WEED SCIENCE TEST ONE

twelve characteristics of weeds

1. germination requirements are fulfilled in many requirements
2. discontinuous germination, which is internally controlled, because of the longevity of the seed. they don't germinate all at one. summer annuals germinate in late spring.
3. there's rapid growth through vegetative phase to flowering
4. continuous seed production for as long as their growing conditions permit
5. self-compatability, but not autogamy or apomixis
6. cross-pollination, when it occurts, bay unspecialized visitors or wind
7. high seed output in favorable environmental circumstances
8. production of some seeds in a wide range of environmental conditions; tolerance and plasticity
9. adaptations for short distance and long distance dispersal
10. if it's a perennial, brittleness so it's not easily drawn from the ground
12. the ability to compete interspecifically by special means (rosette, choking growth, allelochemicals)

do all weeds have the 12 characteristics?

no every weed has all these characteristics of problem weeds, but successful ones do for the most part

what is the most important general characteristic of problem weeds?

number of offspring produced, ability to survive in a range of habitats, multiple means of survival

are weeds dynamic and constantly changing

yes

how plant plant species are there and how many are considered weeds?

250,000 plant species and 1% (2500) are considered weeds

are most noxious weeds native

no

plant competition

compete with desirable plants for valuable resources

complete crop loss can occur if weeds are not managed properly

what resources do plants compete for

nutrients, light, water, and sometimes space

added plant protection costs

they harbor other pests (disease, insects, nematodes)

examples of added protection costs: black nightshade

it's a host for cucmber mosaic virus

examples of added protection costs: yellow nutsedge

host for cotton root knot nematode, which causes losses in cotten (they block nutrients/water, and they disrupt root foundation)

reduced product quality

foreign matter dockage

off-flavors in products from crops

rejected crops due to toxic weeds

increased moisture content

loss of quality due to delayed harvest

example of reduced product quality: wild garlic

not competitive with wheat, but it's a quality issue because wild garlic bulbs mix with wheat

reduced animal quality

made weeds are toxic to animals

can cause poor weight gain

thorns/spines can cause injury

toxicity depends on plant location, environmental conditions, and plant growth stage

increased production and processing costs

labor, equipment wear and tear, fuel, herbicide costs

water management

impeded flow in drainage and irrigation canals

utlized water intended for irrigation

impded river/lake navigation

interferes with recreational uses of water

decrases aesthetic value of water

fish kills (algae)

human health

dermal irritants, allergens

examples of dermal irritants

giant hogweed, poison ivy

examples of allergens

ragweed, various grasses

decreased land value and reduced crop choices

weeds can make land less desirable

cause damage to structures

limited options for weed control in some crops

aesthetic value

sports fields won't look nice with weeds

safety issues

roadsides, rights of way, vegetation management

railroad tracks

power plants

cost of weeds

what are the total losses due to weeds

what are the eleven harmful aspects of weeds

plant competition, added plant protection costs, reduced product quality, reduced animal quality, increased production and processing costs, water management, human health, decreased land value and reduced crop choices, aesthetic value, safety issues, and cost of weeds

weed seedbank

the number of seeds in the soil

how many weed seeds are in the soil?

roughly 30,000 to 350,000 seeds m^-1

120 million - 1.4 billion seeds/acre

how are seeds buried

tillage

worms

insects

other things

seed fate/predation: what is the fate of seeds in the soil seedbank?

predation: eating by insects

decay: fungi

death from old age (seed shelf life): some are only ready to grow for a certain period of time

how do you maximize withdrawals from the soil seed bank?

promote seed mortality, germinate seeds and then kill seedlings, suicidal germination, fall seedbed

how do you minimize deposits in the soil seed bank?

prevent seed dispersal, production, and burial

lots of seeds can die in the seed bank

wild oat seed in wheat/fallow rotation

- mixed 4 inch deep in fall 2004
- soil sampled and seeds extracted
- winter annual - germinates in fall, completes in spring
- summer annual - g
- decay wasn’t measured per se, just number of dead seeds counted
- how do you know if a seed is dead? tests, germination test, blue tetrazolium or red
- crush test: use tweezers to press down on seeds; if it’s deflated it’s dead

seed decay

seed decay

- measured by putting seeds in mesh bags
- either on the surface of buried 5cm deep, no differences in decay
- decay = seed coat remnant left
- no different with cover crops
- we don’t know much about speeding up seed decay
- environmental conditions: temperature, weather, moisture, etc

preventing seed production and burial

- remember high rates of seed “withdrawals” over winter if seeds are left on the surface: rainfall, temperature, bugs
- what’s the best way to prevent seed production? chemicals, removing the weed in some way, weeding, mowers, keep plants from reaching reproductive stage
- assuming you have weeds that are producing seed, or about to produce seed, ways to prevent seed production/seed rain
- spraying/mowing
- pulling/hoeing
- windrow burnings
- weed destructor
- chaff carts on combine

methods of weed propagation

sexual reproduction, asexual reproduction, vegetative propagules

sexual reproduction

1. flowers and pollinating, seed production
2. weedy species prolific seed producers
3. annuals, biennials, simple perennials reproduce entirely this way
4. creeping perennials may also reproduce sexually

asexual reproduction

1. new plants develop from vegetative structures
2. flowering and seed production not involved
3. confined to creeping perennials

vegetative propagules

rhizomes, roots, stolons, tubers, bulbs, creeping roots

rhizome

horizontal underground steam, which roots and develops new shoots at the nodes, has nodes and internodes, and scale leaves

roots

don’t have nodes or leaves

stolon

above ground stem that grows flat on the ground, roots and develops new shoots at the nodes

tuber

enlarged terminal portion of a rhizome which acts as a storage organ

has axillary buds

- potato, sweet potato
- nutsedge

bulb

- underground modified storage organ
- short, fleshy, vertical stem axis and fleshy scale leaves
- thick, fleshy scale leaves enclose a bug
- wild garlic, onion, star of bethlehem

creeping roots

- deep, horizontal roots
- modified for food storage and vegetative reproduction
- horsenettle, bamboo, mangrove, rice, english ivy

what are ways that you have seen weeds move?

- dandelion seeds wind
- people
- farm tools
- animals

weed seed dispersal: natural mechanisms

wind, water, animals, forceful dehiscence

wind dispersal

- seed blown by wind
- whole plant moved by wind
- milkweed and dandelion
- tumbleweed
- horseweed

water

- water runoff and erosion
- streams and flooding

animals

- mucilaginous seed coats
- hooks, barbs
- pass through digestive track
- burdock, beggar’s tick
- beggarweed
- cocklebur
- spanish needles

forceful dehiscence

- yellow woodsorrel
- they pop seed pods

artificial seed dispersal (human)

movement on equipment, movement in crop seed

movement on equipment

combines, lawn mowers, construction equipment

movement in crop seed

weed seeds that mimic the crop shape/size are hard to separate from the actual crop seed

e.g wild garlic, balloonvines and soybeans

balloonvines has extremely toxic seeds--if it shows up in a soybean field, it's hard to separate from soybeans

germination

seedling does photosynthesis on its own and it doesnt depend on seed reserves

periodicity of germination

time period of germination because of ideal conditions

different from environment to environment

why do we care about weed biology/ecology

it's essential to developing effecting management practices

germination timing: periodicity of germination

germination depth: large seeded vs small seeded

soil seed contact

important for proper environment conditions

external factors regulating seed germination

water, oxygen, temperature, light, chemical germination, soil pH parameters

light

pigweed, palmer amaranth

photosensitive, close to soil surface

canopy, light quality have effect on how ****

chemical germination

stimulation from host plant for parasitic weeds (witchweed)

oxygen

the deeper you go in the soil, the less oxygen

temperature

morning glory: likes warm soil temp; more germination

max, min, and optimum temperature varies by species

explains periodicity of germination

soil pH parameters

normal pH in georgia is around 6

tropical soda apple: noxious weed, low pH, just put lime out and germination won't happen

bioindicator

dormancy

inactive state in which seed will not germinate even though environmental conditions are favorable

due to factors internal to the seed

survival mechanism; ensures the reservoir of the ungerminated but viable seed for the future

simple perennials

reproduce sexually

creeping perennials

can reprocude sexually and asexually (they can do both)

types of dormancy

innate induced and enforced

innate dormancy

plant genetics

seed coat thickness or other after ripening requirement

presence of endogenous chemical inhibitors

endogenous chemicals

chemical inside the plant; inside the seed; the plant produces a chemical to keep the plant asleep

when the chemical is washed off or breaks down then it'll come out of dormancy

innate dormancy can differ within a species
- within the same plant
- within different populations
- can differ between species

example of innate dormancy and polymorphism

common lambsquarters seed polymorphism

- different morphologies of seed on the same plant
- brown seeds: thinner seed coat, straight off the plant they ready germinate in many conditions (no innate dormancy), germinate quickly

black seeds: mostly black seeds on the plant, they have a thicker seed coat and they're highly innately dormant (they don't germinate straight off the plant even in the right conditions) they require cold temperatures, often exposure to nitrate after ripening

- this results in higher fitness for the species

induced dormancy

results from plant exposure to specific environmental conditions and persists after environmental conditions change

examples:
drought, high co2 concentrations, high temperatures, shade, nitrogen status, differences between cropping systems

- stress of the mother plant passes on the same stress to the seeds, so the seeds won't germinate until there's tons of water

enforced dormancy

exposure to specific environmental conditions, but it doesn't persist after conditions change

- seed capable of germination, so it's not dormant

- doesn't germinate because environmental conditions are not suitable

- survival mechanism
palmer amaranth: summer annual (enforced dormancy during the winter)

summer annual vs winter annual
- summer annual has enforced dormancy during winter months

can dormancy characteristics very between

seeds on the same plant (polymorphism)

seeds produced on mother plants grown in different conditions (nitrogen status, shade, moisture, may lead to dormancy differences between cropping systems)

mechanisms of dormancy

1. impermeable seed coat

2. hard seed coat

3. immature embryo

4. hormonal balance

dormancy: impermeable seed coat

seed coat won't let water or oxygen enter the seed; velvetleaf

dormancy: hard seed coat

seed coat is permeable to water, but too strong to be broken by the force of swelling; restricts imbibition of adequate water; pigweed species

dormancy due to hard seed coat and impermeable seed coat can be overcome by damage to the seed coat (abrasion from tillage, attack in soil. alternate wetting and drying can break dormancy due to hard seed coat)

dormancy: immature embryo

seed shed before embryo is fully developed

embryo continued to develop after seed shed

smartweed

dormancy: hormonal balance

must have a critical level of germination promoting hormones; germination inhibiting hormones must drop below critican level. after ripening requirement in some species associated with the formation of germination promoting hormones. annual grasses

germination inhibitors may be leached out seed

implications of dormancy

in agricultural systems, dormancy shorter than in burial studies

allowing weeds to go to seed ensures problems for years to come

job security for a weed scientist

dormancy recap

- mechanism to disperse a population through time

- seeds may be born with innate dormancy - a genetic trait

- seeds may develop enforced/induced dormancy in response to improper environmental conditions

- dormancy mechanisms vary widely between species (including presence of innate dormancy)

- within a species, a degree of innate dormancy may depend on a number of factors

weed ecology

- ecology is how organisms interact with each other and their environment

plays into weed science because it brings about how weeds interact with their environments and other weeds around them (competition)

study of plants relates to how they interact with each other

flowering affected by environmental conditions

we can change a lot of factors that can affect what weeds show up, how many seeds they produce, how big they are, etc.

ecology

study of how living organisms interact with each other (the biota) and the environment (abiota)

levels of organization (ecology)

species

population

community

ecosystem

landscape

population

a group of one species, usually within a limited area

community

a group of different populations, typically within a limited area

managing specific area

specific field

ecosystem

community and environment

landscape

many communities on a larger scale

reproductive strategies

r-strategists and k-strategists

r-strategies

live in usntable environments, reproduce rapidly, high fecundity; lots of seeds, smaller seeds, little investment into each progeny, quick to mature

k-strategists

occupy stable environments, often perennials, usually larger in size, grow more slowly, longer lives, invest more resources into their progeny, larger seeds, produce fewer offspring

edaphic (soil) factors

water, aeration, temperature, pH, fertility, fertility source, cropping system, soil management practices

competition

finite amount of resources

- competition occurs when combined resource dmands of crop and weeds exceed available supply

- resources used by weeds are unavailable to crop

- a few weeds do not cause measurable yield loss

how do weeds reduce yield?

they compete with crops

light, water, nutrients, space

interspecific competition

between two species

intraspecific competition

within same species

threshold

point at which stimulus is strong enough to produce response

weed damage threshold

weed population at which a crop response can be first measured
- carpet weed

weed economic threshold

weed population where cost of control is equal to value of crop yield attribute to that control

- weed population which reduces crop value to a level equal to cost of control

- value of loss equals cost of control

- you start to lost money; yield reduction

are weed economic thresholds static?

no

depends on many things

primary factors of weed economic threshold

potential crop yield in absence of weeds

selling price of crop

cost of treatment

calculating economic threshold