny red middles. Pink buttercups thrive in full sun with light shade. This flower originates in the high mountains in Chile and Argentina.
Pussytoes (Antennaria dioica) are semi-evergreen flowers that die back if exposed to freezing temperatures. Pussytoes spread in 8 to 12 inch clumps of silvery foliage. The spatula-shaped leaves form rosettes 2 inches tall. The leaves are white-wooly on the undersides. This dense mat becomes covered with fuzzy white and pink blossoms sprouting from 6-to-12-inch-tall thick flower stalks. Pussytoes do well as groundcover, but don't tolerate foot traffic.
Rock cress (Arabis caucasica) is a low-growing perennial of evergreen leaves. The leaves are oval with serrated edges. The foliage is green or gray with cream markings. Tiny flowers are white or pink and only have four petals that appear in spring and early summer. Rock cress grows well in rock gardens or as a ground cover. Rock cress is tolerant of prolonged arid conditions.
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