Weeds and their importance

What are weeds?

Plants growing where they are not wanted Plants out of place Plants whose virtues have yet to be discovered

Is this a limited definition? Questions

Is there a difference between a volunteer potato plant in a patch of lettuce and an invasive, persistent, noxious plant species like field bindweed (Convolvulus arvensis L.)?

Noxious weeds

What makes a weed ‘noxious’?

A noxious weed is defined by Montana Law (MCA 7-22-2101) as, “any exotic plant species established or that may be introduced in the state that may render land unfit for agriculture, forestry, livestock, wildlife, or other beneficial uses or that may harm native plant communities.” 

Any unwanted non-native plant with potential impact serious to the extent that it has been declared by the state of Montana that landowners must enter into an approved management program to keep it from spreading.

Montana has 32 state-listed noxious weed species

A weed gains ‘noxious’ status through rule-making authority by the Montana Department of Agriculture, as a district noxious weed by a county weed board, and/or following public notice of intent and a public hearing regarding the status of a weed.

If a weed is designated as a statewide noxious weed by rule of the Department of Agriculture, that weed must be considered noxious in every county of the state. 

How are noxious weeds classified in Montana?

State designated noxious weeds are categorized into five different levels or Priorities.

These levels are classified as priorities 1A, 1B, 2A, 2B and 3. 

This ranking system is based on the management techniques used to control the species as well as their presence and population density. 

In Montana, there is only one species of noxious weed listed under Priority 1A, and currently this plant, yellow starthistle, is not present in the state. 

Under each Priority listing there is a brief description of what is to be done if the species listed under that Priority is found.  Also listed under each Priority heading are the species including their scientific name.   

What are weeds?

Plants growing where they are not wanted

Plants out of place

Plants whose virtues have yet to be discovered

Is this a limited definition?

Questions Is there a difference between a volunteer potato plant in a

patch of lettuce and an invasive, persistent, noxious plant species like field bindweed (Convolvulus arvensis L.)?

Are there certain characteristics that a weed possesses that set it apart from other plant species?

Ideal Characteristics of Weeds

Discussion What makes a weed successful?

Germination requirements fulfilled in many environments

Discontinuous germination and great seed longevity

Rapid growth through vegetative phase to flowering

Continuous seed production

Self-compatibility

Cross-pollination by unspecialized visitors or wind

Very high seed output

Ideal Characteristics of Weeds cont’d

Production of some seed in variable environmental conditions

Adaptations for short and long-distance seed dispersal

If a perennial, vigorous vegetative reproduction or regeneration from fragments

If a perennial, brittleness, so as not to be pulled from the ground easily

Ability to compete interspecifically by special means Rosette, choking growth, allelochemicals

The Bottom Line

Weeds are successful because they have special biological features that allow them to:

1. Harm other plants

2. Survive in a variety of environments

3. Produce abundant, long-lived seeds

4. Grow and spread rapidly

Can we come up with a better definition for a ‘weed’? Weeds are plants that interfere with the growth of desirable plants

and are unusually persistent.

They damage cropping systems, natural systems, and human activities, and are therefore undesirable.

The Impacts of Weeds

Crop impacts Loss of yield

12% of crop loss in the US $32 billion annually

85% of cropland receives an annual application of herbicide $6.6 billion annually in chemical control

Doesn’t include the cost of herbicide application, tillage, hand-weeding

Can also have indirect impacts by limiting other resources (e.g., water)

Loss of quality Weeds contaminate harvested crops

Objectionable odors

Wild garlic in wheat, mustards in dairy feed can ruin milk

Crop spoilage from moisture

Staining of crop or edible seeds

The Impacts of Weeds

Crop Impacts Increase processing required for harvested crops

Added expense

Loss of value

Can’t be used as certified seed

May be docked at the grain elevator due to presence of weed seeds

Limit management choices Crop selection

Limited by life cycle of weeds present, herbicide compatibility/carry-over

Tillage options

The Impacts of Weeds

Crop Impacts

Harbor non-beneficial pests

Overwinter in crop debris and underground root systems

Rodents in no-till crops

Interfere with harvesting

Slows hand-harvesting

Vines and thicker-stalked weeds wrap around cylinders of mechanical harvesters

Wear on machinery

Necessitates the use of desiccants

Increase transportation costs

The Impact of Weeds

Human Impacts Allergies/rashes

Weed pollen (from ragweed and perennial grasses) affects more that 45 million Americans

Poison ivy/poison oak

Fires

Obstructions

Poor aesthetics for homeowners

Livestock impacts Poisonous plants in rangeland

Tall larkspur, locoweed, lupine, halogeton, leafy spurge

Milk sickness, abortion, death

Reduced value of rangeland

Invasive Species

Many crop weeds are native, but many others are introduced

As many as 73% of weed species in crops are non-native

Invasive species are:

1) non-native to the ecosystem under consideration

2) species whose introduction causes or is likely to cause economic, environmental, or human health problems

Better known invasive species in the US:

Leafy spurge (Euphorbia esula) Purple loosestrife (Lythrum salicarid)

Yellow starthistle (Centaurea solstitialis)

Giant reed (Arundo donax)

Downy brome/cheatgrass (Bromus tectorum)

Saltcedar (Tamarix spp.)

Spotted knapweed (Centaurea maculosa)

Eurasian watermilfoil (Myriophyllum spicatum)

Questions!

1. Cropland weeds allow farmers to choose from a multitude of crop management options. True or False.

2. In Montana, a noxious weed is considered to be “any ________ plant species established or that may be introduced in the state that may render land unfit for agriculture, forestry, livestock, wildlife, etc.”

a. Native

b. Exotic

c. Non-invasive

d. Undesirable

3. Name one characteristic that helps weed species increase their success, and explain why it makes them successful.

Common lambsquarter Scientific name: Chenopodium album

Family: Amaranthaceae (aka the Goosefoot family)

Life cycle: Annual

Where found: Cropland

Physical description:

Seed: shiny black

Cotyledon: linear to bluntly lanceolate

True leaves ovate to lanceolate

Alternate arrangement

Have a mealy/powdery coating on underside of leaf

Newest leaves of seeding often tinged pink/purple

Stems pink or purple, often striped at maturity

Upright growth habit

Interesting facts:

Latin name describes plant

Chen=goose, podium=foot refers to shape of true leaves

album=white, refers to whitish coloration imparted by the mealy coating

Redroot pigweed Scientific name: Amaranthus retroflexus

Family: Amaranthaceae

Life cycle: Annual

Where found: Cropland

Physical description: Seed: shiny black

Cotyledon: linear to lanceolate, reddish undersides

First true leaves ovate, slightly notched at tip

Subsequent true leaves ovate to lanceolate, small bristle at leaf tip

Alternate leaf arrangement

Leaves feature prominent veins

Upright growth habit

Interesting facts: Thick, red taproot

Inflorescence is characterized by the presence presence of spiny green bracts