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What is a weed?

Weed ID Is a Way to Make Sense of Biodiversity:

Weed identification is a scientific way of making sense of weed biodiversity. We organize weeds by species, but as you gain experience with weeds you will learn that a "species" can often be a slippery concept. For example: The Foxtails are an excellent example of a "species-group"A species-group is a group of closely related species usually with partially overlapping ranges.  Each is its own species and they do not interbreed with each other, but they are closely related and occupy similar niches in a single field.. So as you learn your species in this section, be aware that variants, hybrids, biotypes, ecotypes, phenotypes, mutants and lots of other forms of a species make weed biodiversity the first principle of weed identification.

1st ID: The first step in managing weeds is to correctly identify the plant pests you want to manage. Different weed species, and even variants within a single weed species, respond to management tactics very differently. Correct identification is fundamental to weed management.

Weed Species: The focus in this course will be on weeds commonly found in Iowa, and midwestern US, crop production fields. But we will also look at other weeds commonly found in urban settings, pastures, waste areas, lawns and golf courses, and maybe a few from around the world.

Weed ID Throughout: Weed identification will be included in all sections of this course, not just this section. As we learn about weed biology and ecology, weed habitats and environment, and weed management I will emphasize the individual weed species being discussed. In this way you will integrate weed science concepts with the actual, living pests you will encounter in the real world.

Definitions of a Weed

The "Ideal Weed Concept"

Ideal characteristics of the worst weeds (Baker, 1965, 1974):

Seed bank:
-Germination requirements fulfilled in many environments.
-Discontinuous germination (internally controlled) and great longevity of seed

-Rapid growth through vegetative phase to flowering
-If a perennial, vigorous vegetative reproduction or regeneration from fragments
-If a perennial, brittlenenss so as not to be drawn from ground easily
-Ability to compete interspecifically by special means (rosette, choking growth,

-Continuous seed production for as long as growing conditions permit.
-Self-compatibility but not complete autogamy or apomixy
-Cross-pollination, when it occurs, by unspecialized visitors or wind
-Very high seed output in favorable environmental circumstances
-Production of some seed in wide range of environmental conditions; tolerance and
-Adaptations for short-distance and long-distance dispersal

Adaptive Characteristics of Agronomic Weeds (Patterson, 1985)

Related to physiology, growth, and competitiveness
-High relative growth rates in seedling stage
-High rates of photosynthesis
-Rapid development of exploitative root systems
-Rapid partitioning of photosynthate into new leaf area production
-Rapid vegetative growth to reproductive phase
-Special "weapons" for interference
-Freedom from environmental constraints ("general purpose genotype"); high capacity
for acclimation to changing environment

Related to reproductive phase
-Breeding systems that provide some outcrossing but also allow self-fertilization
-Copious seed production under favorable conditions with some seed production
occurring over a range of favorable and stressful conditions
-Pollination by wind or generalized insect visitors

Related to cultural practices
-Morphological and physiological similarity to crop
-Timing of seed maturity to coincide with crop harvest
-Resistance or tolerance to chemical herbicides
-Resistance to mechanical control; regeneration from rhizomes or other vegetative
-Seed dormancy, longevity in soil; discontinuous germination over long periods of time


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