Flower:

A tight, flat to dome-shaped cluster of 5 to 30 flowers at the top of the stem and the occasional branching stem in the upper plant. Flowers are about 1/3 inch across with 5 broad petals that are toothed or fringed along the tip edge. Color varies from nearly white to bright pink to deep red, often bi-color with lighter or darker spots. In the center are 10 stamens with purplish tips.

The calyx surrounding flower forms a slender, green tube about ½ inch long with 5 triangular lobes. Around the base are several linear to awl-shaped bracts about as long as the calyx. The calyx and bracts are hairless.

Leaves and stems:

Leaves are opposite, up to 3 inches long and to about ¾ inch wide, lance-elliptic, pointed at the tip, toothless, hairless except sometimes finely hairy around the edges.

Leaf pairs are joined at the base and swollen at the node. Stems are erect, round to weakly 4-sided, green, hairless, unbranched or sometimes branched in the upper stem.

Fruit:

Fruit is an oval to cylindric capsule about as long as the calyx, open at the top with 4 teeth around the mouth. Inside are numerous tiny, blackish-brown seeds.

Notes:

Sweet William is an introduction brought in through the gardening trade and occasionally escapes cultivation. It is more common in Wisconsin and New England but is likely more widespread in Minnesota than records indicate. I've encountered it twice in Ramsey County, both times in county parks, once near a lake shore with low traffic and once in the mow zone along a paved trail. Both could have escaped from nearby residential landscapes but neither appeared to be particularly competitive with other weeds in the area. The lake shore population didn't persist more than a few years, likely drowned out from seasonal flooding.

Sweet William is recognized by the opposite leaves joined at the base, tight flat or dome-shaped cluster at the top of the stem, flowers with 5 broad petals toothed or fringed along the outer edge, and the many slender bracts. There are numerous cultivars in the garden trade and flower color varies from nearly white to deep maroon and all shades of pink in between, and variously spotted or not. Its leaves and petals are both broader than the other weedy Dianthus species known to be in Minnesota.