I am amazed by where weeds appear and conditions
related to their sudden appearance. Our family has a small cabin on Lake Martin
in the piedmont region of central Alabama. This reservoir was completed in 1926
to generate electricity and help control flooding downstream. As a result of the
latter use, the lake elevation is intentionally lowered in the autumn by 10 to
12 feet to create storage capacity for winter rain.
In 1986, there was severe
drought in the region and the lake never reached full pool during the summer.
Near our cabin, a small stream flows into the lake, with a delta of sediment deposited
in a broad plain—an artifact from when the surrounding landscape was in
subsistence farming a century ago. That summer, the sediment dried for first time
in recollection. The result was about five acres of cocklebur and small flower
morning glory— common weeds of crop production. Presently, that small stream
drains a watershed that is totally woodland, with no cultivated cropland. Seed
deposited in the delta decades earlier remained dormant since the delta was
normally underwater during the summer months. The dry summer of 1986 featured
conditions that favored germination; in this case adequate oxygen and sunlight.
Weeds appeared where they were not expected.
Weeds appearing
in unexpected places is a perpetual headache for those who manage food plots,
whether for hire or as a hobby. Understanding the phenomenon of the weed seed bank
will help anticipate the problem and minimize losses. There are underlying factors
to consider; previous land-use, seed production, and seed dormancy.
PREVIOUS
LAND-USE PATTERNS
There are few
stands of true virgin timber in the eastern U. S. and much of our current timberland
has either been previously harvested for timber or cultivated as farmland. These
two disruptions will directly influence rapid changes in plant species
diversity and many of these plants we categorize as ‘weeds’. If timberland
becomes reforested, the ‘weeds’ become scarce and eventually disappear as the
forest matures. Similar processes occur in cultivated sites after crop production
ceases. Basically, these two examples are what plant ecologists call old field succession,
but starting at different points in the process. In both examples, weeds
produce large amounts of seed, tubers, and rhizomes that are stored in the seed
bank.
NUMBERS OF WEED
SEED
Have you ever
wondered how many seed a weed can produce? Weed seed production is influenced
by species, weed density, and growing conditions. A large isolated weed will
produce more seed per plant than a spindly plant growing in close proximity to others.
Obviously, that effect can be offset by large number of spindly plants.
Regardless of all the qualifiers, refer to Table 1 that lists seed production
of several weed species. These numbers are the reason why preventing weed seed
production is one of the overall goals of integrated weed management in any cropping
system, including food plots. Consider an occasional escaped pigweed in a food
plot; perhaps an eyesore but not necessarily enough to affect forage growth on
a large scale. However, multiplying the number of escaped pigweeds by 200,000
seed per plant produces an enormous number. This does not take into account the
unknown number of dormant pigweed seed already in the soil. Using this example,
would it not be prudent to pull or chop the escaped pigweed before they produce
seed and make a manageable problem much worse?
WEED SEED
DORMANCY
Dormancy is
controlled by the genetic code unique to each plant species and environmental conditions.
During dormancy, weed seed are in a protected state that may last for many
years (Table 1). A useful strategy to reduce seed bank numbers in fallow sites
is to stimulate large-scale weed seed germination with repeated tillage and
control the emerged weeds, either with subsequent tillage, a non-selective
herbicide like glyphosate, or both. This is termed stale seedbed weed control.
The sequence of tillage to stimulate weed emergence followed by control will
partially deplete numbers of viable weed seed in the soil. The longer this
sequence is practiced, the better the results. Yes — it is costly and
temporarily removes a potentially valuable food plot site from forage
production. However, stale seedbed weed control can take a hopelessly weedy
site and hammer the weed seed bank down to a manageable level.
Advocates of
minimum-till production systems contend that this system brings weed seed from
deep in the soil profile back to the soil surface where germination occurs. While
minimum-till production systems offer many soil conservation and time-savings advantages,
this is not the case with weed control. Research has shown that when fields are
tilled, 80 percent of the weed seed near the soil surface are buried; with
later tillage only bringing 38 percent of those seed back to the soil surface.
In other words, tillage buries far more weed seed than it brings back to the
soil surface — a significant net reduction. In contrast, sustained minimum till
production systems cause an accumulation of weed seed near the soil surface where
they can readily germinate when conditions are right. This phenomenon is presently
occurring nationwide with widespread infestations of herbicide resistant pigweeds,
frequently in minimum-till production systems. As a result, farmers are
desperately reverting to conventional-tillage systems that bury accumulated
pigweed seeds stratified near the soil surface and hopefully lessen incidence
of a weed pest that has very limited control options. This is particularly
unfortunate since many of these sites are highly erodible and need to be in a minimum-till
production system.
Weeds often
appear unexpectedly in food plots. This should be anticipated because of the
large numbers of dormant seed in the seed bank. Aggressive measures are needed to
reduce the weed seed bank and prevent weed seed production in food plots. These
goals are critical for successful and sustainable weed management in food
plots.
Table 1. Seed
production and longevity of common weed.
Annual bluegrass
Common lambsquarters
Common purslane
Pigweeds
Common chickweed
Common ragweed
Eastern black nightshade
Fall panicum
Goosegrass
Jimsonweed
Large crabgrass
Pennsylvania smartweed
Field pennycress
Shepherd’s purse
Curly dock
Dandelion
Johnsongrass
Quackgrass
|
Seed Production
(no./plant)
36,000
500,000
242,500
229,000
15,000
62,000
480,000
500,000
135,000
23,400
150,000
19,300
20,000
38,500
40,000
17,860
28,000
400
|
Seed Longevity
(years)
68b
1,700c
40
40
600c
39
39
10
6
39
50b
30
30
35
80
68b
12
10
|
Summarized data
published in: Regnier, E. E. 1994. Teaching seed bank ecology in an
undergraduate laboratory exercise. Weed Technol. 9:5-16. (Refer to that article
for the original source of data for individual weeds.) Seed collected from soil
beneath a meadow, pasture, or forest. Ages of weed seed were indirectly
determined by the duration of the current land-use pattern. Seed recovered from
archeological sites and longevity determined by carbon-dating.
RED POPPIES —
AN HISTORICAL EXAMPLE OF WEEDS IN UNEXPECTED PLACES
The setting is
World War I, the Somme Battlefield in northeastern France. Like many battles in
World War I, combat was largely in a restricted area for an extended period. What
had once been pastoral pastures and small woodlots was transformed into a wasteland
of trenches and artillery impact craters. War had destroyed the topography of
the countryside. The following summer after hostilities moved elsewhere, miles
of the once barren battlefield were transformed into a sea of red poppies in
full bloom. This was the inspiration of the present- day tradition of red
poppies used to commemorate what we now call Veteran’s Day or Armistice Day. The
Somme Battlefield was also the location of a detailed plant ecology study1 that
catalogued the suddenly changed flora and factors that influenced plant
diversity. Repeated artillery barrages pulverized the soil and when combined
with the weathering effects of rain, snow, and freezing/thawing the net effect
was akin to a gigantic plow that haphazardly tilled an entire region in France.
Dormant weed seed (in this case red poppy seed) were exposed to oxygen,
sunlight, and water. What was once pasture and woodland was destroyed by war, but
later transformed to a sea of red poppies. Once again, weeds appeared in
unexpected places.
