Transcript Using the Biology of Weeds to Leverage Weed Management

Using the biology of weeds to
leverage weed management
• Chuck Mohler
• Cornell University
Weeds are plants that thrive in
disturbed environments
• For example, in a farm field
• Our crops are mostly annual plants - they
live for one season
• We kill off natural vegetation & disturb the
soil to make conditions suitable for crops
• But this also creates habitats for weeds
Many ways to be a weed
• Annuals
– Summer annuals
– Winter annuals
• Perennials
– Stationary perennials
• Taproots
• Fibrous roots
– Wandering perennials
• Bulbs & tubers
• Rhizomes or storage roots
Outline
• Two general examples
• Managing perennials through exhaustion of
roots and rhizomes
• Choosing an appropriate tillage regimen for
depleting a weed seed bank
• A specific example – your choice
Wandering perennials
• Spread by
thickened
storage roots
or by
rhizomes
(underground
stems)
Apical dominance in perennials
Tillage
Shoot above
ground
Shoot below
ground
New rhizomes
Old rhizome
fragment
Management of perennials
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Key is exhaustion of reserves.
Time shoot removal relative to growth stage
Shallow roots & rhizomes – chop & bury,
Deep roots & rhizomes – hit them often
Competitive crops, frequently cultivated
crops, short season crops
Choosing a tillage regimen for
managing a weed seed bank
Seeds of most weeds are tiny – why?
• Disturbed environments are risky
• Tiny seeds spread the risk over many
offspring
• Seedlings can be small because in a
recently disturbed environment they have
little competition.
• Seedlings have limited resources
Small seeded
species only
emerge if near
the soil
surface
Seed longevity
Loss per year (%)
Species
Cultivated Uncultivated
Lambsquarters
31
8
Annual bluegrass
26
22
Common chickweed
54
32
Common groundsel
High
45
1.0
Seed survival (%)
0.8
Seeds
survive
better
deep
in the
soil
Velvetleaf
0.6
0.4
Pigweed
0.2
0.0
0
2
4
6
Depth in soil (inches)
8
10
Death near the soil surface
• Seed predation
• Wetting and
drying
• Freeze-thaw
Plowing vs. minimum tillage?
• Small seeded species with short lived seeds
 plow them under
– Most will die before they find their way to surface
again
– Example: hairy galinsoga
– Needs to be in the top ¼” to emerge
– So if mixed into 8” of soil, the average return time
will be 32 years
– But few live longer than 2 or 3 years.
• Large seeded species with long lived seeds
 keep them near the surface
– Their mortality will be greater at the surface
– And most that are tilled under will come back to
bother you later
– Example: velvetleaf
– Emerges well from the top 2” of soil
– So if mixed into 8” of soil, the average return time
to the emergence zone is 4 years
– 80-90% survival below 4” –
0.85x0.85x0.85x0.85=0.52 so 50+% will make it
back into the safe-to-emerge zone before they die
– <20% survival near the surface
Species with small, long lived seeds?
• Lambsquarters
• Wild mustard
Many other examples
• Germination cues
– Cultivated fallows
– Mulches
– Stale seedbed
• Relative size of crop and weed seeds
– Management of crop competition
• Plant size distributions
– Reduction of weed seed production
“Manage Weeds on Your Farm: a
Guide to Ecological Strategies”
Mohler and DiTommaso, SAN
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Ecology of weeds
Cultural control methods
Physical control methods
Farm case studies
Identification, ecology and management of
the 75 worst agricultural weeds in the United
States
A specific example
• Most farms have many weeds, but only one
or two really problem weeds
• Often need to focus on those
• http://www.css.cornell.edu/weedeco
• http://www.organic.cornell.edu/ocs/index.html