Flower:

Tiny flowers are tightly packed in small round clusters (glomerules) in spike-like and branching arrangements at the top of the stem, at the tips of branching stems, and arising from upper leaf axils. Glomerules are usually crowded on the branch, sometimes more loosely arranged and clusters are usually erect, sometimes nodding. Within a glomerule, flowers may be at different stages of development, some just budding and others with maturing fruit.

Flowers lack petals, have 5 stamens and a round, green ovary with a 2-parted style at the tip that is divided all the way to the base. Cupping the flower is a green calyx with 5 lobes about 1 mm long, triangular to egg-shaped, often strongly keeled, and blunt to rounded at the tip. Bracts are leaf-like or absent altogether. The calyx, stalks and branches are moderately to densely white-mealy.

Leaves and stems:

Leaves are alternate, 1 to 4 inches long, up to about 2½ inches wide, variable in shape, diamond-shaped to egg-shaped to lance-elliptic in outline, pointed to blunt at the tip, wedge-shaped at the base, tapering to a stalk half to about as long as the blade. Lower leaves are largest, usually irregularly toothed, 1½ to 2+ times as long as wide, occasionally with a pair of very shallow lobes near the base (but usually not), becoming smaller and less toothy as they ascend the stem with the uppermost leaves often much narrower, proportionately longer, and toothless.

Surfaces are green often turning reddish with age, hairless, moderately to densely white or pink-mealy especially when young, the upper surface usually becoming smooth, the lower surface usually remaining white-mealy. Stems are erect to ascending, unbranched to much branched, sparsely to densely white-mealy especially on the upper stem, and green to purple striped, sometimes purple at the leaf nodes.

Fruit:

Fruit is a dry seed enclosed in the persistent ovary shell (pericarp) that has a smooth texture and matures from green to mottled brown to blackish and may or may not be tightly adhered to the seed, easily separated from it or not. Fruit in the glomerule is all arranged horizontally and the calyx lobes often close around the fruit, concealing it, but may be spreading, exposing it. Seeds are flattened round to egg-shaped, 1 to 1.5 mm long, shiny black, faintly wrinkled to smooth on the surface.

Notes:

Lamb's-quarters is a well known weed, now spread throughout the world, but does not typically invade high quality habitat. Generally easily recognizable, it is highly variable in form and leaf shape much in part to centuries of selection for cultivation in many cultures as well as frequent hybridization with similar, related species, many of them with shared, overlapping leaf and stem characteristics. Typically whitish green from the dense, white mealy granules, the granules can be quite pink in some populations or selected cultivars, and often wear off with age. This wide variation of characteristics (polymorphism) has led some taxonomists to divide them into an indefinite number of subspecies or related species with little overall consensus or acceptance. Given its inherent variability and propensity to hybridize, trying to sort out what might be what, may not be worth the challenge.

One of the more frequently mentioned varieties, var. missouriense (or C. missouriense), considered native throughout much of the south central and northeastern US, is said to flower in mid September regardless of when seed germinates, has nodding flower clusters and is consistently purple at the leaf nodes. Another frequently mentioned variety, var. lanceolatum (or C. lanceolatum), also considered native, is a robust form with numerous widely spreading to arching branches and leaves mostly lance-elliptic and not much lobed or toothed. Of note are conflicting reports of whether the pericarp is easily removed from the seed or not, though that may just apply to one or more of its many varieties or forms. We'll have to investigate this trait further.

In Minnesota, Lamb's-quarters is most similar to the native Pitseed Goosefoot (Chenopodium berlandieri) with which it is also known to hybridize. While this may add some complexity to separating the two species out, true forms of C. berlandieri can best be distinguished by the mature fruit: the pericarp and seed are both pitted on the surface where C. album is smooth, though magnification is required to see this, and the pericarp is not easily removed from the seed. C. berlandieri also tends to have a lower, more spreading (nearly as wide as tall) and sometimes candelabra-like growth form, larger leaves frequently have a pair of lobes near the base, leaf bases are sometimes straight across, and the style is not always divided all the way to the base. However, these traits are all variable so check the fruit for a definitive ID. Woodland Goosefoot (Chenopodium standleyanum), also a native, is a more delicate, spindly woodland species usually with few-flowered panicles that are smooth to only sparsely white-mealy, the glomerules usually distinctly separated, leaves have few or no teeth, and the pericarp is easily separated from the seed.