Comprehensive Study Guide to Weed Science and Management

Defining Weeds and Their Economic Impact

A weed is most simply defined as "a plant out of place." These plants are responsible for massive expenditures of energy and financial resources annually. Weeds are problematic for several detrimental reasons:

• Competition and Yield: Crop yields are significantly reduced because weeds compete with cultivated plants for essential nutrients, water, and sunlight.
• Efficiency: Land use becomes less efficient when weeds occupy space intended for productive crops.
• Quality: The overall quality of agricultural products is reduced by weed infestation.
• Recreation: The enjoyment of outdoor recreational activities is diminished by the presence of weeds.
• Aesthetics: Weeds spoil the visual beauty of turfgrass and ornamental landscape plants.
• Toxicity: Some weed species, such as poison ivy, are toxic to humans and animals.
• Harvesting: Certain weeds, particularly vines, create physical obstacles during the harvesting process.

Evolutionary and Reproductive Characteristics of Weeds

Weeds possess specific characteristics that allow them to flourish under adverse conditions and successfully compete against cultivated plants. There are four primary characteristics that facilitate their survival:

• Number and Size of Seeds: Weeds produce vast quantities of seeds, which are often much smaller than those of crop plants. For instance, a single tumbleweed plant produces approximately $129,000$ seeds, whereas an alfalfa plant produces around $50,000$. In terms of density, it takes $2,000,000$ tumbleweed seeds to equal one pound (1
  ,lb), compared to $250,000$ alfalfa seeds.
• Lifespan and Longevity: While some weed seeds are short-lived (2
  ,to
  ,5
  ,years), many species remain viable for extended periods. Seeds from species such as Canada thistle, rough pigweed, shepherds purse, mustard, chickweed, purslane, curly dock, and mullein can hibernate in the soil and germinate after $20$ or even up to 70
  ,years.
• Dormancy: Most cultivated plants germinate shortly after maturity. In contrast, many weed seeds enter a state of dormancy, delaying germination until a considerable amount of time has passed and environmental conditions (typically the following spring) are optimal for the growth and development of young plants.
• Vegetative Reproduction: Many weeds do not rely solely on seeds for propagation. They can reproduce vegetatively through various structures, including:
  • Bulbs and aerial bulbs (or bulblets).
  • Corms.
  • Runners or stolons (above-ground stems).
  • Creeping rhizomes (below-ground stems).
  • Creeping horizontal roots.

These reproductive strategies allow weeds to survive and multiply even during periods of unfavorable environmental conditions.

Methods of Weed Introduction and Dispersal

Weeds are spread to new areas through natural processes and human activity. Natural dispersal mechanisms include:

• Animals and Birds: Seeds are often equipped with hooks, barbs, or sticky coatings that allow them to cling to animals passing by. Additionally, both animals and birds consume weed seeds; these seeds can pass through the digestive tract unharmed and are deposited in new locations via manure.
• Wind Dispersal: Wind is a major vector for seed distribution. Seeds designed for wind travel often feature specialized structures such as delicate parachutes, tufts of hair, or wings. Dandelion is a primary example of a weed utilizes these structures to travel miles on even a soft breeze.
• Water (Rains and Flooding): Water can transport seeds over long distances. Some weed seeds possess buoyancy features that allow them to stay on top of the water during rains or floods. Russian thistle is an example of a weed spread via this method.
• Vegetative Spread: Regardless of seed production, weeds can expand across large areas using runners, rhizomes, and creeping roots. Notable examples include:
  • Bermudagrass (Cynodon dactylon)
  • Torpedograss (Panicum repens)
  • Johnsongrass (Sorghum halepense)

Horticultural Glossary: Weed Management and Botanical Terms

• Additive or Adjuvant: A substance added to a pesticide to reduce surface tension between two unlike materials (such as a spray droplet and a plant surface), improving adherence and coverage. Also known as a surfactant.
• Annual: A plant that completes its entire life cycle—germination, growth, flowering, seed production, and death—within a single growing season.
  • Summer Annual: Germinates in the spring and produces seed by the end of the same growing season.
  • Winter Annual: Germinates in the fall, over-winters as a young plant, and produces seed during the following growing season.
• Broadcast: The process of sowing seeds or applying pesticides/fertilizers uniformly over an entire specific area rather than in rows or spots.
• Dicotyledon (Dicot): A plant that produces two seed leaves upon germination.
• Growing Season: The duration between the start of plant growth in the spring and the cessation of growth in the fall.
• Herbicide: A chemical agent used specifically to kill undesirable vegetation.
  • Contact Herbicide: Kills or injures only the plant parts it touches directly. It is effective primarily on green plant tissue.
  • Systemic Herbicide: Taken up by the leaves or roots and translocated throughout the entire plant system to kill it.
  • Selective Herbicide: Designed to kill specific targets (e.g., 2,4-D kills broadleaf weeds) while leaving other plants, like lawn grass, largely unharmed.
  • Nonselective Herbicide: Kills or injures almost all plant species. Examples include glyphosate (Roundup, Touchdown Pro) and glufosinate (Finale).
  • Preemergence (PRE): Applied before weeds emerge from the soil. It often requires rainfall or irrigation to move the chemical into the top few inches of the soil.
  • Postemergence (POST): Must be applied after weed seedlings have already emerged to be effective.
• Monocotyledon (Monocot): A plant that produces a single seed leaf upon germination.
• Noxious Weed: A plant species designated by law as potentially harmful to public health, crops, livestock, or property; also refers to highly invasive, difficult-to-control plants.
• Perennial: A plant that lives for more than 2
  ,years, producing new foliage and seeds every growing season.
• Preharvest Interval: The legally mandated time that must pass between the last pesticide application and the harvesting of the crop.
• Preplant: A product applied to the soil before the crop is actually planted.
• Spot Treatment: Applying pesticide only to a small, specific section where weeds are present rather than treating the whole area.
• Tolerance (Turf): The ability of a specific turfgrass species to withstand a normal dosage of herbicide without suffering injury or death, often due to physiological or anatomical traits.
• Warm-Season Grasses: Turf species that reach optimum growth at temperatures between 80
  ,^{\circ}F and 95
  ,^{\circ}F.
• Weed-and-Feed: A commercial product that combines fertilizer and herbicide for simultaneous application.
• Weediness: The statistical likelihood of seeds germinating into unwanted plants that will require removal.

Herbicide Coverage and Surface Area Dynamics

The effectiveness of herbicide application is heavily influenced by the leaf surface and the use of adjuvants. Coverage varies significantly depending on the solution composition and the physical characteristics of the leaf.

Application Variables

• Water droplets alone: Often bead up on the leaf surface, providing minimal coverage due to high surface tension.
• Water with $\frac{1}{2}\%$ v/v non-ionic surfactant: Dramatically increases the spread of the droplet across the leaf surface.
• Water with $1\%$ v/v crop oil concentrate: Provides extensive spread and adherence, often superior for penetrating difficult surfaces.

Leaf Surface Types

• Waxy Leaf: These leaves possess a thick waxy cuticle over the leaf tissue. Droplets tend to bead on this surface unless a surfactant is used to break surface tension.
• Hairy Leaf: These leaves feature leaf hairs (trichomes) that can suspend droplets above the actual waxy cuticle and leaf tissue. Surfactants are critical here to ensure the herbicide reaches the leaf tissue rather than just sitting on the hairs.