1. Weed Biology and Management
Curtis Rainbolt
Extension Weed Scientist
Everglades REC

2. Weed Biology and its Impact on Management
What makes a plant a weed?
Cost of weeds
Why do weeds always win? Biology
Get to know the enemy
Anatomy of a weed
Common south Florida weeds
Strategies for weed management

3. Definition of a weed: A weed is an undesired plant out of place
Water hyacinth in a aquatic garden: not a weed
Water hyacinth clogging canals: a weed

4. Weed impacts Weeds are costly $24 billion in agricultural crop loss
$3 billion in control costs
Pimentel et al

5. Weeds are costly
It is estimated that without control, sugarcane losses would be 50% from heavy infestations of fall panicum
In 2000, over $51 million was spent for weed control in US sugarcane

6. Why do weeds always win?
Dormancy: broken when conditions favor survival
Rapid early growth and expansion
Early and fast root growth
Efficient uptake and processing of nutrients and water

7. Why do weeds always win? Ability to reproduce early in life cycle
Prolific seed production
Absorb resources in excess
Tolerate low levels of resources
Genetic and environmental adaptability
Ability to develop resistance to control measures

8. Reproduction by seed First infestation is dependant on seed
-First infestations in a field can also occur when vegetative structures are transported into new fields on tillage and harvesting equipment.
-Seed production varies greatly within and among weed species.
-Seed production also varies with environmental variation between locations and years, competition from neighboring plants, and genetic variability.
Estimates of the total number of weed seeds in the soil range from 4 million to 133 million per acre furrow slice

9. Vegetative reproduction
Less longevity in soil than seeds
Very small structures can reproduce
Canada thistle: ¼” piece of root results in new plant
Torpedograss can reproduce from very small segments of rhizomes
Can be as prolific as seed production
Yellow nutsedge: 1,900 new plants and 18,000 tubers in one year from one plant
-Another example of prolific vegetative reproduction:
Whitetop has been observed to spread over an area greater than 10 feet in diameter and produce over 450 shoots in the first year of establishment.

10. Get to know the enemy: weed identification

11. Weed identification goals
Impossible to learn the thousands of weeds found in Florida
Learn the primary weeds
Keep field notes
The goal is to learn how to identify a weed
Plant anatomy
Plant keys

12. Weed classification: life cycles
Annuals- reproduce by seed only
Biennial:
Life cycle completed in two years
Flowering and fruiting in second year
Examples: wild carrot, cudweed
Perrenials:
Simple- reproduce by seed only
Creeping- reproduce by seed and vegetative propagules

13. Differences between grasses and sedges:
Sedges have a solid, triangular in cross section, stem. Leaves are arranged in threes (extend in three directions).
Grass stems may be round or flattened.

14. Purple vs Yellow Nutsedge

15. Purple vs Yellow Nutsedge

16. Common Sugarcane Weeds

17. Fall panicum (Panicum dichotomiflorum)
Most common grass in the area
Relatively easy to identify
Stem can be hairy or smooth (hairy when young)
Ligule fringe of hairs
Round stem
Widely dispersed seedhead

18. Wild oats (Sorghum almum)
Not really an oat
Closely related to johnsongrass
No rhizomes
Large, membranous ligule
Robust plant
Can look like sugarcane seedling when small

19. Broadleaf panicum Panicum adspersum (Urochloa adspersa)
Relatively prostrate growth
Wide leaves with wavy margins
Round stems
Usually dark green in color
Very similar to alexandergrass

20. Alexandergrass (Brachiaria plantaginea)
Relatively prostrate growth
Somewhat wide leaves with straight margins
Round stems
Usually light green in color
Very similar to broadleaf panicum
leaves narrower (usually)
margins straight rather than wavy (usually)

21. Alexandergrass vs Broadleaf panicum

22. Napiergrass (Pennisetum pupureum)
Very robust plant
Forms dense clumps in fields
Long, wide leaves with finely toothed margin
Up to 12 feet tall
Seedhead has “bottle brush” appearance

23. Paragrass (Brachiaria mutica)
Prostrate growing, medium size grass
Long stems covered with hairs
Short hairs on leaf surface
Swollen nodes
Grows in very wet areas
Often moves out of ditches
Pasture grass in Africa

24. Paragrass (Brachiaria mutica)

25. Goosegrass (Elusine indica)
Found in many fields
Low growing
Very white, flattened stems
Looks like it has been stepped on
Probably not competitive

26. Crabgrass (Digitaria spp.)
Very wide first leaf
Initial clumping growth progressing to prostrate, tillering
Visible membranous ligule
Can be very hairy, or hairless, depending on species

27. Torpedograss (Panicum repens L.)
Perennial with robust, creeping, sharply pointed rhizomes.
Leaf blade stiff and erect.
Hairs on upper and lower leaf surface.
Seedheads with stiff, ascending branches.
Occurs in wet areas.

28. Spiny pigweed (Amaranthus spinosus)
Most common pigweed species
Stickerweed
Large, upright growth habit, entire leaves
Very evident spines located at nodes

29. Alligatorweed (Alternanthera philoxeroides)
Common in many areas of the EAA
Prefers wet areas
Often spread by cultivation
Low growing
Hollow stems when growing in wet spots
Opposite leaves
Small white blooms

30. Common lambsquarters (Chenopodium album)
Common some years
Usually during the cooler months (Dec, Jan)
Can be difficult to control
Waxy leaf surface
Small “dots” of wax are useful for ID
Gives leaves a white-gray color
Alternate leaves
Medium size lobes on leaves

31. Common purslane (Portulaca oleracea)
Very common
Probably not competitive
Prostrate growing
Succulent
Leaves small, smooth, opposite or alternate
Small, yellow flowers
Red stems

32. Common ragweed (Ambrosia artemisiifolia)
Often found on ditch banks and field edges
Deeply dissected leaves
Many hairs on upper and lower surfaces
Long seedhead at top of plant
Yellow flowers
Similar in appearance to ragweed parthenium
Different flower type

33. Ragweed parthenium (Parthenium hysterophorus)
Primarily ditchbanks
Less common than common ragweed
Leaves less deeply dissected
Divisions don’t go all the way to the stem
White flowers
Single, not multiples

34. American black nightshade (Solanum americanum)
Occasional weed in EAA
Problematic in vegetables (tomato, pepper)
Same family (Solanaceae)
Resistant to paraquat in some areas
Alternate leaves
Usually entire to somewhat lobed
Purple fruit
Seems quite competitive

35. American black nightshade (Solanum americanum)

36. Sources of Weed ID Information
Picture books:
Southern Weed Science Society ID Guide
Excellent resource
Very thorough (almost too many plants)
Web Picture/Taxonomic Sites tm

37. Weed management strategies

38. Secrets to Successful Weed Control
Prevention

39. Only you can prevent weed invasion!
Be careful what you plant
Consider all points of entry
Keep an eye out for new invaders elsewhere
Prevent reproduction of early invaders

40. Ecological weed management is based on how a plant is built
Annual vs. biennial vs. perennial
Growth stage – perennials act like annuals for a short period
Timing relative to the seasons
Control prior to seed production

41. Management timing relative to the seasons
Perennial weed growth schedule:
Spring: export carbohydrates from roots to new shoots
Summer: capture and assimilate new energy
Fall: “pack it in” for winter – carbohydrates transported to the roots
Winter: usually, minimal growth or activity

42. Management timing relative to the seasons
Perennial weed management – general terms:
Spring: limit new growth – drain the roots
Summer: prevent energy capture
Fall: opportunity to attack the root storage system
Winter: eliminate new seedlings

43. Manual removal Hoeing, Pulling, Cultivation
Success determined by population and distribution – is it feasible?
Annual weeds easily removed
Perennial plants are often “subdivided”
Vegetative root pieces often produce new plants

44. Biological control
Biological control of weeds in cropping systems is a difficult proposition
The control agent must be very host-specific and not injure non-target species
The life cycle of the control agent must match that of the target species
Surrounding habitat should support control agent survival and reproduction
In the future, possibility of bioherbicides

45. Herbicides Several good options for most crops grown in EAA
Applications should be timed to minimize competition with crop
Should be made prior to weed seed head formation

46. Questions??