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record of the Institute for Food and
Agricultural Sciences and should be
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the Institute and its staff. Current IFAS
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Copyright 2005, Board of Trustees, University
D EEW CONTROL IN FOLIAGE S
BY HU LIBEEf Y
j Richard T. Poole
Agricultural Research Center Apopka Li'
ARC-A Research Report RH-80-2
F..S. Univ. of Florida
Weeds increase production costs and compete with desired plants for aval'labFTe1
water, light, carbon dioxide and nutrients. Weeds also harbor insects, diseases
and nematodes which may attack desired plants. Because foliage plants are used
primarily for their aesthetic value, weeds infesting containers must be removed
to preserve plant beauty, quality, and marketability.
Weed control is often obtained as the by-product of certain practices utilized
in the production of vigorous, uniform, disease and pest-free plants under climate-
controlled conditions. Proper fertilization and irrigation, disease and insect
control are important tools for controlling weeds. Nevertheless, the use of
materials to suppress weed growth in and around the nursery has a place in modern
production practices, and even more so in open field conditions found in semi-
tropical and tropical nursery growing areas where environmental controls cannot
be rigidly maintained.
CLASSIFICATION OF HERBICIDES BY ACTION
Soil or Industrial Sterilants. Certain materials are used to "sterilize"
the soil for extended periods and usually remain in the treated-soil for a
considerable length of time. For example, such treatment is frequently used in
areas along railroad or highway rights-of-way, around utility substations or
power poles or along fence rows. Possible uses of these materials in a nursery
would be limited to "root free" loading areas, perimeter fence rows or in or
adjacent to roadways, provided that the effects of these materials would be
limited to the immediate area of placement.
Fumigants. Fumigants are highly volatile materials or gases (sodium azide,
calcium cyanimid, paradichlorobenzene, formalin, chloropicrin, methylbromide)
which are toxic to vegetative portions of plants and most seeds as well as to
many other forms of life in the soil. They generally do not persist in the soil
for more than 1 to 2 weeks. They are applied to moist, well-mixed, loose soil
as granules, drenches, or gases.
Preemergence Herbicides. This group of herbicides consists of materials
which are applied to the soil surface and usually incorporated into the upper
few centimeters of soil before the weeds germinate. These herbicides must be
applied before the weeds emerge, as they have little or no effect on weeds already
present. They are usually non-toxic to the crop for which they are registered,
although safety of a material on corn, cotton or peanuts does not guarantee safety
on philodendrons or dracaenas!
Postemergence Herbicides. Other materials, when applied to growing plants,
kill the plant quickly (within several hours) or slowly (several days to weeks).
Postemergence herbicides are applied to weeds already present in contrast to the
preemergence herbicides. Postemergence herbicides may be divided into two
categories depending on whether or not they kill only the contacted portions of
the plant ("contact herbicides") or are absorbed into the vascular system and
translocated through the shoot or root system, eventually killing one or the
other or both ("systemic herbicides"). To achieve good kill with a contact
herbicide, cover as much of the foliage of the target plants as possible. If
the crop is coexistent with the weeds, the use of a directed spray to keep the
herbicide off the foliage of the crop plant is sometimes necessary.
Herbicides are applied either as sprays or granules; dusts are not used
because of the hazard of drift. Sprays may be prepared with a completely water
soluble crystalline or powdered material (SP), an emulsifiable concentrate (EC),
or a wettable powder (WP) suspension. Wettable powders remain suspended in water
for a certain amount of time, but eventually most or all of the powder will settle.
Constant agitation is necessary when using wettable powders to prevent settling
and uneven application rates. Granular (G) formulations contain a small amount
of active ingredient impregnated on a large volume of an inert carrier such as
clay, ground corn cobs, sand or vermiculite. Coverage with granules may not be
as good as with liquid application, but costly spray equipment is not required
and granules tend to fall off foliage without injuring the crop plant. However,
the use of granules on palms, dracaenas, sansevierias, dieffenbachias, and other
plants forming cups, whorls, or rosettes of leaves carries with it the risk of
funneling granules into the center of the plants and damaging them.
Two methods of describing rate of application of herbicides are: 1) pounds
per acre of formulated material; or 2) pounds per acre of active ingredients
(active ingredients being the specific-chemical(s) responsible for the herbicidal
effects). For example, a herbicide can be formulated as 5G or 50WP. If 5 pounds
per acre active ingredient is required to control weeds, 100 pounds of the 5G would
be used but only 'O pounds of 50WP material would be used. One hundred pounds of
5G contains 5 pounds active ingredient, but 10 pounds of 50WP contains 5 pounds
HERBICIDES LABELED FOR ORNAMENTAL USE
CASORON EPTAM ROUNDUP
DACTHAL LASSO SURFLAN
DEVRINOL PRINCEP. TOK
DIQUAT RONSTAR VEGEDEX
WEED CONTROL IN THE GREENHOUSE AND IN STOCK BEDS
To the author's knowledge Diquat is the only herbicide registered for use
under greenhouse benches. Other contact herbicides have been used to kill
existing weeds but they are not now labeled for use in the greenhouse and their
use is therefore illegal. Preemergent herbicides, also not labeled, have been
used in the greenhouse -- sometimes with satisfactory results, sometimes with
disastrous results. For good weed control in the greenhouse, a preventive program
should be followed and any germinating weeds removed on a regular basis.
SAFETY OF HERBICIDES IN ENCLOSED STRUCTURES
Herbicide Safety Herbicide Safety
Karmex S Pramitol-P ?
Hyvar-X S Treflan ?
Diquat S Pramitol-L U
Princep ? 2,4-D U
The eradication of established weeds in a stock foliage bed is difficult and
must be accomplished by hand. There are presently no known herbicides that will
selectively kill weeds present and not damage the foliage plants. Prevention
and early eradication of young weeds is the best procedure to follow. When stock
beds are growing vigorously and a dense canopy is formed over the bed, few, if any,
seeds will germinate. When vines or cuttings are taken from the stock beds for
propagating material, areas in the bed-are then exposed to light and seed germination
will occur. The easiest and best time to pull weeds is also after harvesting for
propagation material. The weeds should be removed and a registered herbicide for
the crop applied as soon as possible after harvest. CAUTION: Repeated applications
of a herbicide can cause a toxic amount of the material to accumulate in the soil.
DO NOT apply the herbicide more frequently than the time interval specified on
A word of warning is in order regarding where some of these materials might
be used in a nursery. The use of any herbicide in glasshouses or solid structures
which restrict air circulation and exchange with the outside air should be avoided.
Fumes arising from soil-applied herbicides, even to soil under benches, have caused,
damage in greenhouse crops. Unless certain that the material and formulation are
safe to use in such an area, the best recommendation is to avoid use. A CARDINAL
RULE WITH ANY PESTICIDE WHOSE PERFORMANCE IS UNFAMILIAR TO THE GROWER IS TO TRY
IT ON A LIMITED NUMBER OF PLANTS IN A SITUATION SIMILAR TO THAT UNDER WHICH THE
MATERIAL IS PROPOSED FOR USE.