November 2020 - 10

MSUE VEGETABLE NOTES
Effective weed control vital for higher yielding carrots
Sushila Chaudhari
Michigan State University
Michigan ranks fourth in both
acreages of processed carrots and total
carrots (processed and fresh market).
Carrot production successfully occurs in
both muck (pH of 5.5) and sandy loam
(pH 6.5) soils. Carrots are planted from
mid-April to June and subsequently
harvested from July through October
for fresh market and October through
November for processing.
Weed control in carrots is important,
because weeds can easily outcompete
carrot plants due to the slow emergence
and slow initial leaf growth of carrot to
shade the ground. Weeds can compete
with carrot for light, water, nutrients
and space during the growing season,
and at harvesting can cause problem
with digging. Many weeds act as hosts
to insects and diseases that may then do
further damage.
Carrot growers face a multitude of
weed problems including crabgrass,
pigweeds, lambsquarters, wild radish,
common groundsel, common purslane,
chickweed, yellow nutsedge, horseweed
and several others. If weeds are not
controlled, yield losses have been
reported to range from 50 to 100%.
Weed-free carrot field. Photo: Bernard Zandstra/Michigan State University
Weed control in carrot fields starts
in fall after harvest of rotational crop,
and is mainly achieved through the
combinations of chemical, cultural and
mechanical weed control strategies.
After crop harvest in fall, pay close
attention to the weed species those are
present in your field. If there are many
perennial weeds, then it is the best time
to apply glyphosate.
Cultivation can be effective
for annual weeds, but it is not
appropriate for perennial weeds
due to their vegetative re-rooting
following disturbance. Planting cover
crops in fall is also a good strategy to
manage winter weeds. Stale-seed bed
is very important technique to manage
early flushes of weeds in spring before
planting carrots. With this technique,
prior to preparing the seedbed, weeds
are allowed to germinate and then killed
with minimal soil disturbance using
herbicide (paraquat and/or glyphosate),
flame weeder, or shallow cultivation.
Carrots are very sensitive to weed
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competition during the first few weeks
of growth. Therefore, preventing new
weed germination and killing emerged
weeds in the cotyledon stage is critical
to achieve successful weed management
throughout the growing season.
Herbicides always play an important
role for weed management in carrots.
Lorox (Linuron) is the primary
herbicide that is considered as backbone
for weed control in carrots, because it is
efficacious, provides excellent crop safety
and is inexpensive. It can be applied
pre- or post- emergent and is efficacious
on a broad spectrum of weeds and
stages of weed development. However,
as a result of intensive use of Lorox
the number of linuron-resistant weed
biotypes has been reported. Therefore,
it is important to rotate herbicides from
other modes of actions.
A pre-emergence application of
Dual Magnum (S-metolachlor), Prowl
(pendimethalin), Lorox, and Caparol
(prometryn) after seeding but before
carrots emerge helps to control weeds
while carrot germinate. If the plan
is to grow barley cover-crop as wind
breaker in carrots then plant barley at
least 5 and 7 days before applying Dual
Magnum and Prowl, respectively.
Treflan (trifluralin) application
before seeding is labeled to use only in
mineral soils. It should be incorporated
in 2-3 inches of soil layer soon after
application to avoid any losses through
volatilization.
The next preemergence application
may be applied using Prowl at layby as
a directed spray to the soil between the
row middles or Dual Magnum at 3-5
leaf stage; be careful not to exceed label
recommendations on rate. The later
application of Prowl or Dual Magnum
helps to extend the residual control
of weeds; both of these product have
no activity on already emerged weeds.
Lorox, Caparol and Tricor (metribuzin)
can also be applied for postemergence
control of broadleaf weeds.
Lorox can be applied after carrots are
3 inches tall. Tricor application should
made when carrots have 5-6 leaves.
Tricor may cause injury to carrot if
application is made under cool and
cloudy weather, or temperature greater
than 85˚F. A postemergence application
of Caparol may be applied through the
sixth-leaf stage of carrot. Addition of
non-ionic surfactant with Caparol will
improve weed control but may increase
carrot injury.
Fusilade (fluazifop-P), Poast
(sethoxydim) and Select Max
(clethodim) can be used as required
during the season to control grasses;
be careful not to exceed label
recommendations on rate. For effective
control, application should be made
when grasses are small and actively
growing. Select Max is the only
one that has activity against annual
bluegrass. Poast is more effective
for crabgrasses and Fusilade is for
quackgrass and johnsongrass.
Weed control in carrot on mineral
soil is more challenging because of
greater potential for crop injury from
the herbicides. Therefore, it is always
recommended to use low rates of
herbicides on light sandy soils, and
increase the rate on soil containing more
clay and organics. Hand weeding may
be required if weeds escape herbicide
application, but any weeding activity in
carrot fields may be difficult because of
high-density planting and can damage
crop and reduce yields.
Detailed information related to
the herbicide rates and efficacy on
weeds can be obtained from labels and
Michigan State University Extension
bulletin E-433, " Weed Control Guide
for Vegetable Crops, " that contains
lists of all currently labeled herbicides
along with specific remarks for their
use in carrot. VGN
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November 2020

Table of Contents for the Digital Edition of November 2020
