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: Harm other plants Survive in a variety of environments Produce abundant, long-lived seeds 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 Loss of quality 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 Livestock 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

Downy brome/cheatgrass Eurasian watermilfoil 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! Cropland weeds allow farmers to choose from a multitude of crop management options. True or False. 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.” Native Exotic Non-invasive Undesirable 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