Weed Control Recommendations


Fruit crops are high value crops and weeds are often the number one cultural problem faced by many growers. Fruit acreages must be kept relatively clean of weeds to achieve a reasonable return on investment. Every grower should implement a pre-planned integrated weed control program. This program should take into consideration all the factors that influence weed control (cultural, chemical) and crop growth with the aim of maximizing crop growth and minimizing costs.

Weed Biology

Fruit crops are perennial crops, and therefore, can be invaded by weeds at many different times of the year (spring, summer, fall). Weed species have different lifecycles which allow them to establish at different times during the growing season. Weeds are classified into 4 different lifecycles: summer annuals, winter annuals, biennials, and perennials.

Summer annuals include many of the most common weeds, such as wild mustard, redroot pigweed, smartweed, and wild oats. This group of weeds germinates in spring, flowers, and produces seed in one growing season. Therefore, these plants exhibit rapid growth in spring and summer. The best time to control spring annuals is early in spring when the weeds are small (2 to 4 leaf stage). These plants cannot survive frost.

Winter annuals include such weeds as flixweed, stinkweed, and shepherd's purse. Winter annuals are weeds that germinate in fall, form rosettes, go dormant over winter, and resume growth early in the spring. A rosette is a circular cluster of leaves growing close to the soil surface. In spring, the weeds bolt, flower, set seed and die in mid to late summer. The time to control winter annuals is when they germinate in fall. Control in spring (when rosette starts to regrow) is not effective because the weeds are too large at this time. Therefore, it is important to scout fields for winter annuals in the fall.

Biennials include such weeds as nodding thistle, biennial wormwood, absinthe, and wild carrot. Biennial weeds germinate in spring, grow vegetatively during the first growing season, and go dormant as a rosette overwinter. The plant resumes growth the second spring, flowers, sets seed, and dies the second growing season. Therefore, biennial plants survive over 2 growing seasons. The best time to control biennials is when they are small (in their first year).

Perennials include such weeds as quackgrass, Canada thistle, and dandelion. Some perennials, such as dandelion, exist as one plant and reproduce mainly by seed. Others like quackgrass and Canada thistle, exist in clusters and can reproduce vegetatively (ie. underground rootstalks). Perennials can survive over several growing seasons as they regrow from buds below the soil surface. As a result, to control perennial weeds you must try to kill the root system as well. Perennial weeds are very competitive and should be eliminated prior to planting the fruit crop.

Resources Available

To assist in the identification of plants, local agricultural staff, such as an Agricultural Representative or Weed Supervisor, can be approached. Manitoba Agriculture, Food and Rural Initiatives has a Crop Diagnostic Center that also deals with plant identification. Plant samples can be forwarded (through local agriculture offices) for identification.

Two good publications used in weed identification include: The Weed Seedling Identification Guide (available for no charge from Manitoba Agriculture, Food and Rural Initiatives) and Weeds of The Prairies (available from Alberta Agriculture - $15.00).

Site Preparation before Planting

To ensure optimum weed control throughout the entire life of the strawberry field, the crop must be established in soil that is as weed free as possible. The best weed control program starts before the crop is planted. Since herbicide options in fruit crops are limited, steps should be taken to minimize the impact of weeds before planting. Crop rotations before planting the fruit crop should be used to clean up the field. Cultivation and herbicides should both be used to control weeds. Roundup can be used to burn off summer annuals or perennials in spring. Smother crops such as buckwheat or fall rye grown in the season prior to planting are strongly recommended. These crops will compete strongly with weeds and help clean up weedy land.

Field Sanitation

Windbreaks, fencelines, and field borders can serve as a source of weed infestation. Keep these areas as weedfree as possible to prevent escapes. Tillage equipment should be cleaned between fields to avoid introducing weeds.

One of the greatest sources of weeds can be the straw used for mulching. Whenever possible, a field should be inspected prior to obtaining the straw to assess the weed population. Using straw that has been double thrashed is the best practice, however, this is often difficult to obtain. Research has shown that the ammoniation of straw can kill weed seeds. Weedy straw may be improved through ammoniation. However, straw that has been ammoniated is less rigid and may break down faster.

Herbicide Action and Injury

Cultural control measures (tillage, sanitation, mowing, etc.) should be integrated with herbicides when possible.

Prior to planting, pre-emergent products such as Treflan EC, Rival, and Bonanza 400 can be applied. These products require incorporation (twice), and therefore, must be applied to a clean seedbed to ensure maximum contact with soil particles. Pre-emergent products control germinating weeds only.

Several products such as Simazine, Devrinol, and Dacthal act primarily through the roots and will not control emerged weeds. Therefore, if weeds have germinated tillage is necessary before applying these products. After application, irrigation or rainfall is required to activate these products.

Non-selective products such as Gramoxone and Roundup control all green vegetation, and therefore, cannot contact the fruit crop. Roundup is highly mobile within the plant and will move to control buds on roots. Gramoxone will not move within the plant. Therefore, Gramoxone will control only the green vegetation it contacts. Some products leave a residue. Such products can provide residual weed control (Sinbar, Simazine) but can also cause injury to subsequent crops.

Be aware of recropping restrictions that apply with some fruit herbicides.

Some herbicides such as Sinbar and Simazine are selective and can be sprayed over the fruit crops. However, fruit crops can be injured with herbicide applications. With some products, varietal restrictions apply (Sinbar). The condition of a stand will also influence how much injury can occur from a herbicide application. A weakened or stressed stand ie. from disease, moisture stress (too much or too little), temperature stress, insect attack, etc.] is more vulnerable to herbicide injury. Apply herbicides only when the fruit crop is healthy and growing vigorously.

Often herbicides recommended for fruit crops have a range of recommended rates. Rate often differs due to soil type. For instance, light textured or sandy soils require less herbicide than heavier soils. Use the lowest recommended rate on fields low in organic matter.

Tables are included in Weed Control Recommendations for Fruit Crops which illustrate the choice of products that can be used at different times during the planting and fruiting years. Growers should ensure that they do not exceed recommended rates or the recommended number of applications per season. Manufacturers also recommend a water volume that should be used to apply the product. Always use the recommended volume as injury can result if lower water volumes are used.

Application of Herbicides

Herbicides must be applied with calibrated spray equipment to ensure proper application for weed control and crop safety. Backpack sprayers do not give uniform application and can result in herbicide injury. Spray equipment must also be adjusted to avoid overlap. Spray booms must be adjusted to the correct height above the crop to avoid nozzle overlap. Spray marking systems can be used to assist with proper spray swath marking.

Herbicide manufacturers recommend nozzle types, spray pressures, and water volumes for proper application. With some products a range of water volumes may be recommended. Use the higher water volumes if weed populations are dense to ensure adequate coverage is achieved. Always follow manufacturers recommendations to ensure an appropriate spray pattern is achieved.

To delay herbicide resistance, specific herbicides and herbicide groups should not be used more frequently than 1 in 3 years. It is essential that herbicide application records be kept to monitor herbicide use. Record keeping will also allow residual products to be tracked. In addition, in the event of herbicide failure (poor weed control or severe crop injury) application records (time, temperature, rate, etc.) can assist in identifying the problem.

How to Identify Crop and Weed Leaf Stages

Recognition of plant growth stages is essential for effective weed control. Many herbicides are safe on a crop only when applied at a specific growth stage. Similarly, weeds are controlled only when they are at certain growth stages. For most post-emergence products, growth stages are described by number of leaves.

Cereals and Annual Grass Weeds

Leaves are counted, starting at one for the first leaf, and progressing up the primary shoot. Tillers are important but not counted as leaves. A leaf should be counted as soon as it emerges, but may be labelled as early, mid or full leaf. The early leaf stage is when it begins to emerge, the full stage is just before the next leaf emerges.

Tillers, or stools, are the secondary shoots of a grass plant. The first tiller emerges from the axil of the first leaf, the second just above the second leaf and so on. Tillers generally appear at the three to four leaf stage. Be sure to identify tillers, and count only leaves on the primary shoot. As well, do not remove any leaves from the main shoot when separating the tillers.

Leaf Stages of Broadleaf of Weeds and Crops 

Broadleaf Weeds

Cotyledons - These are the seed leaves which usually emerge above ground. On some plants, such as fababeans, lentils and peas, they stay below the soil surface. Cotyledons are not true leaves and are not counted when determining leaf number. They are a different shape than the true leaves and may dry up and disappear at an early stage.

Alternate leaves - Some plants have one leaf at each node on the stem. The next leaf emerges at the next higher node and extends away from the stem in the opposite direction. These plants (lamb's quarters and wild mustard are good examples) are said to have alternate leaves. To determine the leaf stage, simply count the numbers of leaves present.

Opposite leaves - Plants with two leaves at each node, one on each side of the stem, are said to have opposite leaves. The next pair of leaves on the next node are rotated about 45 degrees so that they are not directly over the previous pair. Plants with opposite leaves have even-leaf numbers only. When counting, the leaf number progresses from cotyledons to 2 leaf, 4 leaf, etc. These plants generally appear shorter than plants with alternate leaves at a similar leaf stage. Be sure to count each pair as two leaves. Hemp nettle is a weed which has opposite leaves.

Whorled leaves - More complex plants like cleavers may have whorled leaves. These plants have three or more leaves at each node on the stem. The leaf number in each whorl may vary, so be sure to count each individual leaf unless label recommendation refers to the number of leaf whorls.

Leaf Stages of Broadleaf of Weeds and Crops 

Resistance of Weeds to Herbicides

In recent years, the number of herbicide-resistant weeds and the areas they infest in Manitoba have increased.

Herbicide-resistant weeds arise following repeated use of the same herbicide (or herbicide group) for a number of years on the same field. Growers who have developed weed resistance on their farms will typically see a weed, which is normally controlled by a herbicide, escape uncontrolled after a number of years of use of the same product or product group.

Herbicide resistance should be suspected under the following conditions:

  • A weed species that the herbicide controlled in previous seasons now escapes the treatment, while other weeds which appear on the label continue to be controlled in the field.
  • The escapes cannot be attributed to adverse weather or emergence after application (if a post-emergence product is in question).
  • Irregular-shaped patches of a weed develop where the herbicide gives little or no control.
  • Records of the past history of the field show repeated use of the same herbicide, or combinations of herbicides, which kill the weed in question in the same way.

Herbicide Resistant Weeds in Manitoba

Current Status of Herbicide Resistance


Description of Resistance

Wild Oat

Resistant to a group of herbicides that includes Achieve, Assure, Champion Plus, Fusion, Hoe-Grass, Horizon, Post, Poast FlaxMax, Prevail, Puma, Select, Triumph Plus, Venture. Typically resistant to 10 or more times normal field rates.

Resistant to Avadex BW, Fortress and Avenge, Resistant to 1.5 to 4 times field rates. At present, confirmed in Manitoba only.

Resistant to three different groups of herbicides that includes Achieve, Assure, Champion Plus, Fusion, Hoe-Grass, Horizon, Poast, Poast FlaxMax Prevail, Puma, Select, Triumph Plus, and Venture (Group 1); Assert and Pursuit (Group 2); and Mataven (Ungrouped). Resistant to 2 to 5 times field rates.

Green Foxtail

Resistant to trifluralin products (Treflan, Rival, Advance, Fortress, Bonanza) and to Edge. Resistant to 5 or more times field rates.

Resistant to a group of herbicides that includes Achieve, Assure, Champion Plus, Fusion, Horizon, Laser, Poast, Poast FlaxMax, Prevail, Puma, Select, Triumph Plus, Venture. Resistant to 10 or more times field rates.

Resistant to two different groups of herbicides that includes Achieve, Assure, Champion Plus, Fusion, Hoe-Grass, Horizon, Poast, Poast FlaxMax, Prevail, Puma Select, Triumph Plus and Venture (Group 1) as well as Treflan, Rival, Advance, Fortress, Bonanza and Edge (Group 3).

Wild Mustard

Resistant to a group of herbicides that includes 2,4-D, Attain, MCPA, Banvel, DyVel, Target, Estaprop, Curtail M and Poast FlaxMax. Resistant to 10 or more times the field rates. At present, confirmed in Manitoba only.

Resistant to a group of herbicides that includes Bladex, Lexone, Primextra Light, Sencore and Atrazine. Resistant to approximately 5 times field rates. At present, confirmed in Manitoba only.

Kochia, wild mustard, chickweed, hemp nettle

Resistant to a group of herbicides that includes Ally, Assert, Express, Muster, Odyssey, Pursuit, Refine Extra, Laser DF, Champion Plus and Triumph Plus. Resistant to 10 or more times the field rates. Wild mustard, chickweed and hemp nettle confirmed in Manitoba only.

How to Identify Weed Resistance

It is important to avoid confusing herbicide failure due to resistance with herbicide failure due to various other factors (such as weather or application errors). When a herbicide fails to control weeds due to weather or application factors, that herbicide may work in the field the next season. But when herbicides fail due to the development of resistance, they will fail in subsequent years, regardless of weather or application procedures.

Avoiding Weed Resistance

It is far easier to avoid development of resistant weed strains than it is to eradicate or control them after they develop and infest an area.

To avoid the development of resistance on your farm, take the following steps:

  • Rotate herbicide usage so that the same herbicide is not used year after year. Rather than using the same product on an annual basis, consider using other products on a regular basis.
  • Where practical, use tank mixes where both active ingredients within the tank mix may act to kill the same weed using different mechanisms of action.
  • Be aware that when resistance to one product develops in a weed population, it can often mean the weed population has developed resistance to other herbicides which act in a similar manner.
    For example, green foxtail, which is resistant to Trifluralin, may be resistant to Edge as well; Kochia which is resistant to Ally, may be resistant to Refine Extra; wild oat populations resistant to Hoe-Grass 284 may be resistant to Achieve.
  • Keep accurate records of crop rotation and herbicide use. It will be easier to plan your long term weed management strategies if you have good records of your past management practices.
  • If resistance is suspected, avoid spreading crop seed, weed seed or crop residues from the affected area. This will minimize spread of the problem.
  • Report suspected cases of resistance on your farm to your ag rep, weed supervisor or industry representative.

Resistance Grouping

To help you plan your herbicide program, the table below lists "herbicide groups". To slow down the process of developing weed resistance, use products from different groups from year to year on your fields.

New herbicides do not necessarily have a unique mechanism of action and may fall within the groups listed below.

NOTE: Herbicides that have the same mechanism of action may not control the same weed spectrum or have the same crop safety. For example, Assert and Ally have the same mechanism of action; however, Assert controls wild oats while Ally does not. Remember to read and follow label instructions.

Herbicide Groups Based on Mechanism of Action

Group 1
(contain ACCase grasskiller)

Achieve, Assure, Champion Plus*, Fusilade II, Fusion, Hoe-Grass 284, Hoe-grass II*, Horizon, Laser*, Laser DF*, Poast, Poast Ultra, Poast FlaxMax*, Prevail*, Puma, Select, Triumph Plus*, Venture

Group 2
(contain ALS/AHAS inhibitors)

Ally, Amber, Assert, Champion Plus*, Express, Laser DF*, Muster, Odyssey, Pursuit, Prism, Refine Extra, Triumph Plus*, Unity*

Group 3
(contain mitotic inhibitors)

Advance, Bonanza, Edge, Rival, Treflan, Fortress*

Group 4
(contain growth regulator herbicides)

2,4-D, Accord, Attain, Banvel, Buctril M*, Champion Plus*, Curtail M, Dyvel, Dyvel DS, Embutox, Estaprop, Kil-Mor, Laser*, Laser DF*, Lontrel, MCPA, Mecoprop, Poast FlaxMax*, Prevail*, Target, Thumper*, Tropotox Plus, Triumph Plus*

Group 5
(contain triazines)

Atrazine, Bladex, Lexone, Sencor, Princep Nine-T, Simazine, Sinbar

Group 6
(contain photosynthetic inhibitors - nitriles/benzothiadiazoles)

Buctril M*, Hoe-Grass II*, Pardner, Laser*, Thumper*, Basagran, Unity*

Group 7
(contain photosynthetic inhibitors - ureas/amides)

Lorox, Afolan, Linuron, Stampede

Group 8
(unknown mode of action- inhibition of lipid sythesis)

Avadex BW, Avenge, Fortress, Eptam, Eradicane

Group 9
(contain inhibitors of EPSP synthase)

Roundup, Renegade, Laredo, Victor, Wrangler, Touchdown

Group 15
(unknown mode of action - contains napropamide)

Devrinol 50 DF

Group 20
(inhibits cell wall synthesis Site A, contain dichlobenil)

Casoron G-4

Group 22
(contains paraquat - photo system I - electron diverters)


Group 25
(unknown mode of action - contains flamprop-methyl)


Other Herbicides

The herbicide Dacthal W-75 is in a group of its own.

  • * Some products contain more than one active ingredient and therefore may appear in more than one group. In some instances, both active ingredients act to kill the same weed using different mechanisms of action. In these instances, use of tank mixes may slow down the process of developing weed resistance.

Herbicides - Hazard Ratings & Relative Toxicities

Product or Trade Name

Active Ingredient

Poison Hazard Rating1

Relative Toxicity2

Other Hazard Rating3

Afolan F linuron none very low none
Bonanza 400 trifluralin caution low none
Casoron G-4 dichlobenil none very low none
Dacthal W-75 chlorthal dimethyl none very low none
Devrinol 50DF napropamide none very low warning
eye irritant
Fusilade II fluazifop-p-butyl caution low none
Gramoxone paraquat danger high none
Laredo glyphosate none very low caution irritant
Linuron 480 linuron none very low none
Lontrel 360 clopyralid caution low none
Lorox DF linuron none very low none
Princep Nine-T simazine none very low none
Rival EC trifluralin none very low none
Rival DF trifluralin none very low warning
eye irritant
Roundup glyphosate none very low caution irritant
Roundup Dry glyphosate none very low warning
eye irritant
Simazine 80W simazine none very low none
Simazine 480 simazine caution low none
Sinbar terbacil none very low none
Treflan EC trifluralin none very low none
Victor glyphosate none very low caution irritant
  • 1 This is the degree of risk symbol or poison hazard rating on the product label. Absence of a risk symbol does not indicate that no hazard is involved in handling or applying the chemical.
  • 2 The relative toxicity rating is based on the degree of risk symbol on the label: danger-highly toxic, warning-moderately toxic, caution-lower toxicity, none-very low, the product does not have characteristics requiring a risk symbol.
  • 3 Products without risk symbols on the label may still be hazardous to varying degrees but do not fall into guidelines requiring a risk symbol or key signal words.