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Lamb's quarters produces inconspicuous star-shaped, yellow-green flowers, <1/8" across, sessile on flower stalks in round, tight clusters; the flowers have no petals, five stamens with yellow anthers, and a single divided style at the tip of a round, green ovary, all cupped in a five-lobed, mealy-white, calyx. The flowers are wind-pollinated and can aggravate hay fever symptoms.

Chenopodium

Chenopodium album

lamb's quarters [Blooms: Jul-Sep]

Chicory flowerheads are an unmistakable clear blue, 1-1.5" across, sessile or on very short stalks, with 10-20 (typically 17) blue ray florets with five tiny teeth at their tips; there are no disc florets. Two series of small bracts covered with glandular hairs cup the base of the flowerhead. In the center of the flowerhead are spike-like, deep-blue stamens (one for each ray floret) fused around a style, each style with a light blue, bifurcated stigma. The flowerheads open in the morning but usually close later in the day.

Cichorium

Cichorium intybus

chicory [Blooms: Jun-Sep]

"Canada thistle" is a calumny on poor Canada; this homely plant is native to Eurasia, not our northern neighbor. Flowerheads of Canada thistle are much smaller (1/2-3/4" across) than those of bull or field thistle and often occur in clusters on the plant. The involucre is goblet- rather than vase-shaped, covered by closely appressed green to purplish phyllaries, that have short, sharp, brown tips but no spines; there are only tubular disk florets with five thread-like, 1/4" long apical lobes, pink or pale lavender (rarely white) in color.

Cirsium

Cirsium arvense

Canada thistle [Blooms: May-Sep]

Field thistle is a native weedy plant often found in disturbed areas, up to 8' tall, but usually much shorter. Flowerheads occur on the tips of branches, are light pink, and about 2" across. The base (involucre) of the flowerhead is vase-shaped and covered with multiple series of closely appressed, overlapping phyllaries/bracts (often said to look like fish scales). Each bract has a a medial white line and a single, sharp, 1/4" long golden spine at the tip. Below the involucre are a few slender bracts that look like miniature versions of the stem leaves; these bracts curve upwards around the flowerhead.

Cirsium

Cirsium discolor

field thistle [Blooms: Aug-Sep]

The base (involucre) of the 1.5-2" wide flowerhead of bull thistle is vase-shaped and covered with numerous series of imbricated, spreading phyllaries/bracts that taper into stiff, sharp points. The upper portion of the flowerhead consists of numerous pinkish-purple to purple tubular disk florets that split apically into five thread-like lobes and a single protruding style. This is the only North American thistle with spines (albeit, short) on the upper surface of the leaves and one of the few with spreading, spiny bracts at the base of the flowerhead; native thistles lack spines on the stems.

Cirsium

Cirsium vulgare

bull thistle [Blooms: Jun-Aug]

Asiatic dayflower flowers are 0.5-1" across; they have two thin, blue upper petals and a single, lower, translucent white petal with three lobes. There are 5-6 long stamens with yellow anthers highlighted by a central dot of maroon; the upper three stamens are sterile and probably only serve as an advertisement, while the lower two bear larger anthers with viable pollen. A single long style lies below the stamens. Below and behind the flower is a leaf-like bract (spathe) 1-2" long that is open to its base and embraces the flower like a hammock.

Commelina

Commelina communis

asiatic dayflower [Blooms: Jun-Oct]

Upper stems of poison hemlock bear compound umbels 2-5" wide of tiny (1/8") white flowers with five spoon-shaped petals (one often larger than the others), usually with a longitudinal fold and notched; below the umbels are oval bracts with elongate tips, not divided like the leaves.

Conium

Conium maculatum

poison hemlock [Blooms: May-Jun]

Field bindweed stems produce 1-4 funnel-shaped flowers, each arising from a leaf axil, about 1" across and cupped by five small sepals and attached to a flower stalk; the flower stalk bears two small bracts about 1" below the base of the flower. Flowers range from white to pink, with a clear yellow center containing five stamens and a single style with a split stigma. Each bloom lasts only one day.

Convolvulus

Convolvulus arvensis

field bindweed [Blooms: May-Aug]

Horseweed flowering stems form a pyramidal inflorescence covered with numerous flowers and what look like dandelion seedheads. Panicles of flowers up to 1.5' long 6" across arise from the tip of the main stem and from axillary stems that arise from the upper leaves. Individual flowers are small (~1/8" in diameter and 1/4" long), cylindrical or urn-shaped, and covered with multiple layers of phyllaries; they look more like buds than flowers with only the tips of the 25-45 white or pink, pistillate ray florets peeking out the end. The 12-25 disk florets are yellow; they bear both pistils and stamens.

Conyza

Conyza canadensis

horseweed [Blooms: Jul-Aug]

Whorled coreopsis' flowerheads are a bright golden yellow, 1-2" across, with eight yellow, elongate egg-shaped ray flowers and numerous (30-40) yellow disk flowers.

Coreopsis

Coreopsis verticillata

whorled coreopsis [Blooms: Jun-Sep]

American hazelnut flowers. The male inflorescence is a cylindrical catkin that appears in the fall on the most recent year's twigs and then overwinters; in the spring, the catkin elongates (to 2-4" long) and the male flowers (consisting of two tiny bracts and four stamens) open to release their pollen. The female inflorescence looks like a small bud protected by bracts that is located at the branch tip or in leaf axils; it contains several pistillate flowers seen only as red styles/stigmas protruding from the tip of the "bud."

Corylus

Corylus americana

American hazelnut [Blooms: Mar-Apr]

Cockspur hawthorn produces flat-topped panicles of flowers 2-3" across that arise from leaf axils; the panicle branches are light green and hairless. Individual flowers are 1/2" across; they have a green calyx with five teeth, five spreading white petals, 10-20 stamens with pink or pale-yellow anthers, and a pistil with 2-3 styles. At least in Jackson Park, the upper surface of the receptacle at the base of the pistil is a shiny greenish-yellow to reddish-yellow. The flower's scent is not pleasant to humans but bees and syrphid flies are not deterred. The fruit is a small (1/2" across) crabapple-like pome, red at maturity.

Crataegus

Crataegus crus-galli

cockspur hawthorn [Blooms: May-Jun]

Downy hawthorn produces clusters of 5-17 flowers; the flower stalks are reddish-green and hairy. Individual flowers are 3/4-1" across with five green, triangular sepals with glandular hairs along the margins that are fused at their base; five round, white petals; 17-20 stamens with pale yellow or white anthers; and 4-5 styles. In Jackson Park, the top of the receptacle (at the center of the flower around the bases of the styles) is reddish-green to deep red and shiny.

Crataegus

Crataegus mollis

downy hawthorn [Blooms: Apr-May]

Common hawthorn produces broad, dense, flattened clusters (cymes) of five to fifteen flowers. Individual flowers are up to 1/2" wide with five reflexed sepals, five white petals, 15-20 stamens with red to purple anthers, and a single style. At least in Jackson Park, the upper surface of the receptacle at the base of the pistil is a shiny yellowish green.

Crataegus

Crataegus monogyna

common hawthorn [Blooms: May-?]

Narrow-leaf hawksbeard is a weedy-looking plant that reaches a size of 8-40". Flowers occur in a panicle (usually arising from a leaf axil) with up to 20 dandelion-like flowerheads. Individual flowerheads are about 1" across with 30-70 yellow ray florets (the "petals"); beneath the ray florets are two layers of green bracts — an inner layer of 12-15 3/8" long, sharply-pointed, keeled bracts covered in wooly or short stiff hairs and an outer layer of bracts half the length of the inner layer, sometimes ridged, with hairs like the inner layer. Narrow-leaf hawksbeard can be distinguished from similar dandelion-like flowers (Crepis pulchera, Crepis capillaris, and various sowthistles and hawksweeds) by the presence of auricles on the stem leaves and a layer of appressed hairs on the inside surface of the inner bracts, best seen with a magnifier.

Crepis

Crepis tectorum

narrow-leaf hawksbeard [Blooms: May-?]

Honewort's inflorescence is a compound umbel 1.5-3" across of tiny (1/8") flowers. Each flower has five stamens with creamy-yellow anthers, and one or two light green, conical pistils; the five petals are usually curled in. In Jackson Park, flowers of honewort have two conical styles, whitish to greenish, that are shorter than the petals.

Cryptotaenia

Cryptotaenia canadensis

honewort [Blooms: May-Jun]

Winged pigweed flowers are whitish-green, sessile, and 1/16" across; the flowers have five inward-curving sepals that cover the ovaries, five stamens, and a flattened ovary with 2-3 styles. Flowers may contain only a pistil or both a pistil and stamens; if the latter, the creamy-yellow anthers are prominent.

Cycloloma

Cycloloma atriplicifolium

winged pigweed [Blooms: Jul-Aug]

Shining flatsedge is an attractive plant up to 10" tall with sprawling culms up to 8" long, triangular in section. The flower spike (inflorescence) has 2-7 strikingly laterally-flattened spikelets 1/4-1/2" long; each spikelet has 8-18 reddish-brown floral scales arranged in two opposing rows, each covering a floret consisting of 2-3 stamens, an ovary, and a split style which may protrude 1-2 mm beyond the scales. The plant is wind pollinated.

Cyperus

Cyperus bipartitus

shining flatsedge [Blooms: Aug-Sep]