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Common hackberry flowers may be staminate (male), pistillate (female), or perfect (both), all on the same tree. Flowers are about 1/4" across and bloom before the leaves open, often high in the tree (thus, I have no pictures yet). All flowers are wind-pollinated, yellowish-green with 4-6 spreading sepals joined at their bases; there are no petals. Male flowers have 4-5 stamens with green or yellow-brown anthers; female flowers have a green ovary with a large two-part, densely fuzzy style like an umbrella beneath the ovary. Perfect flowers have both.

Celtis

Celtis occidentalis

common hackberry [Blooms: Apr-May]

Sandburs are no one's friend. They are a form of grass with light green culms, round in section, up to 2.5' long; they are mostly covered by the sheaths of the alternate leaves. Each culm and upper branches (if any) terminate in a 4" long (3/4" across) raceme of 4-20 spiky burs. Each 1/4" diameter bur (excluding the spines) consists of 2-3 spikelets, each containing a fertile flower recognizable only by the stamens or styles protruding from the tip of the spikelets. The flowers are wind pollinated. This is the only sandbur in Illinois, so identification is easy (if not always painless).

Cenchrus

Cenchrus longispinus

sandbur [Blooms: Aug-?]

Spotted knapweed has pink to purplish flowers ~3/4" across composed of numerous tubular, branching, sterile ray florets with five lobes; the outermost ray florets are the longest. Disc florets ARE present, shorter and more erect than the ray florets but similar in appearance, with pink to white-tipped anthers and a divided style. The urn-shaped base of the flower is covered with tightly appressed oval bracts, each green with black tips; long, comb-like pale bristles radiate from the tip giving an appearance rather like a tick (inset).

Centaurea

Centaurea stoebe

spotted knapweed [Blooms: Jun-Sep]

Clusters of 4-8 flowers begin to bloom before the leaves emerge on Eastern redbud; the buds (lower left) are rosy-pink, turning a lighter pink when the flowers open. Individual flowers are about 1/2" long, on pedicels 1/4-1/2" long, pea-like with an upper central petal (the banner), two upper lateral petals (the wings) and two lower petals (usually a darker pink) that form the projecting keel; a short purplish tubular calyx with five blunt teeth lies behind the corolla. The keel encloses ten stamens and a single curved style.

Cercis

Cercis canadensis

Eastern redbud [Blooms: Apr-May]

Wild chervil upper stems terminate in up to three clusters of 3-7 small, white flowers (right image). Individual flowers (upper left) are tiny (1/8" across), each with five white petals, five stamens, and a cylindrical green ovary bearing a split style; the sepals are highly reduced. After fertilization, both the cylindrical ovary and the flower stalk elongate; mature fruits (lower left) are usually hairless, about 1/4" long, and contain a single seed.

Chaerophyllum

Chaerophyllum procumbens

wild chervil [Blooms: May-?]

Partridge pea flowers are ~1" across, noticeably asymmetric, with five yellow petals with red blotches at their base; the lowermost petal is about twice the size of the other petals. One of the lateral petals curls around the stamens. There are ten reddish-purple stamens and a single greenish-yellow style (longer than the stamens) that curves upward towards the tip; nine smaller stamens lie on one side of the style and one larger stamen on the other.

Chamaecrista

Chamaecrista fasciculata

partridge pea [Blooms: Jul-Oct]

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]