1. dal.ca
www.dal.ca
Weed Management under Organic
Production
Andrew Hammermeister
Organic Agriculture Centre of Canada
Dalhousie University, Faculty of Agriculture

2. Ecology – The Science of
Connections
• “In living nature, nothing is
unconnected from the
whole”

3. What is a weed?
• Liebman et al. (2001) define agricultural
weeds as “plants that are especially
successful at colonizing disturbed, but
potentially productive sites, and at
maintaining their abundance under
conditions of repeated disturbance.”
• Schonbeck (2013): any plant not
intentionally sown or propagated by the
grower that requires management to
prevent it from interfering with crop or
livestock production.

4. •
http://www2.ca.uky.edu/agc/pubs/pat/pat1-6/pat1-6.htm

5. • http://www2.ca.uky.edu/agc/pubs/pat/pat1-6/pat1-6.htm

6. Plant community succession
Disturbance
• A disturbance creates a vacuum, unused nutrients, light, water
• Natural ecosystems undergo ‘succession’ after disturbance.
Empty space is quickly filled
Annuals  herbaceous perennials  shrubs  forest
• Cultivated agricultural systems artificially maintain the
ecosystem at an immature state.

7. Ecological function of weeds
http://www.extension.org/pages/18529/an-ecological-understanding-of-weeds#.UlXFI8qDmSo
• Protect the soil from erosion
• Replenish organic matter, feed and restore
soil life
• Absorb, conserve, and recycle soluble
nutrients that would otherwise leach away
• Absorb carbon dioxide from the
• Restore biodiversity
• Provide habitat for insects and animals

8. Weeds have a variety of adaptations to
disturbance (Mohler, 2001a):
• Take up and utilize large amounts of soluble nutrients
• High tolerance to stresses, excess nutrients, drought,
waterlogging; temperature extremes; or repeated grazing,
mowing, or tillage.
• Rapid growth and formation of mature seeds, vegetative
propagules, or both
• Rapid seed germination in response to light, nutrients,
scarification (scratched seed coat), etc.

9. Weeds are adapted to disturbance (Mohler,
2001a):(Mohler, 2001a):
• Prolific seed production (up to hundreds of thousands
per plant)
• Seed characteristics that promote wide dispersal:
small size, pass through digestive tracts, burrs attach to
fur, and easy wind dispersal
• Seed dormancy mechanisms and seed longevity in
the soil that allow seeds to “wait” for favorable growth
conditions before germinating
• Ability to regrow or reproduce from small fragments
of root, rhizome (underground stem), tuber, or other
underground structures
(Mohler, C. L. 2001a. Weed life history: Identifying vulnerabilities. p. 40–98. In M. Liebman
et al. Ecological management of agricultural weeds. Cambridge University Press, New
York.)

10. Weed seeds
• Some weeds produce dozens of seeds per
plant, others may produce 10s of thousands
• Weed seeds can survive in the soil for less than
1 year to several decades depending on the
species
• Some weeds germinate immediately, others may
require several years of burial before
germinating
Seed Longevity of 41 Weed Species Buried 17 Years in Eastern and Western Nebraska.
Burnside et al. 1996. Weed Science , Vol. 44, No.1, pp. 74-86
http://www.jstor.org/stable/4045786?seq=1

11. Diagram of the maximum emergence depth of a range
of weed species according to seed weight (size).
(Based on Roberts,
H. A. (Ed.). (1982). Weed Control Handbook (7th ed.). Oxford: Blackwell Scientific Publications.)http://physicalweeding.com/information/

12. Weed problems occur when a sufficient weed seed
population encounters a favorable environment for weed
growth in the presence of a crop that is susceptible to the
effects of weed competition.
Figure credit: Ed Zaborski, University of Illinois (adapted from Schonbeck and McCann, 2007)

13. Production costs of weeds
http://www.extension.org/pages/18529/an-ecological-understanding-of-weeds#.UlXFI8qDmSo
• Competing directly for light, nutrients, moisture, and space
• Releasing natural substances that inhibit crop growth
(allelopathy)
• Physically hindering crop growth and development, especially
climbing vines like morning glories, Ipomoea spp.; and hedge
bindweed, Calystegia sepium
• Hosting pests or pathogens that may attack crops
• Promoting disease by restricting air circulation around the
crop
• Interfering with or contaminating crop harvest
• Reproducing prolifically, resulting in a greater weed problem
in the future
• Parasitizing crops directly (e.g., dodders, Cuscuta spp.; and
witchweed, Striga asiatica)

14. How do we make money and
reduce pesticide use?
• Creativity
– Look at the problem from another angle
– Why is the weed there?
– Why is it more competitive than the crop?
• Pesticide applications are a reaction to the
problem
– Weed outbreaks are not due to a herbicide
deficiency!!!

15. Integrated Weed Management
• Herbicides
• Roguing
• Mechanical
weeding
• Flaming
Chemical
Cultural
Rotations
Seeding rate
Crop selection
Cultivar choice
Delayed seeding
Physical
Biological
• Few
biopesticides for
weeds

16. Weed management options
Weed Management Strategies
•Enhance crop competitiveness
•Remove or curtail weed growth in
critical early crop development
•Reduce weed seed bank in soil

17. CULTURAL WEED CONTROL

18. Prevention of Weeds
• Diversify your cropping system
• Control weeds in hedge rows, ditches,
manure/compost piles
• Use weed-free seed
• Clean equipment to prevent spread
• Work patches of rhizomatous weeds separately,
and clean equipment
• Clean equipment between fields
• Compost manure properly (60oC for 3 d)
• Catch chaff from combine

19. Preparation
• Prepare the planting site well
– Get rid of perennial weeds before planting
– Optimize fertility for the crop (incl. pH)
– Consider summer fallow if coming out of
grass
– Have a good seed bed
– Have proper drainage

20. Improve crop competitiveness:
seedbed preparation
•
•
•
•
Level fields
Reduce planting speeds to ensure uniform depth
Plant into a firm seedbed and/or pack behind the seeder
Good seed soil contact essential for rapid and even
establishment
• Packing before planting:
– Stimulates a flush of weeds
– Creates a firm seedbed
– Creates uniform and firm soil for fingerweeding
• Packing after planting:
– Improves seed-soil contact
– Can stimulate competitive weed growth
(use a seeder that packs only in row)

21. Improve Crop Competitiveness:
Seed and Seeding
•
Seed
–
–
–
•
Competitive cultivar
Seed purity: damaged seed, weeds, other materials
Viability, germination, vigour
Rate
–
•
Higher rates are more competitive
Depth
–
–
•
Deeper planting delays emergence, can be used strategically,
but not for small-seeded crops or less competitive crops
Deeper seeding allows pre- and post-emergent tillage
Timing – Crop and weed problem dependent
–
–
•
Early planting – competes with late emerging weeds
Delayed planting allows control of early weeds, better vigour
for some crops
Spacing
–
Optimize competitiveness, want even seed distribution

22. Soil Nitrogen Status and
Seeding Rate
High N, low seeding rate
Low N, high seeding rate

23. Features of Good Crop Rotations
For Weed Management
For weed suppression
•
•
•
•
Crops that develop slowly or are less competitive
should follow weed suppressing crops
Alternate leaf crops with straw crops
Alternate between spring sown and winter sown
crops
Select crops that are competitive with weed problem
Row crops can be used to control some weed
problems
Intercropping to increase crop competitiveness,
compensate for variability in soil

24. Rotation Examples:
Prairie Organic Farm (Ian Cushon)
• Alfalfa x2: seed + bees
– Seed 50-100 lbs/ac @ $2-3/lb
– Bees $100/10,000
– Gross $250-$500/ac
• Hemp
– 500-1200 lbs/ac @$1/lb for oil
• Lentil
– 600 lbs/ac (10bu/ac) $0.6/lb
• Flax (underseeded to alfalfa)
–
10 bu/ac @$30/bu
• Total: $1660-$2860/ac/5yrs
– $332-$572/yr (3,500 acres)

25. Rotation example:
Quebec Dairy Farm - 30 cows
–
–
–
–
–
Forage
Forage
Forage
Forage
Forage
•
•
•
–
–
Break after 1st cut
2 months of weed (couch
grass) control
Apply manure & lime
Spelt or corn for feed
Mixed crop for feed
underseeded to forage
• Critical success factors:
– Optimize soil pH, drainage,
land levelling
– Forages for weed control
and soil building
– Roguing weeds in corn
– Would like to add soybean
• Grows short season corn

26. Canada thistle
• Alfalfa for 3 years
• Mow when in
flower
• Frequent shallow
spot tillage
• Till patch and
clean off
equipment

27. Green Manure Crops
• Pest Management
– Smother weeds
– Break pest cycles
•
Weed control:
–
–
–
–
–
–
–
Ryegrass
buckwheat
fall rye
winter wheat
oilradish
mustard
legumes (alfalfa, sweet clover,
white clover once established)

28. MECHANICAL/PHYSICAL
WEED CONTROL

29. Critical weed free period

30. Critical weed free period
http://www.extension.org/pages/18529/an-ecological-understanding-of-weeds#.Uooq2sqDmSo

31. Critical weed free period
http://www.plant.uoguelph.ca/research/weedsci/extension/ext_iwm.html

32. Critical weed free period
http://www.plant.uoguelph.ca/research/weedsci/extension/ext_iwm.html

35. Earthworm: numbers & biomass
Nelson and Lynch, 2008
Numbers
Biomass
c
ab
c
200
a
a
a
ce
yr
s
2
1
yr
sin
at
o
a
a
b
sin
ce
3y
rs
sin
4/
ce
5
yr
ss
in
ce
Re
fe
re
nc
e
0
c
bc
b
100
a
c
a
0
a
a
c
a
b
yr
sin
ce
2
yr
ss
in
ce
3y
rs
sin
4/
ce
5
yr
ss
in
ce
Re
fe
re
nc
e
b
200
at
o
400
c
Phase of the Rotation (years)
1
2006 data
2007 data
c
2006 data
2007 data
Po
t
c
Biomass (fresh mass gm -2)
600
Po
t
Numbers (individuals m -2)
300

36. • False seed bed: A seedbed is prepared, weed
seeds in the top 5 cm / 2” of soil germinate and then
emerge, the soil is then re-tilled (cultivated) with the
minimum disturbance necessary to kill weed
seedlings, the crop is then sown, germinates and
emerges from mostly weed free soil.
http://physicalweeding.com/information/

37. Stale seed bed: Final seedbed is prepared, weed seeds
in the top 5 cm / 2” of soil germinate, crop is sown, weed
seedlings emerge, immediately prior to crop emergence
weed seedlings are killed by a thermal weeder, crop
emerges from weed free soil.
http://physicalweeding.com/information/

38. Stale seed-bed
• Control weeds before the crop emerges or
just prior to transplanting.
• Burn down herbicides
• Flame weeding?
– Kills small weeds
– Doesn’t disturb soil

39. Hit weeds when small; light cultivation

40. Finger/tine weeder

43. http://www.youtube.com/watch?v=AqwfX3zF-t8

45. Camera guided wide blade

46. Wet conditions… major challenge

47. Roguing

49. Reigi Weeder
– 6 acres a day, 2 people & small tractor
– Best on small weeds
– http://www.youtube.com/watch?v=k_R-fTC7uYk

50. Till and mow
Discourage rodent damage by removing weeds

51. Mulching in apples

52. Black Currant Weed
Management Research
• The Questions
– What is the most
cost effective
means of
controlling weeds in
small bush fruits?
– How do these
treatments affect
soil biology?
• The Treatments
•
•
•
•
•
Mowing
Cultivation
Flaming
Acetic Acid + tilling
Mulches
• Black plastic
• Black woven cloth
• White woven poly
• Straw
• Bark

53. Weed management trial in Dalhousie orchard
(Note: the crop in the following slides is black currants but the following discussion should apply to haskap
also.)

54. Cultivation - 6wks after planting

55. Acetic Acid herbicide - 6wks after planting

56. Mowed – 6 weeks after planting

57. Plastic - 6wks after planting

61. Plastic vs. mowed
14 months after planting

63. Bush size (m3 plant-1)
0.7
a
0.6
ab
0.5
ab
ab
b
b
0.4
b
0.3
0.2
0.1
c
0
Figure 1. Bush size at the end of 2012. Error bars show standard error of the mean. Bars
with the same letter group are not significantly different (p<0.05).

64. 400
Black Plastic
Soil Degree Days
350
300
Black Fabric
250
200
Mowing
Cultivation+A
cid
Hay+Plastic
Cultivation White Fabric
150
Straw
100
50
0
0.140
0.150
0.160 0.170 0.180 0.190 0.200 0.210
Average Soil Moisture (m3/m3)
Figure 2. Soil temperature (degree days) vs. average soil moisture from JuneSeptember 2012 as measured by soil probes buried 10 cm deep in each plot
(n=32).

65. 1.0
.
Speci2
M. subterranea
MediTanP. carbonarius
B. rupestris
D. filiformis
SmalAtht
Black plastic + hay
Black plastic
C. obscura
T. corticinus
S. notangulus
L. intermedius
D. globulosus
Speci3
Small-bl
MediBlac
A. familiaris
H. capillaricornus
A. apicalis
Straw
-0.6
E. anceps
Cultivatated soil
-1.0
Slender1.0
Fig. 5. Relationship between extracted beetle taxa and plot
types (cultivated soil, black plastic, black plastic + hay, or straw)
as shown by a principal component analysis biplot.

66. Cost Effectiveness (cm3/$)
450
400
350
300
250
200
150
100
50
0
a
b
c
cd
c
cd
cd
d
Figure 3. Cost effectiveness (bush volume per dollar spent)
at the end of two years of growth. Columns with the same
letter are not significantly different (p<0.05).

67. Summary
• Organic weed management requires an
integrated approach
– Prevention
– Cultural control
– Physical control
– Chemical or flaming control
• More approaches used at the same time
dramatically increase success of weed
management
• Keep weeds in perspective

68. Visit
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