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Thirty-five days (five weeks)
Identify weeds found in different habitats.
Describe the role of habitat and past history on weed infestations.
Weeds are one of the most persistent pests plaguing crop producers, land managers, and
homeowners. What makes weeds so persistent? Well, they have a unique ability to
capture niches created
by humans and produce a considerable amount of seed. Single plants of some weed
species, such as pigweed, can produce more than 200,000 seeds. Not all of these seeds
survive to produce plants, but many remain in the soil for years waiting for the right
conditions for germination. Because
of the prolific
seed-producing capacity of weeds, a few survivors of weed control
tactics can quickly increase the number of seeds in the soil.
The soil can be thought of as a bank. As a weed matures, seed is dispersed from the plant and some of the seed ends up in the soil. This can be thought of as a deposit using our bank analogy. The deposits of seeds into the seed bank can vary considerably among habitats and even within similar habitats. Weed numbers and weed seed production are generally less in natural settings, such as woodlands or prairies. In disturbed habitats, such as farm fields, the weed seed bank can range from 200 to 54,000 seeds per square foot depending on field history. The seed bank can increase rapidly during a year of poor weed control, but diminshes quickly during years when good weed control is obtained.
Withdrawls from the seed bank can occur in one of several ways. The most obvious withdrawl is germination of these seeds into new plants . But, the majority of weed seeds fail to develop into mature plants. This is because seeds are an important food source for many creatures, including birds, small mammals, and insects. In natural settings, animals may eat more than 70 percent of the weed seeds. In agricultural settings, lack of habitat for predators often limits this weed predation. Many weed seeds are also lost through decay after being infected with fungi or various microorganisms.
Weed seed can be physically moved from one area to another. Many weed seeds have special anatomical features that allow them to be blown by the wind or attached to the coats of animal or clothes of humans. Some seeds can survive the digestive tracts of animals and are carried away with the animal until they are excreted, often far from where they were consumed. Water is an effective transporter of weed seed. Habitats that are occasionally flooded frequently have a more diverse weed population due to deposition of new species. Field machinery, especially tillage and harvesting equipment, can carry seeds out of an area to be re-deposited in a different area. Losses from the seedbank due to movement are generally small when compared to the total seed bank, but these seeds are important because they can introduce weeds into areas where they weren't previously found.
Each weed species has certain biological charateristics that allow it to survive and thrive in specific habitats. Likewise, each habitat contains weeds that are adapted to the specific environmental conditions available. The weed species found in soils that experience frequent tillage, such as gardens and crop fields, are much different than those found in undisturbed habitats, such as prairies and woodlands. Soil from lawns and athletic fields contain weed seeds of species uniquely suited to frequent mowing.
The following demonstration allows for visual evalutation of differences in the weed seedbank from soil of different habitats. Weed seeds rain into the soil each year, where they can remain in the seed bank for many years. When you remove the soil from these areas and place it under ideal conditions, you encourage germination of some of the weed seeds.
The weeds that germinate from the soil collected in each habitat will
be quite different form one another. The lawn will contain seeds of low growing plants
that survive and produce seed even with frequent mowing. They may be both perennial
and annual weeds. Some example include dandelion, planatain, and crabgrass. The crop
field or garden soil will mainly contain seed of fast growing annual weeds that take
advantage of the disturbed conditions of these habitats. Depending on the past
history of the area, the weeds may be very abundant or quite sparse. If the cropped
area was not kept free of weeds in previous years, a subtantial amount of seed will have
been shed into the seed bank. On the other hand, there will be few seeds in the soil
if the area was meticulously weeded. It is most likely that soil from the wooded area will
have very few weeds present. This is because weeds do not grow well in the shaded,
woodland environment. If weeds do germinate from this soil, it may be that they blew
in on the wind or were carried there by animals.
The results of this demonstration can be quite variable depending on the habitats selected
and their history. If many weeds appear in the flat, it is likely that the habitat
was disturbed in the past allowing for at least one season of vast amounts of weed seed
production. These seeds can remain dormant in the soil for many years waitng to
germinate when the conditions are right. Soil from habitats that have not been
recently disturbed will have fewer weed seeds. Seeds that germinate from this soil
may have blown in on the wind or were carried there by animal or water.
Iowa State University Extension Publication IPM-48 "Weed Seeds and the Seedbank:
Implications for Weed Management" is an excellent resource for understanding the
concepts behing this demonstration. It is available from:
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| Copyright 2000, Iowa State
University Agronomy Department All rights reserved. Contributed by Dr. Lance R. Gibson, Agronomy Department Photo credits: Dr. Lance R. Gibson, Agronomy Department Questions, comments, feedback - Email us ! |
