Consider that no plant is native to a container, since a container
is a totally artificial environment created for our convenience.
What works in Mother Nature's soil rarely works the same way in
the unique environment of a container.
The mix, or growth medium,
in a modern container nursery contains NO field soil, but is rather
a mixture of pine bark, sand , peat or other material. Becasue of
the ever-present, perched water table at the bottom of the container,
coarse materials are necessary to provide drainage and avoid root
suffocation from excess water, while retaining sufficient water
to support the crop between irrigation cycles. Further, because
we have restricted the root system of the plant to the volume of
the container (and that root system is quite limited relative to
the root system the plant would develop in the landscape) water
must be reapplied frequently. Water loss from the container is accelerated
by the fact that the temperature of the growth medium in the container
is typically 20 to 50 degrees warmer than soil in a landscape, and
this heat "cooks out" the water and restricts root growth.
Taken altogether the result is that lots of water, applied frequently,
is necessary to grow plants in containers.
In my early research of
trying to sort out the factors involved and most effective ways
to control weeds in containers, two key points surfaced:
1. The high content
of organic material in the growth medium (pine bark, peat, etc.)
binds or adsorbs, like iron filings sticking to a magnet, substantial
quantities of herbicides, thus the rates must be higher than for
the same herbicide applied in the field
2. The more water soluble
the herbicide, the faster it moves downward through the mix, and
the greater the likelihood of damaging the crop.
More than 30 years of
study in the area have amplified my understanding to give the following
simple rule for container nurseries.
Use only preemergent
herbicides with water solubilities LESS than ONE ppm.
None of the compounds listed (see sidebar) will last more
than about two months on the surface of a container due to
elevated temperatures and microbial activity. YOU MUST watch
the calendar carefully, and also check for the first presence
of germination of weed seeds.
REAPPLY REGULARLY. In nearly all cases it is better to be
a bit early than to wait too long and have to pull weeds by
and Barricade must be applied at high rates in order to have
sufficient soluble herbicide to control weeds. Remember, if
too little of the herbicide is soluble, weed control will
be poor, but if too much is soluble, weed control may be good,
but there is a danger of stunting the crop.
compounds currently available that fit this requirement
Factor (prodiamine) @ 0.013 ppm water solubility
Goal (oxyfluorfen) @ 0.1 ppm
Treflan (trifluralin @ 0.3 ppm
Prowl, Pendulum, Southern WeedGrass Control (pendimethalin)@
Ronstar (oxadiazon) @ 0.7 ppm
Gallery (isohexaben) @ 1.0 ppm
is a combination of Goal and Surflan
Snapshot 80 DF is a combination
of Gallery and Surflan
Snapshot 2.5 TG is a combination
of Gallery and Treflan
Ornamental Herbicide II is a combination of Goal
(oryzalin), which appears in Rout and Snapshot 80DF,
has a solubility of 2.6 ppm. I do not recommend its
use either alone or in combination with other compounds
since crop stunting is common.
The practical sequence
that works well is to water the plants in well by hand after potting
up new liners or shifting to a larger pot. Then the next day,
apply the preemergent herbicide
and water it in by sprinkler irrigation. Because of the low
solubility of the suggested compounds, they do not move below about
1/2" deep into the growth medium. Since nearly all weed seeds
require light to germinate, it is essential that the herbicide remain
on the surface where it is needed, and away from the roots of the
control of weeds in the whole area of plants being grown in containers
is also a critical part of the weed control program. The more weed
seeds available to travel onto your plants in containers, the more
likelihood you will have weed problems.
If you recycle your water,
my very strong advice is that ANYWHERE on the property you shoulduse
only those herbicides with water solubilities less than one ppm
. For example, the soil sterilant herbicide Hyvar X has a water
solubility of 815 ppm, and goes wherever water goes, causing damage
wherever it shows up. Roundup is not likely to cause damage because
it adheres to soil and organic particles very strongly and is broken
down quickly by microbes.
NEVER, NEVER let anyone
talk you into using an herbicide at your nursery until you know
its solubility. Further, ALWAYS do a small test area before treating
the entire nursery. When reading the results of an herbicide test,
look to see if they had a double control. A double control is one
where one control plot receives no herbicide and the weeds are allowed
to grow, while the second control receives no herbicide but is kept
free of weeds by hand. Only with this second control can you judge
whether or not the herbicides in the test are stunting the crop.
Weeds will always be with
us, but with a good control program, they need not be a serious
Ph. D. is president of Lacebark, Inc., Publications & Research,
Tel. 405 377 3539
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