Managing Wild Radish in Wheat
Stanley Culpepper, Extension Agronomist
Wild Radish Biology
Wild radish (Raphanus raphanistrum) is the most common weed infesting small grains throughout Georgia.
Wild radish is classified as a winter annual, but recently it has been germinating in Georgia during the fall, winter and spring. The weed germinates most commonly when soil temperatures drop below 65˚ F. Studies indicate
that both a chilling requirement and a shattering of the fruit pod must occur to break dormancy.
Seedling wild radish has heart-shaped cotyledons. The first true leaves are slightly serrated and indented, and
are about two to three times as long as they are wide (Figure 1). They are covered with stiff, bristly-feeling
hairs. As the leaves mature, the serrations become more jagged and deeply indented.
Wild radish remains in the rosette stage for much of the year, reaching 10 to 14 inches across the base. When
conditions are favorable and the plant matures, it bolts. Bolting is a process in which the internodes (regions of
the stem between leaves) begin to lengthen and a flower stalk forms at the top (Figure 1). In wild radish, multiple flower heads form on several branches arising from a single flower stalk. The flowers are generally yellow
but occasionally may be white.
Figure 1. Wild radish from emergence through bloom.
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Economic Importance
Wild radish competes with crops for water, light, nutrients and physical
space. In addition, the seed stalk can interfere with harvesting by causing an inefficient combine operation. Wild radish seed pods often contaminate harvested grain, thereby reducing profits due to excess cleaning
costs.
Instead of shattering and releasing seeds, the seed pod dries down and
fragments into small sections, each containing a single seed (Figure
2). These seed pods are very close in size and shape to wheat and other
small grains and are extremely difficult to remove in a routine cleaning
process.
Figure 2. Wild radish seed pods (left)
and wheat seeds (right).
In addition to having a direct impact on small grain yield and/or quality, wild radish can harbor many serious
insects and diseases, including blackleg (Leptosphaeria maculans), a major disease problem for canola and
cabbage. Heavy wild radish infestations could provide enough inoculum to cause problems in some rotational
Wild Radish Control
Cultural Control Methods: Weeds are often controlled most effectively through cultural practices that
result in rapid small grain stand establishment and canopy development, which provides an undesirable environment for weed growth. Cultural practices include:
1)
2)
3)
4)
5)
Planting certified seed (free of weed seeds)
Good, weed-free seedbed preparation
Proper fertilization
Seeding at the proper rate and time
Disease and insect management
Mechanical Control Methods: Wheat and other grains are either planted in narrow rows (drilled) or
broadcast seeded and disked in, making mechanical cultivation impossible. Mowing would help reduce seed
production, but wild radish does not begin to bolt until the grain begins to joint. Because of these factors,
mechanical control is not considered an option for small grain weed control; however, mowing could be a fair
alternative in small grains used for grazing.
Chemical Control Methods: If applying herbicides, read and follow label recommendations. Generally,
larger weeds are more difficult to control than smaller weeds; therefore, timely herbicide applications are critical. Many of the herbicides used in wheat affect growth processes within the target weed. In essence, the more
actively the plant grows, the better the control. Applications made to stressed weeds (e.g., drought, wet, cold,
heat) will often result in decreased control.
Many herbicides should be applied only during certain stages of wheat development to avoid crop injury (Figure 3, Table 2). Although the stage of development provides a good indicator for application timing, factors such
as environmental conditions, crop health and variety (early vs. late maturity) also impact crop tolerance.
2,4-D or MCPA. These phenoxy herbicides are extremely effective in controlling wild radish (Table 1). Applications of 2,4-D (3.8 lb active/gal) or MCPA (4.0 lb active/gal) at 1.0 to 1.25 pt/A are recommended for wild
radish 8 inches or less in diameter. These herbicides often do not adequately control chickweed and henbit.
Mixtures of 2,4-D or MCPA with Harmony or Express are advised for most fields.
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Application timing for these herbicides is critical. The critical period for 2,4-D applications is after wheat is
fully tillered but before jointing (Feekes stages 4 and 5, Figure 3). Application before this growth stage may
cause a “rat-tail” effect whereby the leaf does not form and unfurl properly. The crop may appear stunted and be
delayed in maturity, and tiller development may be adversely affected. Conversely, application after jointing has
commenced may result in malformed seed heads.
Apply MCPA after wheat tillers (preferably 2+ tillers) but before jointing (Figure 3). Injury (similar to 2,4-D injury) can be observed if MCPA is applied before or after the critical period of application for wheat. In general,
MCPA causes less injury to wheat than 2,4-D, but it is also less effective on larger weed species.
Harmony Extra. Harmony Extra is a prepackaged mixture of the sulfonylurea herbicides thifensulfuron-methyl
and tribenuron-methyl. Harmony Extra controls many winter annual broadleaf weeds, but wild radish must be
small (less than 3 inches in diameter) for adequate control (Table 1). 2,4-D or MCPA at 0.25 to 0.375 lb active
ingredient/A may be mixed with Harmony Extra for improved wild radish control. A nonionic surfactant at the
rate of 1 qt/100 gallons of spray solution is recommended when Harmony Extra is applied in water.
An advantage of Harmony Extra compared to 2,4-D is its wide window of application. Harmony Extra can be
applied in wheat after the two-leaf stage but before the flag leaf is visible (Figure 3).
PowerFlex (active ingredient pyroxsulam) can be applied in wheat at 3.5 oz/A from the three-leaf stage until
jointing. In addition to ryegrass, the PowerFlex label claims control of several broadleaf weeds including Carolina geranium, common chickweed, hairy vetch and wild mustard, and suppression of henbit. The label does not
mention wild radish but numerous Georgia studies suggest good to excellent control of radish up to 6 inches in
diameter. Add crop oil concentrate at 1% v/v (1 gal crop oil/100 gal spray solution) with the application.
Osprey (active ingredient mesosulfuron) is a postemergent herbicide applied at a rate of 4.75 oz/A in winter
wheat from emergence up to the jointing stage of growth. The manufacturer recommends adding a nonionic
surfactant at 0.5% by volume (2 qt/100 gal spray) plus 1 to 2 qt/A of urea ammonium nitrogen fertilizer (28-0-0,
30-0-0, or 32-0-0) or ammonium sulfate fertilizer at 1.5 to 3 lb/A (21-0-0-24). Osprey plus this adjuvant system
will normally control wild radish 4 inches in diameter or smaller.
Osprey will also provide good control of henbit and common chickweed if applied when these weeds are smaller than 2 inches at time of application. Osprey may be mixed with Harmony Extra to improve control of broadleaf weeds, including wild radish.
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Table 1. The Effect of Stage of Growth on Wild Radish Control in Wheat.
Stage of Growth1
Herbicide
0-4 inches
4-8 inches
8-12 inches
Bolting/Flowering
2,4-D
2
>95%
>95%
>85%
60-75%
MCPA
>95%
>90%
>70%
50-60%
Peak
>90%
>85%
75-85%
<40%
Express
85-90%
>60%
50%
<40%
Harmony Extra
85-90%
70-80%
50%
<40%
Express + MCPA
>95%
>95%
>85%
60-75%
Osprey
>95%
70-80%
60-70%
50-60%
PowerFlex
>95%
>85%
>80%
60-75%
Wild radish size in diameter of leaf rosette.
Percent control as compared to untreated areas.
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Table 2. The Effect of Stage of Growth on Wheat Injury by Various Herbicides.
Stage of Growth1
Herbicide
0-1 tiller
2-3 tillers
full tiller
Jointing
2,4-D
2
>80%
35%
0-10%
70-90%
MCPA
>30%
10%
0-5%
50-70%
Peak
0-5%
0-5%
0-5%
5-10%
Express
0-5%
0-5%
0-5%
5-10%
Harmony Extra
0-5%
0-5%
0-5%
5-10%
Express + MCPA
>30%
5%
0-5%
50-80%
Osprey
0-5%
0-5%
0-5%
0-5%
PowerFlex
0-5%
0-5%
0-5%
0-5%
Refer to Figure 3 and the Small Grain Production Guide for growth stages.
Percent injury (visual chlorosis or necrosis or tiller malformation).
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Circular 839 Revised October 2009
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