Introduction
The critical period to control weeds in corn is the 2- to 8-leaf
over stage (3 to 10 leaf tips or V1 to V6). For seed corn and
hybrid corn yield potential to be realized, optimum weed control
is required. Extra management in no-till cropping systems may
be needed to control perennial weeds and weed species that are
new (due to a shift in weed populations). Spring pre-plant burndown
treatments are critical in allowing the crop to develop without
interference during critical early growth phases.
Critical Period for Weed Control
Season-long weed control is not necessary to protect yield.
Maintaining good weed control during the period from the 2- to
8-leaf stage has been shown in research to keep the yield loss
due to weeds to less than 5%. Weeds germinating later have minimum
impact on crop yields and result in relatively low production
of weed seeds. The herbicide label indicates the growth stage
of the weeds when optimum control can be obtained. The timing
of weed control should optimize the critical period for the crop
and the optimum growth stage of the weed. Weed control should
occur within the labelled timing for the crop but should be timed
primarily on the weed leaf stage and density. The critical period
for weed control will vary within the 2- to 8-leaf stage depending
on factors such as soil type, soil moisture, precipitation, weed
species and density, growing conditions, etc. The timing is earlier
within the window for conditions that result in greater stress
potential to the crop. For example, weed control in seed corn
grown on coarse-textured soils with high weed densities and low
soil moisture will require weed control to occur at an earlier
stage within the critical weed-free period.
Seed Corn Leaf Stages
Counting the leaves on a seed corn plant sounds like an easy
task, but there are a few complications that can cause one to
miscount. It is important when reading pesticide labels or other
information to know which leaf counting method is being referred
to.
There are several methods of counting leaves:
- One method counts all leaves including any leaf tip that has
emerged from the whorl at the top of the plant (leaf tip method).
- The second method only counts those leaves that are fully
emerged and are arched over with the next leaf visible in the
whorl but standing straight up (leaf over method).
- The leaf collar method used extensively in the U.S. refers
to the leaf collar being visible, where the leaf collar is the
light green to whitish band that separates the leaf blade from
the leaf sheath, which wraps around the stem. The corn stages
for corn are referred to as V1, V2, V3, etc., where the V3 stage
is a plant with three collars showing.
In OMAFRA
Publication 75, the Guide to Weed Control, leaves are counted
that have emerged from the whorl and are starting to arch over
(leaf over method). This normally occurs when leaves are about
50% emerged. Most herbicide labels refer to this method of leaf
counting, but check the label or contact the product representative
to confirm.
hint: Start counting
from the bottom leaf, and check that the first leaf visible is
the one with the rounded leaf tip.
Another complication with leaf counting is that it starts with
the bottom leaf known as the first leaf. The first leaf is shorter
than other leaves and has a round leaf tip. As the plant grows,
the bottom leaves start to die and drop to the ground. A 10-leaf
corn plant may have one to three leaves that have died, and these
dead leaves may or may not be visible. Care must be taken to account
for them as well in leaf counts.
It takes approximately 75 to 80 Crop Heat Units to produce each
corn leaf. Therefore at temperatures of 30°C (86°F) during
the day, and 20°C (68°F) at night, there is one new leaf
every 2 to 3 days; and at 20°C (68°F) during the day,
and 10°C at night, one new leaf appears every 5 to 6 days.
Some herbicide labels may use plant height to indicate crop or
weed growth stage. Actual plant height may vary due to genetics
and the weather or other stesses of the season. Table 5-1, Comparative
Growth Stages, gives some comparative heights for each leaf stage,
but plants may vary from the stage in the table. The standing
height is measured from the ground surface to the top of the plant
as it stands. "Leaf extended" refers to the height of
the plant with the leaves pulled up to their full height.
See Table
5-1, Comparative Growth Stages
See Table
5-2, Percent Corn Yield Loss Due to Weeds at Known Populations
Crop Losses Due to Weeds
Certain weeds are more competitive than others. Table 5-2 Percent
Corn Yield Loss Due to Weeds at Known Populations, shows some
comparative yield losses due to weeds, assuming that the weeds
emerge with the crop. (Note: These numbers
are for field corn since limited information is presently available
for seed corn).
Crop scouting is essential to determine the weed species and weed
populations present in the field. Consider the competitiveness
of weed species when deciding whether to treat escapes. Effects
on crop quality and harvest procedures also need to be considered.
These estimates are based on normal weather conditions and adequate
soil moisture. Yield losses may increase under drier soil conditions
and be variable under conditions of plant stress.
Mechanical Weed Control
Small annual weed seedlings can be partially controlled by
blind harrowing, before the corn has emerged, with a set of light
harrows operating at a shallow depth or by using a weeder harrow
(with L-shaped flexible tines) when the crop is 5-10 cm tall.
High speed (10 km/h or 6.2 mph), shallow (2.5-3 cm) cultivation
with the rotary hoe when corn is 7-8 cm (2 3/4-3 3/16 in.) high
will help control small weed seedlings. These techniques will
not reduce herbicide action and may in some years enhance chemical
weed control. Inter-row cultivation can be used to complement
other weed control measures. Row cultivation is most effective
when weeds are small. Cultivation should be shallow to reduce
germination of new weed seeds, soil moisture loss and corn root
injury. Inter-row cultivation may be required when weeds escape
a herbicide treatment. Weeds should be considered as escapes when
they are 5-7 cm (2-2 3/4 in.) high.
Band treatment of chemical over the row will reduce cost by half
to two-thirds, depending on the row spacing and the width of the
band. Shallow inter-row cultivation will be required to control
weeds between the bands. Consider the combination of the two operations
when evaluating the economics of treating weeds in this manner.
Herbicide Resistance
Herbicide resistance has been found in Ontario. Resistance
to triazine herbicides was first found in 1974 and is now present
in several species of broadleaf weeds and grasses. In 1997, resistance
was found when using ALS (Group 2) herbicides to control pigweeds
and in 2000, ALS-resistant Eastern black nightshade and common
ragweed were found. Herbicide resistance usually develops from
repeated use of the same type of herbicide. Using principles of
integrated weed management helps to reduce the onset of resistance
and to manage resistance when it is found.
See Resistance
Table
Integrated Weed Management
Weed control in the seed corn corn crop is influenced by:
- crop rotation
- seeding rate
- row width
- inbred characteristics
- tillage practices
- use of custom crop harvesting or planting equipment
In general, the agronomic practices that produce a healthy,
fast-growing crop will provide the best competition with weeds.
Crop rotations can be effective in reducing certain perennial
weeds that may grow best in a particular crop. Rotating to a different
crop can allow different herbicides or tillage practices to be
used, which may be particularly effective on specific weed species.
Narrow rows and higher populations tend to increase the crop canopy
density and reduce the growth of weeds, but care should be taken
to balance the benefits of weed control with the other requirements
of the crop.
Fertilizers tend to stimulate the germination of some plant species.
Using banded phosphorous and potassium tends to concentrate the
nutrients most where the crop has access to them. Side-dress nitrogen
applications disturb the soil, which may stimulate the germination
of weeds but also places nitrogen in a narrow band and below the
depth from which most weeds germinate and grow.
Equipment can carry weed seeds from field to field. Combines,
tillage equipment, wind and soil erosion can all transport weeds.
Weeds can also infest a field from manure application or other
soil amendments. Proper sanitation and cleaning of equipment along
with maintenance of field border areas all benefit long-term weed
management in the field. Birds may also carry seeds of weeds from
field to field.
Please refer to OMAFRA
Publication 75, Guide to Weed Control, for more information,
including herbicide products and rates. Table 5 -3 lists many
of the common herbicides for seed corn and commercial hybrid corn.
Also check with your seed corn company to determine if any herbicide
restrictions are known for your inbred or for specific recommendations.
See Table
5-3,Corn (Field, Seed and Sweet) Herbicide Weed Control Ratings
See Table
5-4,Additional Weed Control Ratings for Corn (Field, Seed and
Sweet)
Specific Weeds
Chickweed
Stems & Roots:
Stems - prostrate, spreading or nearly erect, much-branched,
5-50cm (2-20in.) long, soft, delicate, bright green, with swollen
nodes, smooth except for a single, narrow lengthwise line (about
1mm, 1/25in. wide) of fine white hair on one side of each branch,
this line of hair alternating from one side of the branch to the
other on successive internodes; stems rooting from nodes which
touch the ground, and the plant spreading by this means to form
dense, matted patches.
Leaves:
Opposite (2 per node), stalked near the base, stalkless near
ends of branches, blades oval with pointed tips, smooth or slightly
hairy.
Flowers & Fruit:
Flowers small, white, produced at tips of stems and in angles
between branches; petals white, shorter than the 3-4mm (1/8-1/6in.)
long green sepals; each of the 5 petals is 2-lobed so the flower
may appear to have 10 tiny petals; seedpod somewhat egg-shaped,
about as long as or slightly longer than the sepals, the tip splitting
into 6 tiny teeth and releasing the reddish-brown somewhat spherical
seeds which are about 1.2mm (1/20in.) in diameter. It may start
blooming in early spring and produce flowers and seeds throughout
the growing season.
Habitat:
Chickweed occurs throughout Ontario in a wide variety of habitats
and soil textures. It is one of the most common weeds in lawns
but is equally at home in gardens, cultivated fields, pastures,
waste areas and even under deciduous forests.
Similar Species:
It is distinguished from similar plants by its bright green
colour, its ovate-pointed leaves, and the single lengthwise line
of fine white hair on one side of the stem but switching sides
above and below each node.
See Chickweed
images
Field Horsetail
Stems & Roots:
The rhizomes are dark brown or blackish, spread out for long
distances and are often 1 m (3 1/3 ft) below the ground surface.
They send up numerous aboveground shoots but of two different
types at different times of the year. In early spring, the shoots
are ashy-gray to light brown, unbranched, hollow, jointed stems;
each node (joint) surrounded. by a toothed sheath; and the tip
of stem ending in a brownish, spore-producing cone. After the
cones have shed their spores (early May) these whitish to light
brown stems wither and die down. At the same time, the second
type of shoot emerges from the ground. These are green, slender,
erect, hollow stems, leafless but with whorls of 6 to 8 branches
at nearly every node; each branch may branch again with whorls
of smaller branches; stems and branches surrounded by a small,
toothed sheath at each node but never end in a spore-producing
cone. Both kinds of stems are easily pulled apart at the nodes
and can be fitted back together like sections of a stove pipe.
Flower & Fruit:
The cones shed their spores in early May.
Habitat:
Field horsetail occurs in all parts of Ontario in depressional
areas with poorly drained soils, as well as in sandy or gravelly
soils with good drainage such as railroad embankments and roadsides.
An intense competitor, it can severely suppress crops and other
plants.
See Field
Horsetail images
Caution: Horsetail contains a substance which
destroys vitamin B in animals. It is especially poisonous to young
horses. Hay containing this weed may be more poisonous than fresh
plants in the field.
Wire-Stemmed Muhly
Stems & Roots:
Rhizomes usually in a tangled mass at or just below the soil
surface and consisting of many short, scaly, much branched, beige
to pink or purplish, brittle segments; stems slender, wiry, up
to 100cm (40in.) long, varying from upright to nearly prostrate,
usually much branched and bushy in aspect, smooth; exposed stem
internodes yellowish-green above each node and grading upward
to pinkish-purple below the next node; leaf sheaths green, smooth,
shorter than the stem internodes, with separate margins that open
and expose the internodes of the stem; ligule membranous, its
outer margin ragged, 0.5mm (1/50in.) long; no auricles.
Flowers & Fruit:
Leaf blades rather thin, 2.5-15cm (1-6in.) long and 2-8mm (1/12-1/3in.)
wide, tapering to a long, thin point; inflorescence of small,
soft, somewhat silky panicles, these at first green, then becoming
greenish-purple to purple at maturity, very numerous, being produced
at the ends of stems and from most leaf axils, each with clusters
of crowded spikelets; spikelets soft, hairy, 2-3.3mm (1/12-1/8in.)
long, each spikelet with 1 floret and an awn 4-12mm (1/10-1/2in.)
long or awnless. Flowers from July to September.
Habitat:
Wire-stemmed muhly is a native plant found in southern and
eastern Ontario in moist, rich soil in woods, thickets, shores,
banks and flood plains, from which it has spread into cultivated
fields, roadsides and waste places.
Similar Species:
It is distinguished by its tangled mass of beige-purple, scaly
rhizomes, its slender somewhat purplish stems and its numerous
compact inflorescences from upper leaf axils as well as ends of
stems.
See Wire-stemmed
Muhly images
Proso Millet
Stems & Roots:
Stems stout, up to 1m (40in.) or more high, smooth or somewhat
hairy for several cm below each node, especially where not enclosed
within the leaf sheath; leaf sheaths densely hairy, the hairs
somewhat harsh and standing more or less perpendicular to the
surface; leaf sheaths split, their margins membranous, overlapping
just above each node but becoming separate upwards nearer the
leaf blade; leaves smooth to sparsely hairy, elongate, to 30cm
(12in.) or longer, and 5-25mm (1/5-1in.) wide, widest just above
the rounded base and tapering towards the tip; ligule a band of
hairs 2-5mm (1/12-1/5in.) long, the bases of the hairs united
and more or less membranous; no auricles;
Flowers & Fruit:
Panicle either dense and arching or nodding to one side (usually
associated with cream-, orange- or reddish-seeded forms) or erect
and loose or open (usually associated with black-seeded forms),
8-30cm (3-1/5-12in.) long; spikelets ovoid, 4-5.5mm (1/6-1/4in.)
long and 1/2 to 2/3 as wide; seeds 3-3.5mm (1/9-1/7in.) long by
about 1.6-2.0mm (1/15-1/12in.) wide, hard and usually shiny, varying
in colour from white through shades of yellow, orange and brown
to black, the darker coloured seeds with 5 parallel beige veins.
Flowers from July to September.
Habitat:
Proso millet was introduced from Europe. Some forms are cultivated
for bird seed and some of these along with other wild forms are
major weeds of grain fields in parts of southern Ontario.
Similar Species:
It is distinguished from the very similar Witch grass by its
much larger seeds that may vary in colour from white through cream,
orange, reddish or brownish-green to black, and by the arching
or nodding panicle in some forms (the erect, open-panicled forms
usually being larger, coarser and having fewer seeds than Witch
grass); and it is distinguished from Fall panicum by its hairy
leaf sheaths and its larger spikelets. Seedlings of Proso millet
can be distinguished from those of Witch grass only by the size,
shape and colour of the mother seed clinging to the primary root.
Seed Proso
Millet images
Lamb's Quarters
Stems & Roots:
20-200cm (8in.-6 1/2 ft) high, branched or unbranched, smooth,
green or with reddish or purplish lengthwise stripes and ridges
Leaves:
First 2 or 4 true leaves apparently opposite (2 per node),
but all later leaves and branches distinctly alternate (1 per
node); leaves stalked, the blades 3-10cm (1-4in.) long, lance-shaped
or more often broadly triangular with irregular, usually shallow
teeth; leaves green or grayish due to a covering of a white mealiness
or powderiness, sometimes with reddish undersurface on young plants.
Flowers & Fruit:
Flowers very small, greenish, densely grouped together into
small, thick, granular clusters along the main stem and upper
branches, having 5 green sepals but no petals; seeds small, rounded
in outline, somewhat flattened, 1-1.5mm (1/25-1/16in.) in diameter,
enclosed in a very thin, membranous, smooth, whitish covering
(pericarp) which is readily fractured and lost when dry. Flowers
from June to August.
Habitat:
Lamb's-quarters is very widespread throughout Canada, occurring
in cultivated fields, pastures, wasteland, roadsides, gardens
and almost anywhere the soil is disturbed.
Similar Species:
It is distinguished from the Atriplexes by having only the
first 2 or 4 leaves arranged in opposite pairs, and from most
other weeds by its broadly triangular leaves with irregular, shallow
teeth, its smooth, occasionally mealy or scurfy leaves and stem,
and its inflorescence of small, greenish flowers in granular clusters.
See Lamb's
Quarters images
Green Pigweed
Flowers & Fruit:
Compared to Redroot pigweed, older plants of Green pigweed
have a brighter green, thinner, looser inflorescence in which
the individual bristly, finger-like spikes are usually longer
(4-12cm, 2-5in.) and tend to point upwards. The terminal spike
is much longer (10-25cm, 4-10in.), narrower, about 1-1.5cm (2/5-3/5in.)
thick, and either stands erect or frequently hangs over a bit.
Flowers and flower bracts are about the same length as in Redroot
pigweed, but male flowers usually have only 3 stamens, and the
chaff-like sepals are broadest at or below the middle and taper
towards the tip. Compared to Smooth pigweed, older plants have
thicker, spinier spikes and slightly larger flowers. Flowers from
July to September.
Habitat:
Green pigweed occurs throughout southern Ontario but is more
abundant in the southwest, in some areas it is probably more important
than Redroot pigweed.
Similar Species:
Young plants distinguished from Redroot pigweed by their somewhat
shiny green or slightly reddish-green colour and somewhat less
hairy stem and leaves.
See Green
Pigweed images
Redroot Pigweed
Stems & Roots:
Stems erect, 10cm-2m (4in.-6 1/2ft) high, but usually 50-90cm
(20-36in.), simple or branched, lower part thick and smooth, upper
part usually rough with dense short hair, greenish to slightly
reddish but usually red near the roots; leaves alternate (1 per
node), long-stalked, ovate with a shallow notch at the tip on
young plants but on older plants somewhat diamond-shaped, dull
green above but lighter green and with prominent whitish veins
below, and somewhat hairy.
Flowers & Fruit:
Inflorescence a coarse, branching, bristly panicle made up
of a short, thick terminal spike and below it several to many
short, lateral finger-like spikes, these pointing upward if not
crowded or outward if densely crowded, and smaller spikes in some
lower leaf axils, each spike made up of many tiny flowers and
spiny-tipped bracts up to 8mm (1/3in.) long; each flower unisexual,
having either 1 pistil or 5 stamens but never both (similar to
Prostrate pigweed); seeds black, shiny, round, flattened with
a narrow, thin margin, and about 1mm (1/25in.) in diameter. Flowers
from July to August.
Habitat:
Redroot pigweed is a common weed in cultivated fields, gardens,
pastures, waste places, roadsides and other disturbed areas throughout
Ontario.
Similar Species:
It is distinguished from Tumble pigweed and Prostrate pigweed
by its tall, erect habit of growth, its larger and broader leaves,
and its flowers crowded into a thick, terminal panicle as well
as in some of the lower leaf axils; from Smooth pigweed by its
coarse, harsh inflorescence; and from Green pigweed by the somewhat
dull green colour of its leaves, the dense covering of short hair
on its upper stem, its thick, coarse, bristly terminal panicle
with the uppermost central spike extending only a short distance
above the rest of the panicle, and by two features requiring magnification
to see: the sepals of each flower are broader above the middle
and rounded or somewhat flattened at their tips, and its male
flowers usually have 5 stamens each.
Seed Redroot
Pigweed images
Canada Thistle
Stems & Roots:
Stems erect, 30-150cm (1-5ft) high, usually branched, slender,
smooth or occasionally with a few, narrow, spiny-margined leaf-like
wings on the lower part; leaves more or less lobed and spiny,
alternate (1 per node), elliptic to oblong in outline, stalkless
and often clasping the stem; the wide variations in lobing, spininess,
hairiness, texture and colour of leaves divide the species into
4 botanical varieties.
Flowers & Fruit:
Flower heads numerous, comparatively small, 5-15mm (1/5-3/5in.)
wide and about twice as long, the involucral bracts weakly spiny
or almost smooth; ray florets absent but disk florets prominent
with purplish or sometimes white corollas; plants unisexual; although
stamens and stigmas are sometimes present in the same flower,
the flowers are functionally unisexual, all the flowers in 1 head
and all the heads on 1 plant being either male or female; heads
with male (pollen-producing) flowers somewhat shorter and narrower
than heads with female (seed-producing) flowers; seeds light brown
or straw-coloured, smooth, 2.5-4mm (1/10-1/6in.) long. Flowers
from June to late autumn.
Similar Species:
Four botanical varieties of Canada thistle occur in Ontario.
They are distinguished by differences in leaf characteristics.
Common names have not gained general acceptance for them but descriptive
terms are applicable as follows: Spiny Canada thistle; Entire-leaved
Canada thistle; Gentle Canada thistle; Woolly Canada thistle.
All four varieties of Canada thistle are distinguished from the
biennial thistles in non-flowering stages by their perennial habit
reproducing from spreading underground roots, by the absence of
a distinct circular rosette of basal leaves, and by their mostly
non-winged stems with generally slender stature, and from Bull
thistle by the absence of prickles from the surface of the leaf
blades (apart from spines along the margins); flowering stages
are distinguished by their generally smaller flower heads, mostly
less than 25mm (1in.) long and 15mm (3/5in.) wide, these unisexual,
and the involucral bracts surrounding each head either without
spiny tips or with very short weak ones. They are distinguished
from the Sow-thistles by the absence of white milky juice from
stems and leaves.
See Canada
Thistle images
Velvetleaf
Stems & Roots:
Stems 1-2m (3 - 6 1/2ft) tall and occasionally taller, much-branched
in the upper part, finely soft-hairy.
Leaves:
Leaves alternate (1 per node), broadly heart-shaped, large,
7-20cm (3-8in.) wide with a sharp-pointed apex, shallowly round-toothed,
soft-hairy and very velvety to the touch.
Flowers & Fruit:
Flowers single or in small clusters form in the leaf axils,
each with 5 large sepals and 5 yellow to yellow-orange petals,
1.3-2.5cm (1/2-1in.) wide when open; the filaments untied to form
a central column as in the mallows; the fruit form each flower
is a circular cluster of 12 to 15 seedpods about 1.3-2.5cm (1/2-1in.)
long, at first green but turning dark brown to black at maturity,
each individual pod opening with a vertical slit down its back
and containing several purplish-brown, V-shaped seeds about 1mm
(1/25in.) long. Flowers from late July until autumn.
Habitat:
Velvetleaf occurs in southern Ontario where it is increasing
in corn, soybeans and other annually tilled crops and in waste
places.
Similar Species:
It is distinguished by its erect habit of growth, large, alternate,
valentine-shaped leaves which are very soft-velvety to the touch,
its yellow to yellow-orange flowers, each with a central column
of staments, its ring of several seedpods produced from each flower,
and in late autumn by the rather grotesque to blackish stem with
many erect clusters of seedpods.
See Velvetleaf
images
Fall Panicum
Stems & Roots:
Stems 1 to many from a coarse fibrous root system, 10-180 cm
(4 in.-6 ft) long, erect or spreading or lying nearly prostrate
on the ground; often rooting from nodes in contact with the soil,
usually bending in a zigzag manner at each node or joint; lower
leaf blades and leaf sheaths of very young plants finely hairy,
blades and leaf sheaths produced higher up the stem usually completely
hairless; nodes of stem enlarged and prominent; leaf sheath somewhat
inflated or loosely fitting around the stem just above each node;
leaf sheaths split, their margins very thin, membranous, white
or colourless, and separate or overlapping each other; ligule
a fringe of hair 1.5-3 mm (1/16-1/8 in.) long; no auricles; inflorescence
of several much-branched panicles; terminal panicle at the end
of each stem large and bushy, 15-50 cm (6-20 in.) long and almost
as wide, the branches thin but quite stiff; panicles on lower
branches small, compact, and often remaining partially enclosed
by the leaf sheath from which they emerge.
Flowers & Fruit:
Spikelets borne singly at the ends of the tiny branches, about
2.5-2.7 mm (1/10 in.) long by 1.0-1.1 mm (1/25 in.) wide, each
containing a single floret ( "seed") about 1.8-2.0 mm
(1/14-1/12 in.) long by 0.9-1.0 mm (1/25 in.) wide, gray-brown
with 5 parallel beige veins. Flowers from August to September.
Habitat:
Fall panicum is a native plant in eastern North America but
has become a weed of significance in cultivated land only since
about 1968. It now infests many thousands of acres of prime corn
land in the 6 southwestern counties of Ontario and is present
in many more farms in all the counties south and west of a line
from Hamilton to Grand Bend. Outside this region it is known to
occur only occasionally in cultivated land in central Ontario,
in streamside locations along Bear Creek southeast of Ottawa,
and in several waste areas in Ottawa city and along the St. Lawrence
River.
See Fall
Panicum images
Quackgrass
Stems & Roots:
Stems erect, 30-120 cm (1-4 ft) tall, either not flowering
during the whole growing season or producing a slender, unbranched
inflorescence called a spike; stem nodes (joints) distinct and
often purplish. Leaves flat, nearly smooth; lower leaf sheaths
hairy, upper ones often smooth, sheaths split with margins overlapping;
auricles present and clasping the stem like little hooks. Spike
(seed head) with 1 (rarely 2) unstalked spikelet at each node
or joint.
Flowers & Fruit:
Spikelets alternating from one side of the rachis (central
stalk of the spike) to the other, and with their flat or broad
side towards the rachis; each spikelet made up of 3 to 7 florets
("seeds") side by side between 2 outer glumes (empty
chaff), and either with short awns (bristles) or awnless. Flowers
from June to September.
Habitat:
Quack grass occurs in cultivated fields, pastureland, waste
places, rights-of-way, lawns and gardens in almost any soil texture
throughout Ontario. This is the most troublesome perennial weedy
grass in Ontario and throughout Canada.
See Quackgrass
images
Lady's Thumb
Stems & Roots:
Stems erect from a taproot, 20-100cm (8-40in.) high, green
or reddish, smooth except for slightly swollen at the distinct
nodes; each node with a hairy ocrea (cylindrical membranous sheath
surrounding the stem).
Leaves:
Leaves alternate (1 per node), narrowly elliptic, 2-15cm (4/5-6in.)
long, greenish above and slightly paler below, usually with a
reddish to brownish or purplish blotch near the middle; undersurface
of leaf often slightly rough with tiny bumps, but never glandular
or hairy, ocrea arising with the leafstalk at each node, membranous
and somewhat papery, its surface covered with short, upward slanting
hair and its upper margin ciliate with a fringe of short, erect
hair about 1-2mm (1/25-1/12in.) long.
Flowers & Fruit:
Flowers small, densely crowded into narrow cylindrical spikes
(1-4.5cm, 2/5-2in.) long at ends of stems and branches; each flower
with 5 pinkish sepals 2-4mm (1/12-1/6in.) long, sometimes nearly
white; fruits ("seeds") more or less enclosed by the
sepals when mature, shiny, smooth, black, broadly ovate in outline,
about 2mm (1/12in.) long; of 2 kinds, either rounded-triangular
or flattened or somewhat lens-shaped in cross-section; the seed
often slightly thickened near the middle. Seedling with cotyledons
(seed leaves) about 8-12mm (1/3-1/2in.) long by 2-3mm (1/12-1/8in.)
wide, tapered towards both ends, reddish on the undersurface;
stem below the cotyledons often reddish to brownish-green; cotyledons
soon withering on developing stems. Flowers from June to September.
Habitat:
Lady's-thumb is an introduced weed which occurs in cultivated
land on nearly all soil textures throughout Ontario as well as
along roadsides and waste places.
See Lady's
Thumb images
Green Smartweed
General Description/Similar Species:
Introduced from Europe but almost identical to the smaller
plants of Pale smartweed with ocrea and undersurface of upper
leaves as in; distinguished from Pale smartweed by having its
flowers in shorter and plumper pale green to greenish-white spikes
(1-5cm long by 0.8-1.5cm wide; 2/5-2in. by 1/3-2/3in.), its lateral
spikes unstalked (no bare stem) in the axils of leaves or on stalks
mostly less than 1 cm (2/5in.) long, its sepals only as long as
or slightly shorter than the "seed," and its flattened
"seed" usually 2mm (1/12in.) in diameter or larger.
Habitat:
Green smartweed occurs in fields, pastures and gardens throughout
Ontario but is far less common than Pale smartweed.
See Green
Smartweed images
Tufted Vetch
Stems & Roots:
Stems 40cm-2m (16-80in.) long, weak, wiry, trailing on the
ground or climbing on nearby objects; leaves alternate (1 per
node), pinnately compound with 8 to 12 pairs of bristle-tipped
leaflets and branching tendrils at the end; the plant climbing
by means of these tendrils.
Flowers & Fruit:
Flowers bluish-purple, pea-like, about 12mm (1/2in.) long,
often 30 or more crowded together on one side of a long bare stalk;
seedpods pea-like, 10-25mm (2/5-1 in.) long by 4-6mm (1/6-1/4
in.) wide, partly flattened, light brown, containing 2 to 8 rounded
to oval reddish-brown seeds; seeds 2.5-3mm (1/10-1/8in.) across
and marked with a prominent, long, whitish or reddish-brown scar.
Flowers from early June to late autumn.
Habitat:
Tufted vetch occurs throughout Ontario in cultivated fields,
pastures, waste places, roadsides and gardens.
Similar Species:
It is distinguished by its spreading underground rootstalks,
compound leaves with 8 to 12 pairs of leaflets and branching tendrils,
many flowers clustered on one side of a long stalk, and its flattened,
brownish seedpods containing up to 8 rounded seeds, each with
a scar extending 1/4 to 1/3 of the way around it.
See Tufted
Vetch images
Three-Seeded Mercury
Stems & Roots:
Stems erect, 7.5-100cm (3-40in.) high, simple or branched,
slightly hairy; leaves green to bronze-green, 1-9cm (2/5-3 1/2
in.) long, lance- to rhombic-ovate on petioles that are 1/3 the
length to almost as long as the leaf blade; margins with irregular,
rounded teeth.
Flowers & Fruit:
Flowers in greenish clusters in axils of leaves, each cluster
composed of one or more palmately cleft bracts with 5 to 9 lobes,
one or more stalked male spikes 4-15mm (1/6-2/5in.) long and one
or more shorter female flowers; seedpods deeply 3-lobed (similar
to those of Leafy spurge) and containing 3 seeds; seeds tan coloured,
1.6-1.8mm (1/15in.) long. Flowers from July to September.
Habitat:
Three-seeded mercury occurs in dry or moist soil in open woods,
fields, waste places, ditches and roadsides throughout south-central
Ontario.
Similar Species:
It resembles young plants of Redroot pigweed but is distinguished
by its flowers borne in axillary clusters with bracts having 5-9
lobes and its leaves occasionally a bronze-green colour.
See Three-Seeded
Mercury
Prickly Lettuce
Stems & Roots:
Stems erect, 30-150 cm (1-5 ft) high, whitish-green, usually
smooth with a few prickles on the lower part, rather finely branched
at the top: leaves variable in size and shape. usually deeply
lobed or nearly divided with backward-curving lobes but sometimes
with irregularly shaped lobes or without lobes; the outer (convex)
margin of each lobe usually weakly spiny-toothed, the inner (concave)
margin usually without teeth or with much smaller teeth; leaves
clasping the stem with basal lobes; underside of the midrib nearly
always with a single row of stiff, sharp prickles (hence "prickly"
in the common name), these usually absent from the upper leaves
among the inflorescence and occasionally absent from the lower
leaves of second growth after mowing; leaves alternate (1 per
node), usually twisted near the stem so the leaf blade is oriented
with the margins pointing vertically and the flat surfaces facing
horizontally, leaf tips often (but not always) pointing north
and south.
Flowers & Fruit:
Flower heads small and very numerous on fine stalks in much-branched
inflorescences; each head about 7-8 mm (1/4-1/3 in.) long and
about 3 mm (1/8 in.) across, with 5 to 12 yellow ray florets,
the yellow colour often fading to bluish on drying; disk florets
absent; seeds narrowly oval with a long beak tipped with a tuft
of white hair (pappus); whole plant with sticky white juice. Flowers
from June to late autumn.
Habitat:
Prickly lettuce occurs throughout Ontario in waste places,
pastures, roadsides, cultivated fields, and occasionally in gardens.
See Prickly
Lettuce images
Common Ragweed
Stems & Roots:
Stems erect, 15-150cm (6-60in.) high, usually much-branched,
hairless or hairy throughout; lower leaves opposite (2 per node)
but becoming alternate (1 per node) higher on the plant, bright
green to slightly yellowish-green on young plants, becoming grayish-green
on older plants, compound and finely pided, the final divisions
usually coarsely toothed.
Flowers & Fruit:
Flower heads not showy, individually small, 2-5mm (1/12-1/5in.)
across, green and inconspicuous but very numerous and forming
distinctive inflorescences; individual florets either male or
female, but never both; all flowers within one flower head either
only male or female, but both male flower heads and female flower
heads usually present on the same plant; heads of male (pollen-producing)
flowers in raceme-like elongated clusters at ends of branches,
each male head hanging downwards on a short stalk like a tiny
inverted umbrella; female (seed-producing) flower heads in axils
of short, narrow, green bracts near the base of each long cluster
of male flower heads, each female head with only a single flower
and producing a single, hard, somewhat triangular or diamond-shaped
seed with several, short, sharp spines around the upper shoulder,
the whole seed 3-5mm (1/8-1/5in.) long. Flowers from August to
October.
Habitat:
Common ragweed is one of the most abundant weeds of cultivated
land throughout southern Ontario, but is rare or absent in northern
and northwestern parts of the province. It also occurs in gardens,
flower borders, poorly kept lawns, edges of sidewalks, roadsides,
fencelines, waste places, and in disturbed areas in pastures and
meadows.
Similar Species:
It is distinguished by its finely divided leaves, which are
opposite in the lower part and alternate in the upper part of
the plant, these being yellow-green at first, later gray-green
with age, and its very numerous, tiny, non-showy, greenish male
flower heads clustered along slender branches in the upper part
of the plant.
It is essential that Goldenrod, Solidago spp., [verge d'or, solidage],
not be confused with Common ragweed. Several species of Goldenrod
occur throughout Ontario in meadows, pastures, woodland, river
flats and roadsides, and have very conspicuous bright yellow inflorescences
during the ragweed hayfever season of late summer and autumn.
Goldenrods do produce pollen but only in small quantities, and
their pollen is heavy and sticky. It is not carried on the wind
and the plants are pollinated by insects. Because Goldenrod pollen
is not carried on the wind, it must not be blamed as the source
of irritation for ragweed hay fever sufferers.
Caution:
Common ragweed is the most important cause of hay fever during
August and September. Although inconspicuous and not recognized
by most people, the tiny male flower beads hanging on their slender
stalks produce huge quantities of very light pollen. As the pollen
falls from these hanging flowers, it is caught by the wind and
may be carried for distances greater than 200 km (125 miles).
Hay fever sufferers, therefore, may be affected by pollen from
ragweed plants far away.
See Common
Ragweed images
Bur Cucumber
History:
A member of the melon or gourd family. Typically this has been
a weed that grows in fencelines, along creeks and in waste areas.
However, more recently it has been creeping into agricultural
fields, particularly where no-till production systems are in place.
Life Cycle:
Annual, reproducing only by seed.
Distinguishing Characteristics:
Bur cucumber has a vine habit and branched tendrils opposite
each leaf. Sometimes it can be confused with wild cucumber or
domestic cucumber. However the leaf shapes are quite different
as wild cucumber has more deeply lobed leaves whereas bur cumber
leaves range from heart shaped to that which resembles more of
a pentagon type shape. Also burcumber, when fruiting will have
clusters of 3 to 10 spiny fruits, whereas wild cucumber will only
have 1 at each node.
See Bur Cucumber
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