From early spring to the first frosts of autumn, thousands of wild flowers brighten the prairies. Many species are general throughout the State, while others are typical only of certain sections. Before the last patches of snow are gone, the blue-gray pasque flower, so like the crocus that it is often called by that name, appears on the rolling prairies and the northern slopes of hills. It is soon followed by the wild parsley, Nuttall's or yellow violet, and the vivid plumes of the purple avens. Most of the spring flowers are of soft, delicate hues, such as the white meadow rue, parsley, false-Solomon's-seal, silverberry, squaw-weeds, meadow parsnip, blue-eyed-grass, and harebell. With the coming of midsummer, the colors become more brilliant. The fragrant prairie rose, the State flower, blossoms profusely in fields and along roadsides. The showy oxeye or false-sunflower, the flaming prairie mallow, wild blue and yellow flax, the vivid flame lily, the purple coneflower, and the black-eyed Susan emblazon the summer fields. Along the Pembina and Sheyenne Rivers, and in Sully's Hill National Game Preserve, grow the wintergreen and ladyslipper. Water lilies float on pools and shallow streams in the western part of the State. In the Badlands grow the rabbit brush, butte primrose, false-lupine, and prickly pear, and the scoria lily which resembles a thistle during the day and opens its fragile, waxy petals only after the sun has gone down. Yellow is the color of the prairies in autumn, as amid the fading foliage the goldenrod, sunflower, aster, and blazing star dominate the scene. Some wild flowers, such as the wild morning-glory, are so common that they are regarded as weeds. These are not so obnoxious to the farmer, however, as the Russian-thistle, pigeon grass, quack grass, pigweed, mustard, burdock, and sow thistle which often invade the grainfields. The seeds of most of these plants were brought in with seed grain from European countries, and their eradication is a difficult process. Another obnoxious plant, against which a strong campaign has been conducted by farmers, is the common barberry, on which thrive the parasitic fungi that cause wheat rust. Many weeds, however, are considered a valuable asset to the fields and pasture lands where they grow. These include the American vetch or wild sweet pea, which forms an important addition to hay, and the white and violet prairie clovers, which, although too tough to be used for fodder, serve to enrich the soil.