1. Agronomy
Weed Plants and Seeds

2. What is a Weed? Plant that is out of place
Plant not intentionally grown
Plant that decreases aesthetic value

3. Why are Weeds Troublesome?
Reduce Crop Yield
Steal light, water, nutrients, space, and time
Can produce toxic substances (Allelopathic)

4. Why are Weeds Troublesome cont’d?
Hosts for plant diseases
Shelter for insects and diseases
Reduce crop quality/taste
Multiply rapidly

5. Why are Weeds Troublesome cont’d?
Clog equipment
Decrease future stands and longevity
Add to frustration and safety concerns

6. Groups of Weeds 1 year 1-2 years > 2 years Annuals Biennials
Summer (germinates in May and June)
Winter (germinate in late summer or fall)
Biennials
Perennials

7. Groups of Weeds cont’d. Grassy Weeds Broadleaf Weeds
Simple leaves, monocots
Broadleaf Weeds
Complex leaves, dicots

8. Groups of Weeds cont’d.
Early Season
Late Season

9. Monocots VS. Dicots Simple leaves One seed cotyledon Complex leaves
Two seed cotyledons

10. How To Manage Weeds Field Scouting
Proper Identification and Recording of Weeds
Economic Threshold

11. Economic Threshold
The amount of weeds in a given area that is needed to justify taking action to reduce their impact

12. Weed Control
Good Weed control requires an integrated approach system

13. Cultural Controls
Forces the plant being grown to be more competitive against the weed

14. Cultural Control cont’d.
Seed Preparation
Planting date
Fertilization
Crop Rotation
Row Spacing
Seeding Rate
Seed Variety

15. Mechanical Control
Physical action completed to reduce the rate of weed growth and spreading

16. Mechanical Control cont’d.
Plowing
Disking
Field Cultivating

17. Chemical Control
The use of chemical pesticides (Herbicides) to control weeds

18. Chemical Control Issues?
Safety and Handling
Cost of Chemical and Equipment
Crop and Chemical Rotation
Environmental Hazards
Carryover and Residual Effect
Herbicide Resistance
Reduced and Conservation Tillage

19. Chemical Herbicides Pre-Emergent
Applied before the crop or weed plant emerges through the soil
Must be timed perfectly

20. Chemical Herbicides Post-Emergent
Applied after the crop or weed plant emerges through the soil
Timing not as critical

21. Noxious Weeds
A noxious weed is a plant defined by law as being troublesome, undesirable, and difficult to control

22. List of Noxious Weeds in PA
There are currently 13 noxious weed species on PA’s list

23. Herbicide Characteristics
Contact Herbicides
A chemical that kills on contact of any plant tissue

24. Herbicide Characteristics
Systemic Herbicides
A chemical that is absorbed or translocated in a plant

25. Selective VS. Nonselective
Selective Herbicides
Selective herbicides kill weeds without significant damage to desirable plants

26. Selective VS. Nonselective
Nonselective Herbicides
Nonselective herbicides kill ALL plants present when applied

27. Herbicide Resistance
Herbicide resistant plants are uninjured or unaffected by certain chemical applications

28. Herbicide Resistance Resistant plants are becoming more common
Begins with a few tolerant plants that survive
These plants then reproduce and spread

29. Herbicide Resistance Prevention
Minimize tillage of the soil
Rotate crops
Use various chemicals
The perfect herbicide does not exist!!!

30. Carryover
Chemical carryover or residue may affect future crops in the rotation
Occurs due to excessive applications

31. Managing Organic Weeds
Organic Farming is the growth of crops without the use of chemicals and synthetic fertilizers
How can an organic farm manage weeds without pesticides and synthetic fertilizers?

32. ARS photo by Scott Bauer.
Waging war with weeds
Weeds compete with crops for moisture and plant nutrients.
Gardeners pull or hoe weeds.
Farmers use tillage and cultivation.
Conventional farmers use chemical treatments.
ARS photo by Scott Bauer.

33. Farmers are in a fight with nature …
Nature wants a diversity of plant growth.
Farmers want to limit nature to grow just one crop.
Conventional tillage and fertilization increases crop production and also encourages weeds.
The battle heats up!

34. The organic approach is different …
The organic farmer tries to work with nature.
An organic grower tries to "manage" the crop ecosystem to help the crop thrive while suppressing weeds.
This requires observing and understanding nature.

35. A weed is a “plant out of place.”
Perennial weeds:
Grow for several seasons
Reproduce from roots, stolons, bulbs, tubers, rhizomes or fewer seeds
Do best in areas with less cultivation
Annual weeds
Grow one season
Reproduce from many seeds
Thrive under same conditions as annual crops.

36. Defining succession:
Natural diversity will result in either forest or prairie.
First step: annual weeds
Second step: perennials
Third step: brush and young trees in forest regions or perennial grasses in prairie regions
During succession plants compete for moisture, nutrients and light.
Organic farmers grow crops and create conditions in which weeds cannot compete.

37. ARS photo by Scott Bauer
Using cover crops …
Cover crops hold down weed growth and add organic matter to the soil.
Allelopathic cover crops produce a chemical in the soil that inhibits the growth of other plants.
Rye is allelopathic, winter hardy, and grows almost anywhere.
Rye crop residues leach allelopathic chemicals into the soil and prevent the germination of small- seeded weeds.
ARS photo by Scott Bauer
Shows rye cover crop.

38. More on allelopathic effects:
Some large-seeded food crops can tolerate the allelopathic effects (corn, cucumbers, peas, snapbeans).
Smaller-seeded crops had less tolerance (cabbage and lettuce).
More crops with allelopathic effects: sunflowers, sorghum and rapeseed.

39. Mulches and cover crops:
Cover crops can be used to physically smother weeds by growing faster and out-competing the weeds.
Mulches can be black plastic, or plant material such as wheat straw.
ARS photo by Scott Bauer
shows wheat straw mulch.

40. More weapons: crop rotation and intercropping
Crop rotation changes the growing conditions each year, breaking the weed cycle.
A good rotation for weed control will include clean-cultivated annual crops, tightly spaced grain crops, and mowed or grazed perennial sod crops.
Add a short-season crop to smother weeds, if needed.

41. Intercropping is another alternative …
Growing two or more crops together to give weeds more competition
Often established by planting a row crop into a standing grain crop
Example: interplant soybeans into standing green wheat

42. Ideal organic farming minimizes tillage …
Tillage leaves soil exposed, allowing annual weeds to sprout and grow.
Can lead to soil erosion
Speeds decomposition and increases runoff
Can damage soil structure
Adds equipment, fuel and labor costs.

43. Ways tillage can be used:
Preplant tillage clears away annual weeds and reduces perennial weeds.
Blind cultivation works the soil to kill weeds with minimum damage to seedling crops.
Between-row cultivation cleans out more weeds emerging as crop begins to grow.

44. More weapons in the war on weeds:
Flame weeding works on broadleaf weeds.
Hand weeding and topping require hand labor.
Weeder geese are good for grassy weeds.