1. Weed Control
Plant Science 280
Mr Gomes

2. Seed Production
A live source of life! A mature fertilized ovule (embryo) that has stored energy reserves
A small plant packaged for shipment…just needs the right environment to go
Prevention of seed production, especially in annuals, is one of the biggest keys to reducing weed pressure

3. Seed Size/Abundance
Seed size is generally extremely small, most weeds can produce a tremendous amount
Shepherd’s purse can have as many as 5 million seeds/pound!!
Weed seed has a unique ability to survive in the soil based on archaic genetic traits
Estimates vary, but numbers vary anywhere from 120 million to as high as 1.4 billion seeds/acre

4. Seeds Produced by Common Weeds
Common Name Seeds/Plant Seeds/Pound
Barnyardgrass , ,286
Common cocklebur ,270
Curly Dock , ,286
Field Dodder , ,806
Lambsquarters , ,570
Shepherd’s Purse 38, ,729,166

5. How can they be Dispersed??
Mechanical
Wind
Water
Human Aided
Animal Aided
Machinery
Mimicry
Other

6. Mechanical Dispersal Think the seeds ability to move itself
Cocklebur’s ability to attach to socks and fur
PUNCTURE VINE!!! Bye bye bike tires
Weeds use these methods in order to move across greater territory

7. Wind Dispersal Most seed is small and light…high mobility
Can move great distances with the slightest breeze
Dandelion and Tumbleweed are great examples

8. Water Dispersal This is a bigger one than most people realize!
Where does most of our surface water come from in CA??
A study found that of 82 species in irrigation water, 27 endured linger than 12 months and some lasted up to 60 months

9. Human Aided Dispersal
Human inability to properly screen out seeds from irrigation or mechanical means
Imported in for livestock feed for years and years. Much of our “weeds” are non natives from Europe and Asia
Ex. Penny Newman and other grain processors use the screenings from grain crops to sell as cattle feeds. This is not cooked, thus weed seeds survive

10. Animal Aided Dispersal
Hay fed to cows in Ca is a big one
Screenings as a cheap feedstuff
In Ca, most feed won’t stay in the rumen longer than 24 hrs, viability remains easily
Liquid and dry manure for fertilizer spreads those viable seeds
After dry is spread, what do most farmers do right after???
DISC and Plow, essentially inviting germination
Attachment to animals and displacement

11. Machinery Dispersal
Moving equipment without proper cleanout, like combines
Conventional tillage
Less of an issue in Ca in 2018, but for years a farmers grain drill
Dodder, wild oats, bindweed, etc. can easily be missed if not professionally conditioned
Big problem outside Central Valley

12. Mimicry
Very common where weeds have evolved to mimic seed size, shape, or color of the crop they infest
This can also be the mimicry of plant morphology or growth habit.
Very difficult to distinguish visually, thus they are not removed.
A great example of this is barnyardgrass in closely related wild and domestic long grain rice

13. Other
Literally any other method that helps propagate any unwanted plant.
Downy Brome first entered Ca in packing material for glassware from Europe
Nursery Stock and Ornamentals. Ex. Morning Glory in corn!!
Highway Construction where “fill dirt” is brought in

14. Soooooo… Consequences?!?!
Challenges with control make eradication nearly impossible
Beauracracy is an ever challenging thing with chemistry development in CA
Rapid infestation
US Bureau of Land Management estimates that alien plants are expanding at a rate of 14% each year
Quick recovery in most cases
Weeds generally recover quicker after disaster than native species and definitely crops.
Most recent, CA fires where cattle graze
Ecological impact
Increased weed pressure generally means increased erosion from shallow root zone.

15. Seed Germination-Dormancy
Two important aspects of understanding weed biology
Germination is the act of going from seed to plant
This is the actual process of development from embryo to plant
Dormancy is a state of suspended animation – alive but not actively growing
Describes the state at which the seed waits in the soil profile for the ideal environmental conditions
Unique trait to more land race weed genetics

16. Germination
Most seed will germinate when proper environmental conditions exist, however not all do. Soil disturbance may or may not initiate germination
Are generally driven by;
Temperature
Moisture
Soil disturbance

17. Germination - Light
At least half of the annual weeds in crops have seed that requires light for germination, especially small seeded annuals
Light only can permeate into the top few mm’s of soil
Chickweed, purslane, johnsongrass, lambsquarters, etc require light
Wild onion and jimsonweed prefer darkness

18. Germination Triggers Immature Embryo
Some plants, like smartweed and bullrush, shed immature embryo’s that are unable to immediately germinate. This prevents germinating at the wrong time
Impermeable Seed Coat
Many weeds like pigweed, wild mustard, and shepherd’s purse have seed impermeable to water, oxygen, or both
This is referred to as “hard seed”
They require tillage, microbes, and certain soil acids to drive the germination process

19. Triggers Cont… Oxygen Temperature
Much of the reason most weed seed germinates near the surface is driven by Oxygen.
Higher oxygen concentrations generally starts the process
Counter to that, compaction reduces this and can usually inhibit germination. This is why tillage can start a varying degree of weed species
Minimum and Maximum temps help dictate timing
Individual species dictate this
Wild oats can germinate in 35 degree temps while pigweed requires it substantially warmer

20. Dormancy
Conversely, dormancy is driven by a higher number of circumstances, they are;
Soil temperature
Soil water content
Light
Surface drying and wetting
Percentage oxygen or carbon dioxide in soil air

21. Dormancy cont… Soil Temperature Soil water content
Over the course of hundreds of years, weed species have evolved to ensure seed germinates at the right time, a constant soil temp, cold or hot, drives this
If this was not the case, climate differences would allow for easier control
In cases like barnyardgrass, the seed goes dormant until enough moisture ripens the seed
However, some seed requires higher oxygen levels during emergence, and will wait until the soil profile dries

22. Surface drying and wetting
Dormancy cont…
Light
Surface drying and wetting
Successional Species
Soil disturbances mix with increased light, telling the seed that resources are more readily available
Reduction of competition also
Seed will wait for these triggers
Often times this is characteristic of disturbed sites where higher resources are available
This correlates perfectly to cultivated fields
Drying and wetting breaks the exterior coating of the seed over time, which brings the seed out of dormancy and starts germination

23. Classes of Dormancy Innate Dormancy
Can be dependent on receipt of specific environmental stimuli. i.e. seed ripening requirement
Induced Dormancy
When seed goes dormant because of exposure to specific environmental conditions such as dryness, Hi CO2 concentrations, high temps, etc
i.e. seed buried to deep stays dormant until it gets tilled up
Enforced Dormancy
Also depends on environment, but does not persist when conditions change

24. Consequences of this!!! “1 years seeding equals 7 years weeding”
Weeds are designed by nature to persist!!
They’re ability to germinate immediately OR sit dormant for periods of time make proper planning crucial
Seed can hang out in the profile for decades in some cases

25. Burial Study 1 yr 71 species 91% of Jimsonweed 6 yrs 68 species
Burial Period Germinated
Germination of weed seed after 38 years
1 yr species
6 yrs 68 species
10 yrs 68 species
20 yrs 57 species
30 yrs 44 species
38 yrs 36 species!!
91% of Jimsonweed
48% of mullein
38% of velvetleaf
7% of lambsquarters
1% of foxtail
1% of curly dock
That means after 38 years of being buried and dormant, you would still have 3,242 Curly Dock Seeds/acre and 6,485 Lambsquarters seeds viable enough to germinate!!!!

26. Take Care of the Problem!!!