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Weed Biology
How Do Plants Evolve?
Evolution results from a mutation in the genes of
an individual plant which directly or indirectly
gives the individual plant greater reproductive
success and the trait is inherited.
The better the adaptation, the more predominant the
gene becomes in the population.
Weed Biology
How Do New Plant Species Evolve?
Gene mutations occur randomly within the same
population. This population does not share the
mutations with other populations because of some
isolating factor, and a new species appears which
eventually cannot breed with the original
population, because too many of the genes are
different.
Weed Biology
What is the Nature of Plants as Weeds?
(Cultivated crop example)
Approx. 300,000 plant species
Approx. 30,000 are weeds to somebody,
somewhere
Approx. 300 are serious in cultivated crops
Approx. 10 to 30 weed species must be controlled
to avoid yield reductions in cultivated plants
Annual Bluegrass
Mar 27, 2009
Geranium, Dandelion, Common Chickweed, Buttercup
Weed Biology
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10.
Ten Most Common Weeds in North Carolina Turf
Bluegrass species
WA, P
Crabgrass species (smooth, large)
SA
Chickweed species (common, mouseear)
WA
Henbit
WA
Goosegrass
SA
Dallisgrass
P
Clover species (hop, white)
WA, P
Dandelion
P
Wild garlic
P
Cudweed species
B
2008 Proceedings - SWSS
Weed Biology
Ten Most Troublesome Weeds in North Carolina Turf
1. Dallisgrass
P
2. Bluegrass species
P, WA
3. Bermudagrass
P
4. Kyllinga species (green, false-green)
P
5. Bahiagrass
P
6. Crabgrass species (smooth, large)
SA
7. Goosegrass
SA
8. Nutsedge species (purple, yellow)
P
9. Violet species (common blue, field pansy)
P
10. Henbit
WA
2008 Proceedings - SWSS
Weed Biology
Four Reasons Why We Have Weed Problems
Weed Biology
Why Do We Have Weed Problems?
1. Monocultures (single species)
True in agriculture as well as the turf industry
Monocultures tend to be man-made.
Weed Biology
Why Do We Have Weed Problems?
2. Lack of Sanitation and also natural movement
People (shoes) and equipment (mowers,
sprayers, tillage implements) movement from
site to site spread weed seeds, tubers, bulbs,
rhizomes, stolons, etc.
Animals, wind and rain also spread weeds.
Weed Biology
Weeds are spread by…
humans, equipment, animals, wind, water
Weeds reproduce by forming…
seeds, tubers, rhizomes, bulbs, stolons
Tubers
Thickened portions of
rhizomes or roots, serving
for food storage and also
for propagation (yellow
nutsedge example)
Rhizomes
Underground stems that
can emit roots from the
lower side and leafy stems
from the upper side (green
kyllinga ex.)
Weed Biology
Tubers
Weeds that produce tubers include…
purple nutsedge, yellow nutsedge, Florida betony
Weed Biology
Rhizomes
Rhizomatous turfgrasses include bahiagrass,
bermudagrass, Kentucky bluegrass, tall fescue
and zoysiagrass.
Rhizomatous weeds include dallisgrass,
nimblewill and kyllinga species.
Wild Garlic
Bulbs
Short, vertical,
underground organs
for food storage or
reproduction on
which specialized
leaves prominently
develop
Weed Biology
Stolons
Aboveground stems that grow horizontally and
take root at the nodes to develop new plants
Turfgrass examples include creeping bentgrass,
bahiagrass, bermudagrass, centipedegrass,
carpetgrass, St. Augustinegrass & zoysiagrass.
Turfgrass weed examples include crabgrass species
and nimblewill.
Weed Biology
Why Do We Have Weed Problems?
3. Cosmetic Standards
Definitely true in turfgrass environments
We want our turf to look nice!
(At least some of us do…)
Mar 27, 2009
Apr 6. 2009
Apr 6, 2009
Weed Biology
Why Do We Have Weed Problems?
4. Weeds Fill Niches Created by People
Old field succession is the last plant population
that develops in a field. Very few plants can
survive under this vegetation. When trees /
forests are cleared and the land is tilled, mowed
(disturbed), old field succession is eliminated.
Weed Biology
Old Field Succession in North Carolina
If weeds are allowed to grow in a fallow field…
Crabgrass and other annual weeds will be the first
weeds to invade for the first 2 years.
Broomsedge and other perennial weeds will occur
in 3 to 5 years.
Pine seedlings will occur in 8 to 10 years and in
150 to 200 years will have converted to an oak
– hickory hardwood forest.
Weed Biology
What is a Weed?
Weed Biology
What is a Weed?
A weed is a plant that forms populations able to
enter habitats disturbed or occupied by
mankind, and potentially suppress or displace
resident plant populations which are
deliberately cultivated or are of ecological or
aesthetic interest.
Simpler explanation…
A weed is a plant out of place.
Weed Biology
Seven Characteristics of a Successful Weed
1. High reproductive output
2. Bimodal reproduction (more than one way to
reproduce – biennial and perennial weeds)
Weed Biology
Seven Characteristics of a Successful Weed
3. Discontinuous germination or sprouting
Crop and turf species are bred to NOT have
dormancy – hence all seeds germinate at the
same time.
Weed seeds can remain dormant in the soil for
many years and germinate sporadically. If
weeds germinated all at once (they don’t), they
possibly could all be killed at one time.
Weed Biology
Seven Characteristics of a Successful Weed
4. Highly dispersible
Weeds must be able to colonize and invade.
Weed Biology
Seven Characteristics of a Successful Weed
5. Adaptability
Weeds must be able to survive unexpected
events. This is where genetic diversity
(different genes) is important.
Weed Biology
Seven Characteristics of a Successful Weed
6. Earliness
Many weeds germinate early and grow faster
and taller than desired turf. Thus they are
capturing resources (light, moisture, nutrients)
also needed by the turf. Weeds can grow in
dormant or actively-growing turf at any time of
the year.
Common Chickweed
in dormant bermuda
Annual Bluegrass
in actively-growing
bermuda
Weed Biology
Seven Characteristics of a Successful Weed
7. Ability to Withstand Environmental Extremes
Aug 16, 2006
Aug 15, 2007
Weed Biology
Seven Characteristics of a Successful Weed
7. Ability to Withstand Environmental Extremes
May 3, 2008
Apr 6, 2009
Weed Biology
Weed Seed Banks
The number of seeds (or propagules) present in the
soil at any given time
Seed banks are replenished continually by seed rain
from parent plants within the area. Seed bank
populations can change daily or even less.
Weed Biology
Factors Affecting Seed Bank Populations
1.
2.
3.
4.
5.
Germination: causes loss to seed bank
Predation: insects, animals eat seeds
Seed death: seeds aren’t indestructible
Dormancy: seeds present but won’t germinate
Dispersal: wind, water, animals, man help
move seeds into or out of an area
Weed Biology
Competition
Competition will occur if there is one limiting
resource needed by two or more plants.
Plants compete for the following resources.
Light
Water
Nutrients
Space
Weed Biology
Competition
Generally, weeds capture resources efficiently
because of the following characteristics.
Early germination
Rapid seedling growth
Large leaf area
Massive root systems
Weed Biology
Review
Explain importance of gene mutations in plant evolution
Know common and troublesome turfgrass weeds
Know why weed problems are mostly man-made problems
Know examples of turfgrass monoculture situations
How do weeds spread, how do weeds reproduce
What are tubers, rhizomes, bulbs, stolons – know plants
that have these parts
Weed Biology
Review - continued
What plants grow in correct sequence when old field
succession is eliminated in NC
What is a weed
Know 7 characteristics of a successful weed
Understand discontinuous germination and why it is
important for the survival (success) of weeds
What is a weed seed bank… how is it replenished, depleted
Plants compete for what resources
Why do weeds capture resources efficiently
Weed Biology
Weed Life Cycles
Weeds are classified as…
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•
•
Annuals (summer and winter)
Biennials
Perennials (warm-season and cool-season)
Weed Biology
Summer Annual Weeds
Germinate in spring or early summer
Produce seed in the summer
Die with cold weather or frost
These weeds live for only ONE growing season!
Weed Biology
Summer Annual Weed Examples
Grasses
Crabgrass species
Goosegrass
Foxtail species
Sandbur
Broadleaves
Spotted spurge
Common lespedeza
Prostrate knotweed
Sedges: annual sedge
Summer Annual Grasses
Weed Biology
Smooth crabgrass: No hairs present except at base of
leaves, roots at nodes, shiny membraneous ligule, light
green leaves, produces many tillers
Large crabgrass: Similar characteristics as above except is
hairy all over
Goosegrass: Whitish stems at center of plant, shiny
membraneous ligule with a fringe of hairs, many tillers
Sandbur: easy to id with seedheads, not so when mowed
Foxtail species: short hairy ligules, usually have reddish
stems at base of plant
Summer Annual Broadleaf Weeds and Sedge
Weed Biology
Annual sedges: have 3-sided triangular shaped
leaves that are glossy in appearance
Common lespedeza: Each leaf is made up of 3
leaflets (trifoliate), mat forming, taproot
Spotted spurge: usually have burgandy or maroon
colored spots on leaves, contains milky
substance when stem is broken, mat forming,
taproot
Prostrate knotweed: bluish-green appearance, has
swollen nodes, common in compacted areas
Weed Biology
Winter Annual Weeds
Germinate in fall or early winter
Overwinter (very slow growth) during coldest
months
Actively grow and produce seed in the spring
Die in summer as temperatures rise
These weeds live for only ONE growing season!
Weed Biology
Winter Annual Weed Examples
Grasses
Annual bluegrass
(poa annua)
Broadleaves
Chickweed species
Henbit
Lawn burweed
Speedwell species
Knawel
Hop clover
Annual Bluegrass
Prolific seed producer
Withstands putting
green mowing heights
Light green appearance
Produces many tillers
Boat-shaped leaf tips
Two parallel veins run
through center of leaves
Common chickweed
Flowers with ten white
petals
Shiny leaves, no hairs
Henbit
Square stems
Purple flowers appear in
early winter through spring
Winter Annual Broadleaf Weeds
Weed Biology
Lawn burweed: produces spine-coated seedpods
beginning in March – April
Knawel: Wiry stems when germinating in the fall,
very small bluish-white flowers when mature
Ivyleaf speedwell: Three-lobed hairy leaves, heartshaped seedpods when mature
Hop clover: Each leaf consists of 3 small leaflets
(trifoliate), yellow flowers usually form in
March through May
Weed Biology
Biennial Weeds
Can germinate in fall or spring when conditions are
favorable
Usually emerge and survives the first year in a
rosette stage
Produce seeds and die in the SECOND growing
season
Can keep from blooming (mowing) to extend life,
but once it blooms, it dies
Shiny cudweed
Purple cudweed
Weed Biology
Warm-Season Perennial Weeds
Germinate or sprout in spring or summer
Can produce seed in the first summer
Can produce seed or reproduce by vegetative means
indefinitely (survive for many years due to
underground structures such as taproots,
rhizomes, tubers, bulbs and aboveground
structures such as stolons)
Weed Biology
Warm-Season Perennial Weed Examples
Grasses
Dallisgrass
Bahiagrass
Smutgrass
Broadleaves
Virginia buttonweed
Dollarweed
Sedge / kyllinga species: green and false-green
kyllinga, purple and yellow nutsedge
Warm Season Perennial
Grassy Weeds
Weed Biology
Dallisgrass: vegetatively – similar to smooth
crabgrass except that leaf edges tend to be
somewhat wavy or krinkled, seedheads
distinctly different (larger) than crabgrass
Bahiagrass: V-shaped seedheads but can have up to
3 per stalk, thick scaly rhizomes, pinkish-red
stolons
Smutgrass: Seedhead usually infected with black
smut fungus that rubs off readily when touched
False green kyllinga
Purple, yellow nutsedge
Warm season kyllinga and nutsedge species
Weed Biology
Kyllinga species: 3-sided triangular stems, usually
thick-matted in wet areas, minty odor when cut
Purple nutsedge: 3-sided triangular stems, rounded
blunt leaf tips, black hairy tubers that form
chains, purple-colored seedheads
Yellow nutsedge: 3-sided triangular stems, sharp
pointed leaf tips, chestnut colored tubers with
no chains or hairs, yellow-colored seedheads
Virginia buttonweed
Dollarweed
Warm season perennial broadleaf weeds
Weed Biology
Virginia buttonweed: usually bright yellowishgreen opposite leaves that appear virus-infected,
star-shaped white flower with 4 petals, hairy
seed capsule containing 2 seeds
Dollarweed: dark green glossy leaves with
scalloped edges, petiole attached to center of
leaf and not the leaf edge
Weed Biology
Cool-Season Perennial Weeds
Germinate or sprout in fall or winter
Can produce seed in the first spring
Can produce seed or reproduce by vegetative means
indefinitely (survive for many years due to
underground structures such as taproots,
rhizomes, tubers, bulbs and aboveground
structures such as stolons)
Weed Biology
Cool-Season Perennial Weed Examples
Grasses
Perennial ryegrass
Wild garlic
Broadleaves
Ground ivy
India mock strawberry
Dandelion
Wild violet
Yellow woodsorrel
White clover
Perennial ryegrass
clumps
Survive low mowing
Glossy, wet-looking
leaves, auricles
Wild garlic
Form underground bulbs
Hollow stems
Cool Season Perennial Broadleaf Weeds
Weed Biology
Common dandelion: deep taproot, distinct yellow
flower and white puffball seedhead, jagged
leaves
Mock strawberry: spreads by stolons and roots at
nodes, trifoliate leaves, yellow flower with fruit
pod resembling a strawberry
Yellow woodsorrel: trifoliate heart-shaped leaves,
purplish-green appearance, spreads by rhizomes
White clover: trifoliate leaves with white water
markings, white flower, spreads by stolons
Wild violet
Spreads by rhizomes and
stolons
Large heart-shaped leaves
Ground ivy
Square hairy stems
Roots at nodes
Opposite kidney-shaped
leaves
Weed Biology
Review
Understand the life cycle of a summer annual weed, winter
annual weed, biennial weed, perennial weed
Know summer annual weedy grasses, broadleaves, sedges
Know winter annual weedy grasses, broadleaves
Know warm-season perennial weedy grasses, broadleaves,
sedges, kyllingas
Know cool-season perennial weedy grasses, broadleaves
Learn identifying characteristics of the weeds discussed