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

• 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

0

0.2

0.4 Velvetleaf

0

0.2

0.4

Lambsquarters

0

0.02

0.04

0 2 4 6 8Depth (cm)

Redroot pigweed

Seed longevity

Loss per year (%)

Species Cultivated Uncultivated

Lambsquarters 31 8

Annual bluegrass 26 22

Common chickweed 54 32

Common groundsel High 45

Seeds survive better deep in the soil

Depth in soil (inches)

0 2 4 6 8 10

See

d s

urv

ival

(%

)

0.0

0.2

0.4

0.6

0.8

1.0

Velvetleaf

Pigweed

Death near the soil surface

• Seed predation• Wetting and

drying• Freeze-thaw

             

               

               

                                                                                                                                                      

Dzier włochaty (Harpalus rufipes Dej.) w pełnym biegu.

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

• 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