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S3 31 /v CENTRAL FLORIDA EXPERIMENT STATION
SFS 7, Sanford, Florida
Mimeo Report CFS71-4 November 11, 1970
WEED PROBLEM CHANGES AFFECTING
CENTRAL FLORIDA VEGETABLE PRODUCTION
W. T. Scudder
Observations over a 15 year period in the Sanford and Zellwood vegetable
producing areas have revealed several marked changes in the predominant
problem weed species. Many weeds of serious concern in 1956 no longer
present an economic problem, while others which were relatively unimportant
or unknown at that time are serious pests today.
Among the weeds unrecognized as crop pests in 1956, but serious now,
are burning nettle, wild celery, broadleaf signalgrass, and Texas panicum.
Other new problem species, known earlier but not considered serious, include
common lambsquarters and common ragweed on the sand soils and curly dock
and smooth pigweed on the Zellwood mucky-peat soils. In contrast, species
such as giant amaranth and spiny amaranth have become relatively unimportant
in recent years.
These population changes are due to many ecological factors. Most of
the changes may be attributable to the introduction of new species into these
areas or to the usage of selective chemical herbicides.
During the 15 years from 1956 to 1970, there have been several changes
in the weed problems confronting Central Florida vegetable growers. Some
weed species have declined in importance, while others which did not occur
as serious crop pests before 1956 have become important economic problems.
Several of these weeds were unknown in the area before 1956, while others,
through present, were not nearly as serious as they are today.
The changes reported here were observed in commercial and experimental
row-crop fields in the Sanford and Zellwood vegetable production areas.
Common names used in this paper are those accepted by the Terminology Sub-
Committee on Standardization of Common and Botanical Weed Names of the
Weed Science Society of America, as published in the October 1966 issue of
Weeds. In a few cases, where the species were not included in the WSSA list,
locally prominent names are used. In most cases, the botanical names have
been verified by Dr. Daniel B. Ward of the University of Florida Herbarium.
Florida Agricultural Experiment Stations Journal Series No. 3808. Sub-
mitted for publication in Florida State Horticultural Society Proceedings
Volume 83, 1970.
CHANGES OBSERVED IN WEED POPULATIONS
In the Sanford area, with predominantly Leon fine sand soil, the most
important population changes have involved the introduction of new species.
Most of these species were already present elsewhere in Florida, but did not
occur as problem weeds in the Sanford area until recently.
New Weeds of Economic Importance in the Sanford Area
Of the above introduced species, narrowleaf signalgrass and horse purslane
were first noted about 1950. They were identified by Professor Erdman West
of the University of Florida Herbarium. The others have all appeared as
problem weeds since 1956.
Five species, indigenous or long-time residents in the Sanford area, have
become increasingly more important during recent years. Conversely, the
populations of two species have declined. These are listed below.
Old Weeds Increasing in Prominance in the Sanford Area
Old Weeds Declining in the Sanford Area
On the Everglades mucky-peat soil in the Zellwood area of Orange and Lake
Counties, eight newly introduced species have become serious problems since
1956. Most serious of these are burning nettle and wild celery, both of which
have appeared and spread very rapidly during the last six years.
New Weeds of Economic Importance in the Zellwood Area
Cressleaf groundsel butterweedd)
Increasing in importance at Zellwood are several weed species which were
known to occur in the peat soils before 1956, but which were of negligible con-
cern to the vegetable growers before that time. All of these were common on
the Sanford mineral soils, but they were not serious pests on the organic soils
during the early years of this study.
Old Weeds Increasing in Importance in the Zellwood Area
Annual smooth groundcherry
Although smooth pigweed populations have increased during recent years,
this has occurred to the almost total exclusion of two other Amaranthus species
which formerly were very prevalent on these organic soils. These are giant
amaranth and spiny amaranth. The three other weeds in the following list have
persisted, but are of much lesser importance today than they were in 1956.
Old Weeds Declining in the Zellwood Area
Common Name Botanical Name
Giant amaranth Amaranthus australis
Spiny amaranth Amaranthus spinosus
Goosegrass Eleusine indica
Wandering cudweed Gnaphalium pensylvanicum
Narrowleaf cudweed Gnaphalium falcatum
FACTORS AFFECTING WEED SPECIES POPULATIONS
Weed population changes, such as those indicated here, reflect alteration
of the natural balance of the plant species native to the area. Some of the
ecological factors causing these changes are listed below:
1. Introduction of new species
2. Competition between species
3. Use of chemical herbicides
4. Crop culture methods
5. Genetic adaptation of species
Each of these factors will be discussed briefly to show how they have
functioned to bring about some of the changes mentioned in this paper.
Introduction of New Species The effect of new introductions on the
population balance of species in an area is self-evident. In many of the
cases noted here, the new weed was well adapted to the Central Florida
environment and soon established itself as a successful competitor with both
crop plants and other weeds. Most of these new species came from other
areas of Florida. For example, the Phyllanthus and Eclipta species have long
been common in South Florida. Wild celery is abundant in native hammock
and other wet land areas, but it has become a problem in the Zellwood vegetable
area only recently. Likewise, narrowleaf signalgrass, a native of Australia,
and horse purslane, from the Carribbean region, have been in Sanford fields
approximately 20 years. They appeared in the Zellwood vegetable area ten
years ago, presumably transported along with vegetable transplants. The
movement of farm equipment and field boxes also has helped the spread of
weed species from one vegetable producing area to another.
There are several known cases where new weeds have been introduced
from other areas as contaminants of seeds. In both the Sanford and Zellwood
areas, umbrella dodder, now a serious pest, appeared first in carrot plant-
ings. Several species which have appeared by this means have been unsuccess-
ful in becoming established. Among these are coffee senna (Cassia occidentalis)
- 5 -
found in soybeans, and dwarf mallow (Malva rotundifolia) and spurred anoda
(Anoda cristata) found in spinach at Zellwood. Burning nettle, one of the most
vigorous and objectional weed pests in both areas, is believe to have been
introduced in hay imported from other states for livestock feed to supplement
the poor pastures following the severe 1957 Florida freeze. It quickly spread
throughout many pastures and cultivated fields at both Sanford and Zellwood.
Its natural tolerance to low temperatures leaves it with very little competition
following winter, frosts, ;
dqmpetition Between Species. The define of a species once prevalent in
cultivated areas often can be explained onlr by its inability to adequately compete
with the rhore vigorous introductions. When two weqds have the same require-
ments for seed igeriinaiion and grobthi the i oire vigorous species will dominate,
competing successfully with the other ior moisture, nutrients, and sunlight. It
then produces greater growth and more seed for the next generation. Among
the species declining due to competition from other weed species are spiny
amaranth, goosegrass, and the cudweeds.
Use of Chemical Herbicides. The widespread use of chemical herbicides
during recent years is responsible for several changes in our weed populations.
As the more susceptible species are eliminated, the more tolerant ones survive
and dominate. Black nightshade, which is highly chemical-resistant, has
created a well-known problem in South Florida, particularly where Solonaceous
crop herbicides are used. Likewise, in Central Florida, both black nightshade
and annual smooth groundcherry populations have increased because of their
tolerance to herbicides. The problem with both species of nutsedge has also
been aggravated during recent years due to the use of selective chemical weed
killers. The nutsedges, notably poor competitors, have often developed large
populations in fields where good selective herbicides have prevented the
growth of most broadleaf weeds and grasses. Two other examples of serious
problems resulting from the expansion of populations of chemical-resistant
species involve common ragweed, which is tolerant to most cabbage and
carrot herbicides used in the Sanford area, and wild celery, vh ich is tolerant
to all of the herbicides registered for use on carrots and celery. These two
species are rapidly being recognized as the most serious vegetable weed pests,
with the exception of nutsedge, in Central Florida.
Crop Culture Miethods. Long established cultivation and other crop culture
methods generally have little influence on the proportions of different weed
species present in the fields. Over a long period of time, if other factors
are not involved, weed populations remain fairly constant. On the other hand,
with increased attention to ditchbank and roadside sanitation or with better
cultivation practices, fewer weed seeds of certain species are produced and
relative populations change. A notable example of this on the organic soils at
Zellwood is the near eradication of giant amaranth, a native plant in the area.
This weed, the largest succulent annual that grows in the United States, is very
vulnerable to all types of mowing and tillage operations. It is large and brittle
and does not flower until fall. This permits mechanical destruction of the
plants before seeding both in the fields and along ditchbanks.
Genetic Adaptation of Species. Built-up chemical resistance, or changes
in the inherent growth characteristics of the plant, have been responsible for
changes in the economic importance of some species. For example, curly
dock has been recognized as a perennial ditchbank weed of minor importance
at Zellwood for many years. Though seeding profusely at the field margins, it
was readily controlled in the crops by cultivation. In recent years, however,
seedling curly dock has loomed as a serious winter weed throughout many
vegetable fields, presumably because of genetic adaptation. It has caused
considerable economic loss, particularly in spinach and other leaf crops grown
for processing. Some weeds appear to have developed resistance to certain
chemical herbicides after long-time use. This has been attributed to genetic
adaptation, through natural selection and the elimination of the susceptible
plants from the population.
During the relatively brief 15 year period during which these observations
were made, there have been numerous changes in the problem weed species
in our Central Florida vegetable crop fields. Most of these ecological changes
have been due to the introduction of new species through the transportation of
contaminated crop seed, transplants, or equipment, but others have been
caused by the use of new cultural methods and chemical herbicides, and by the
effect of competition between weed species. Genetic alterations also appear to
have occurred, permitting the adaptation of some species to better fit their
environment or tolerate adverse situations such as those created by the use of