The Weed Saver:
Others Want Gone
Flower head of Canada thistle,
Cirsium arvense, a
perennial weed throughout
the United States. Canada
thistle seeds are among
the many specimens in ARS'
extensive weed seed collection,
located in Beltsville, Maryland.
While the farm crew struggles to kill every weed possible,
Ruth Mangum often steps in to save some.
Last summer, the weed scientist actually planted pigweed,
irrigated it, and hand-pulled other weeds so they couldn't outcompete
it. "The nice thing about growing weeds is that they don't need
to be fertilized," Mangum says. She does feel a little guilty about
the pigweed escapees that will plague researchers who use her one-time
pigweed field for years to come. "I did my best, but you can't
prevent every seed from falling," she says.
This year marks Mangum's 23rd year as weed "librarian"
for the Agricultural Research Service
in Beltsville, Maryland. She curates one of the oldest and most extensive
weed seed collections in the United States. Most of the collection's
290 seed lotsrepresenting 71 weed species and stored in freezers
in small plastic containerscome from the usual uninvited weeds,
like crabgrass and chickweed, that sprout everywhere, including the
7,000-acre Henry A. Wallace Beltsville Agricultural Research Center
where she works. The rest were shipped to her from ARS labs in the South
A patch of Canada thistle,
Cirsium arvense, a
perennial weed throughout
the United States.
"We like to have a pint of seed for each seed lot. These fit in
one to three containers, depending on how large or small the seeds are,"
Mangum says. "Some, like purslane, are like specks of dust, while
others are much bigger, like wild morningglory seeds, which are about
one-eighth inch long." She either strips the seeds from weed plants
in the field or brings the whole plant in and dries it on paper in the
greenhouse. "Then we have a machine for cleaning the seeds, and
we sieve them after cleaning to filter out any debris," Mangum
Mangum, an ARS plant physiologist, learned from colleague Ray Taylorsonwho
started the collection in the 1960s and retired in 1990that the
best way to fight weeds is to know them. Taylorson taught her that "there
are only two basic ways to deal with weeds," she says. "Get
all the seeds in the soil to sprout at the same time, and then kill
the weeds in one fell swoop, or stop the seeds from sprouting in the
first place." To do either, Taylorson had to learn what makes seeds
sprout after they lie dormant for a year or more. Dormancy is a complicated
process that depends on many internal and external factors.
Using a seed counting
device, plant physiologist
Ruth Mangum prepares foxtail,
Setaria, weed seeds for
a greenhouse experiment.
Continuing Taylorson's work, Mangum, weed scientist John Teasdale,
and colleagues at the ARS Sustainable Agricultural Systems Laboratory
are studying how seed dormancy and other aspects of weed biology are
affected by light, temperature, and various farming practices. "The
seeds we collect have different dormancy periods based on the conditions
their parent plants grew under," Mangum says. "Their response
to their environment as well as to control practices will determine
their ultimate spread or decline in agricultural as well as natural
systems. Our research focuses on understanding the dynamics of weed
populations in response to various agricultural practices, including
herbicides, tillage, crop population and spacing, cover crops, and rotations.
The weed characteristics we're studying include germination, emergence,
competition for resources, seed production, seed survival, and spatial
Buckhorn plantain, Plantago
lanceolata, a common
weed in lawns and abandoned
areas throughout the United States.
Last summer Mangum grew samples from all 30 pigweed seed lots. She
sent them to an outside expert to verify their identifications, since
mistakes are easy to make with plants that look alikeeven to experts
and plant librarians. It is important to positively identify species
thought to be tolerant to some major herbicidesa potential threat
"The biggest surprise we got was that a lot of hybrids were created
by natural cross-breeding of pigweed in the field, something this expert
told us was quite common," Mangum says.
Mangum plants weed seeds not only for identification but also for testing
their viability and for research both in the field and in the greenhouse.
Researchers at Beltsville, for example, use the weed seeds in tests
of vinegar as a herbicide, studies of global warming effects on weeds,
and evaluations of various cover crops for their ability to suppress
weed sprouting and growth.
Plant physiologists Ruth
Mangum and John Teasdale
examine pigweed, Amaranthus,
seedheads grown from different
seedlots from their weed seed
The weed librarian says the farm crew has grown used
to her interest in weeds and always honors her request to save favored
patches of them.By Don
Comis, Agricultural Research Service Information Staff.
This research is part of Integrated Farming Systems
(#207) and Crop Protection and Quarantine (#304), two ARS National Programs
described on the World Wide Web at www.nps.ars.usda.gov.
and John Teasdale are with the USDA-ARS Sustainable
Agricultural Systems Laboratory, 10300 Baltimore Ave., Beltsville,
MD 20705-2350; phone (301) 504-7199, fax (301) 504-8370.
"The Weed Saver: ARS Scientist Preserves Weeds Others Want
Gone" was published in the August
2003 issue of Agricultural Research magazine.