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Weed Ecology, Biology, and Identification

Understanding weed ecology can help in designing management systems to minimize their impact.

Updated:

December 9, 2009

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Weed Ecology

Weed ecology might be described as the weed's lifestyle - where does it tend to thrive, what are its particular needs and adaptations to its environment? People have a major influence on weed ecology such as weed transport/spread and creating various cropping and management techniques that favor a particular weed species. Also, weedy plants have the capability to survive under adverse conditions and use adaptive mechanisms that enable them to thrive under conditions of extreme soil disturbance. Understanding weed ecology can help in designing management systems to minimize their impact.

Weed Biology

Effective weed management is dependent upon the biology of the weed including type and lifecycle. Weeds are more easily identified and/or managed at certain times during their lifecycle.

Life cycles

Summer annuals

Seeds germinate in spring, vegetative during spring/summer, and flowers and sets seed in mid to late summer, die in fall

Examples:

common lambsquarters
pigweed
hairy galinsoga
velvetleaf
large crabgrass
common ragweed
giant foxtail
common purslane
smartweed

Winter annuals

Seeds germinate in fall and winter, vegetative during spring, and flowers and set seed in late spring/early summer before it dies

Examples:

common chickweed
downy brome
shepherdspurse
henbit

Biennials

Plants live more than one year over two growing seasons; seeds germinate and overwinter in a rosette stage, then complete life the following year

Examples:

wild carrot
common teasel
bull thistle
common burdock

Perennials

Simple: generally spread by seed; large root but no spreading horizontal roots

Examples:

dandelion
common pokeweed
dock
plaintain

Creeping: reproduce by rhizomes and stolons; some seed reproduction

Examples:

Canada thistle
poison ivy
multiflora rose
quackgrass
ground ivy
hemp dogbane
yellow nutsedge
Japanese knotweed
groundcherry

Weed Identification

Effective weed management requires proper identification of weeds - otherwise tactics chosen may not work and end up costing money and frustration. Below are some of the key features to look at when identifying a plant accused of being a weed! Many good books, fact sheets and websites now feature photo galleries of weeds at various stages to help with identification (see end of section).

Botanical Review

Grassy weeds and sedges

monocots, one cotyledon (coleoptile in seedling grasses)
parallel veins
grasses-round, oval, or flat stems with hollow internodes
sedges-generally, triangular, solid stem

Broadleaf weeds

dicots, two cotyledons (seed leaves)
usually have wider leaves than grasses or sedges
netted veins
branched stem

Species	Distinguishing Features
smooth crabgrass	membranous ligule, no to few hairs around collar otherwise hairless, bluish color
large crabgrass	membranous ligule, hairy blade and sheath
barnyardgrass	no ligule, flat stem
goosegrass	membranous ligule, flat stem, few hairs near collar, distinct parallel veins, "flattened"
quackgrass	clasping auricles, white rhizomes
wirestem muhly	toothed membranous ligule, scaly rhizomes, bushy appearance
nimblewill	toothed membranous ligule, scaly rhizomes, bushy appearance
witchgrass	hairy ligule, very hairy sheath and blade (otherwise similar to fall panicum)
fall panicum	hairy ligule, prominent white mid-rib in blade
annual bluegrass	slightly pointed membranous ligule, light green, folded leaves, boat-shaped tips
giant foxtail	hairy ligule, fine hairs on upper blade
yellow foxtail	hairy ligule, long straggly hairs near collar, flat stem, purplish near base
green foxtail	hairy ligule, no hairs (smooth on blade)
shattercane	membranous ligule, prominent mid-rib, shiny black seed
johnsongrass	membranous ligule, prominent mid-rib, reddish-brown seed or rhizomes
yellow nutsedge	triangle stem, shiny, V-shaped leaves, rhizomes and tubers (nutlets)
prostrate knotweed	elliptic, dull blue-green, leaves, ochrea, flowers in leaf axils
wild buckwheat	rounded arrow shape leaves, ochrea at petiole base, triangular seeds
Pa. smartweed/ladysthumb	true leaves have purple blotches, stems are reddish-purple, ochrea has no hairs vs. ladysthumb ochrea which has hairs, otherwise smaller plant but similar
mile-a-minute	triangular leaves, waxy-look, prominent ochrea, recurved spines
red sorrel	arrow/spade shaped leaves with lobes at base, ochrea, long slender seed stalk, sour taste
curly/broadleaf dock	wavy leaf edge vs. broad, flat heart-shaped base on broadleaf dock, ochrea
common lambsquarters	greyish cast, frosted appearance (esp. on new leaves)
tumble pigweed	light green, bushy, small wavy leaves, flowers clusters in leaf axils
redroot/smooth pigweed	notch on end of first true leaves, reddish hue on under leaf, red stem, both can have hairs on stems, look similar until mature---rr pig. has long spike seed heads, sm. pig. has shorter spikes w/ finger-like projections
common pokeweed	reddish-purple stem, dark purple berry clusters, large poisonous taproot
common purslane	reddish-purple stem, dark purple berry clusters, large poisonous taproot
chickweedspp.	common: broadly ovate leaves, single row of hairs on stem, winter annual; mouse-ear: very hairy stems and leaves, perennial
mustard spp.	most have heart to kidney-shaped cotyledons, most have long narrow seed pods, variable features for each species yellow rocket-winter annual, smooth dark green leaves, lobes at base wild mustard-w.annual, may have hairs on leaf, wavy. loby leaves shepherdspurse- w. annual, rosette at base, coarsely lobed leaves that clasp around stem, triangular, heart-shaped seed pod f. pepperweed-finely toothy basal leaves, leaves up the single stalk, seed head branched with small boat shaped seed pods Va. pepperweed-similar to field pep. but smooth leaves and stem, branches lower on stem wild radish-deeply lobed/cut wavy leaves, coarse hairs on leaves f. pennycress-large flat round, notched seed pods, plant can look similar to pepperweed
multiflora rose	like garden rose, 5-9 leaflets of compound leaf, feathery stipules at base, white flowers
black medic	looks like spindly alfalfa, 3-leaflets, yellow flowers and black seed clusters
white clover	trifoliate leaves w/ V-shaped, greenish-white leaf blotch, white flowers
prostrate spurge	prostrate to ground, pink stems and green leaves with purple blotches, milky sap
velvetleaf	heart-shaped, toothed leaves and cotyledons, stem and true leaves densely hairy
spurred anoda	3-lobed leaves, triangular-ovate w/purple veins, hairy, 6-pointed seed pods, in south
common mallow	round, wavy leaves w/base near center, white flowers, fruits like button or "cheese"
prickly sida	toothed oblong leaves, small spines at leaf base, yellow flowers, seedling-like velvetleaf
poison hemlock	lacy leaves, large plant, white flowers in umble, purple spots on hollow stalk, hairless
wid carrot	lacy leaves like garden carrot, white flowers clusters, biennial
hemp dogbane	elliptical leaves, purple stems, branchy, rhizomes, milky sap
common milkweed	oblong leaves with parallel veins that connect near the outer edge, hairy on under side of leaf, milky sap, larger in stature than dogbane
bindweed spp.	field: rounded tips and pointed lobes; hedge: arrow-shaped (triangular) and squared off lobes (i.e., hedge has edge), both have large trumpet-like white/pink flowers
morning glory spp.	tall: hairy heart-shaped leaves, butterfly shape cotyledons; ivyleaf: hairy, three-lobed leaves, butterfly shape cotyledons; pitted: smooth heart-shaped leaves, purple edges, deeply indented butterfly shaped cotyledons; all may have purple, white, pink flowers
ground ivy	creeping, square stem, toothed kidney/rounded leaves, common in lawns
henbit	round, hairy, toothed leaves, square, branched stem, flowers in whorl of upper leaves, winter annual
purple deadnettle	resembles henbit but leaves are more triangular and crowed at top, top leaves purple
jimsonweed	smooth leaves, prominent mid-rib, purple stem, unpleasant odor when crushed
smooth groundcherry	similar to EBNS but flower petals are fused, lantern-type seed pod and is perennial
horsenettle	toothed leaves, yellow spines on petiole, mid-rib and veins, grape sized fruits
eastern black nightshade	some hairs on stem and leaf margins, toothed edges, purplish color on under leaf, flowers star shaped with five petals, green berries turning black at maturity, annual
creeping speedwell	opposite, rounded/kidney shaped leaves w/ toothed edges, prostrate, stolons, in lawns
plantain spp.	buckhorn: long narrow leaves, tall, slender seed stalk, broadleaf: large oval leaves
wild cucumber	5-7 lobed leaves (star shape), large egg shaped seed pod
burcucumber	3-5 lobed leaves (pentagon shape), fruits in seed cluster
ragweed spp.	common: spoon-shaped cotyledons, finely lobed and hairy leaves; giant: large, spoon-shaped cotyledons, first leaves irregularly toothed or lobed, older leaves 3-5 lobed, 5-10+ft. tall
mayweed chamomile	lacy leaves, daisy like flower, strong unpleasant odor
common burdock	large, heart-shaped base leaves, stem and leaves w/ whitish hairs, spiny seed pod, biennial
musk thistle	lobey leaves and stems spiny, spiny "wings" down stalk, pretty, dense flower head
Canada thistle	crinkled leaf edges with spiny margins, horizontal roots, perennial
bull thistle	deeply cut, hairy leaves with large spines, biennial
horseweed (marestail)	many leaves (w/ scattered white bristles) around and up the stalk, branchy seed head
hairy galinsoga	leaves egg shaped to rounded triangle, hairy, yellow w/ white petal flowers, in gardens
common groundsel	irregular, long loby leaves, yellow flower clusters, in flower beds
dandelion	deeply toothed leaves, yellow flowers, milky sap (need I say more?!?)
common cocklebur	smooth, waxy cotyledons, true leaves wavy, large triangle, rough, sandpaper feel, purple specks on stem, spiny seed pod