Weed Seed Dispersal

Weed control, management, ecology, and minutia Weed Seed Dispersal Reading assignment : Harper: Ch. 2: pp. 33-60; summary p. xiv Introductory concepts Seed rain as a process in time Characteristic seed dispersal patterns

Weed Seed Dispersal

I was out in the field today and drove by this fallow field and did two things. First, since I’m a weed scientist, I stopped to take a photo of this weedy streak. Second, because I know the site manager and know he hates weeds, I called to give him a hard time about “missing one”.

Any guesses on what happened here? It might help if I told you I’m facing south and this area often has a prevailing wind out of the northwest during winter and early spring. Another clue would be the construction site to my back with minimally managed weeds. One more clue might be the fence behind me still loaded up with Russian thistle carcasses and debris.

Here’s a link to the UC-IPM Pest Note (Pub 7486) on Russian thistle. That document indicates ” Russian thistle normally matures in late summer. The seed is spread when mature plants detach at the base and are blown along by the wind. A large Russian thistle plant may produce more than 200,000 seeds. In spring, months after their dissemination, it is possible to trace the paths of tumbleweeds across plowed fields by the green trails of germinating Russian thistle seedlings .”

I think we have a winner! Looks like a handful of Russian thistles, one of our common “tumbleweed” species in California went rolling across this landscape earlier in the spring scattering seeds as it made its merry way to the next fenceline.

Good photo. Probably one of the “tumbleweeds” meandering across the field and spreading its seed on the way. FYI, Harry Agamalian has a similar great photo meandering across a seedling vegetable field in the Salinas Valley.

See also  Dill Weed Seeds

Weed Seed Dispersal

Reading assignment: Harper: Ch. 2: pp. 33-60; summary p. xiv

  • Introductory concepts
  • Seed rain as a process in time
  • Characteristic seed dispersal patterns and mechanisms: Non-animal
  • The role of animals in dispersal
  • Post-Dispersal movement of seed

Two demographic ways of looking at dispersal: two parts of the same whole

the expanding range and increasing population size of of an invading weed species into a new area

the part of the process by which an established and stabilized weed species in an area maintains itself within that area

  • dispersal (leaving mother plant)
  • post-dispersal events (subsequent movement)


androchorous seed: dispersed by the agency of humans; synonymous terms: anthropochorous; brotochorous

anemochorous seed: dispersed by the agency of wind

barachorous seed: dispersed by its own weight; synonymous term: clitochorous

bolochorous: seed dispersed by the agency of propulsive mechanisms

: seed dispersed by the agency of animals; synonymous terms: synzoochorous; zoochorous

entomochoric: seed dispersed by the agency of insects

: seed dispersed by the agency of water

: seed dispersed by the agency of ants

Seed rain as a process in time

Dispersal of seed occurs in 4 dimensions (not just 2-D)

2-D: Land/habitat/soil surface area phenomena: distance, width

  1. Differences in time seed ripens on mother plant
  2. Differences in time seed retained on mother before its dispersed: seed shattering
    a. example: weeds with long flowering period and corresponding long period of ripening and release: seed shatters immediately after ripening on mother
    b. weeds flowering and seed production in narrower time period, often require harvesting activity to release seed: evolved with crop, maximize chance of dispersal with crop
  3. Weed seed ripening over short time not imply release and dispersal in corresponding short time
  4. Selection for time of seed release: adaptation to release quick or retain on mother plant?
    a. quick release: oversaturate predator demands of feeding and leave enough after that for new colonizers
    b. slow release: maximize chance some seeds land on ground during favorable time for quick germination or escape from predators
  5. Dormancy: delay in conjunction with dispersal, post-dispersal mechanisms to optimize time between seed maturity and germination
See also  Seed Weed Fertilizer

Characteristic seed dispersal patterns
and mechanisms: non-animal

Gravity dispersal:

many of our common, unspecialized weed seeds don’t move very far from mother plant without animals, humans

preventative weed control is very important

invasion from the outside overemphasized

  1. Wind dispersal is a function of:
    a. how fast seed falls: weight, density, ability to float in air
    b. height of release: may be most important factor in distance spread; mullein seed
    c. speed, direction and turbulence of wind between release point and ground
  2. Very light, dust-like seed (poppies; fungal spores)
  3. Specialized seed structures
    a. plumed seeds or fruits: e.g. dandelion, pappus; milkweed, plume; thisles, pappus; sowthistles; pappus is characteristic of Compositae family seeds
    b. winged seeds: maple seeds: lift provided by wings; asymetrical wing can be an advantage
  4. Movement of mother plant (with seeds) on ground after dispersal, blow with wind; seeds dispersed in rolling action; examples: kochia on mother plant, forms rolling ball; tumbleweed; velvetleaf capsules

Seed ejaculation; example: mistletoe seed propelled by mother plant; also has sticky seed surface to adhere to tree host, bird foot vector

  1. Analogy of invasion of a habitat by colonizing plant (weed) with spread of pathogenic infection in crop
    a. Crop infection: logarithm of infection density to the logarithm of distance from source
    b. If the slope is that of the inverse square law, or steeper, a population will spread into a colonizable territory as an advancing front: “horizon of infection”; the steeper the slope the more sharply defined is the infection front
    c. Dispersal curves with slopes less than than that of the inverse square law will lead to a spread-out pattern of isolated colonists, which may subsequently act as new foci for new infections
  2. Poverty of seed fall in the immediate neighborhood of the parent characteristic of of plants in isolation; e.g. mullein, musk thistle is pasture
  3. Species with specialized wind dispersal mechanisms generally don’t colonize as a horizon but as isolated individuals over a greater distance
  4. Dense stands will colonize as an advancing front
See also  Weed From Bird Seed

The role of animals in dispersal

Animal Dispersal:

Species-specific feeding patterns

Territorial and migratory behavior

Eating, digestion, viability changes

Animal storage of seed: distance, concentration, location

Birds: ones with hard gizzards that destroy seeds; soft gizzards that pass on viable seed

Ants play important role, especially in storage concentrations

Specialized structures:
a. burrs: cocklebur, sandbur seed
b. appendages: beggarticks seed

  1. New continent, region invasions
  2. Local dispersal: machinery, crop seed contamination
  3. Crop mimicry dispersal
    a. weed seed adaptations to look like crop seed: plant body or seed same size, shape, morphology as crop
    b. example: barnyardgrass biotype looking like rice escapes handweeding and is dispersed with rice
    c. example: nightshade fruit (“berries”) same size, shape as dry beans, harvested and dispersed with beans

Post-Dispersal movement of seed

Wind-blown, or soil surface movement by water: accumulate near obstacles (fences, furrows, etc.) or fall in soil crevices, dried soil cracks: composite family weeds with pappus, etc., seed held at constant distance in crack (soil) by pappus not entering crack

Specialized mechanisms of some weed species to bury themselves: wild oat hygroscopic awns

Dynamic interactions with soil surface roughness, by species