Managing Weeds in Vegetable Crops: Introduction

Table of Contents

1. Introduction
2. Preplanting (PP) Treatments
3. Preplant Weed and Cover Crop Control
4. Preplant Incorporated (PPI)
5. Preemergence (PRE)
6. Postemergence (POST)
7. Inter-row Weeding
8. Stale Seedbed Technique
9. Wiper Applicators for Selective Weed Control

Introduction

In April of 2009, the Ministry of Environment ammended the Pesticides Act with the Cosmetic Pesticides Ban Act, 2008 and Ontario Regulation 63/09. Pesticides are now classified for sale and use under 11 different classes. It should be noted that agriculture (e.g. nursery
production) is exempted. For more information on the legislation see the Ministry of the Environment’s website at www.ontario.ca/pesticideban.

More that 50 vegetable crops are grown commercially in Ontario. Some are seeded, some transplanted, some are annuals, and other crops are perennial, presenting a very diverse group of situations to manage weeds. Weed management involves all aspects of weed control in vegetable crops including scouting, herbicides, cultivation, hoeing, mulches, etc.

Perennial weeds are often a more severe problem in perennial horticultural crops, such as asparagus and need to be controlled prior to planting.

Recommendations for snap beans and sweet corn are listed in the field crop section (Beans, page 87 and Corn, page 105) because they are similar to those for field corn and field beans. Differences are noted where necessary.

Treatments listed for horticultural crops in this publication are based on extensive research trials. Do not use herbicides in cold frames or greenhouses unless specifically recommended.

Horticultural crops are often high value crops. Weed competition is costly. Improper application of herbicides can also be costly. Sprayers used for hormone type herbicides (2,4-D, etc.) should not be used to apply insecticides, fungicides or other herbicides on susceptible horticultural crops.

Herbicide Application Timings

Preplanting (PP) Treatments

Preplant treatments are applied before the crop is sown or planted. Some of these herbicides kill seedlings soon after germination while others also kill weed seeds. Most herbicides used for these treatments must be thoroughly incorporated into the soil by cultivation soon
after application. Check the label. Also see Preplant Weed Control, Preplant Site Preparation Prior To Any Crop on page 81 for details of products, rates and remarks.

Preplant (PP) Weed and Cover Crop Control

Control cover crops and emerged weeds before seeding or transplanting vegetable crops. Refer to Table 6-1. Non-Selective Herbicides Available for Preplant Site Preparation, page 77. Alternatively, a grower may choose to kill the cover crop and/or emerged weeds just before planting the vegetable crop and either till the area or leave the seedbed untilled.

Preplant Incorporated (PPI)

Two incorporations at right angles operating at a depth of 10 cm using a double disk (7–10 km/hr) or vibrating shank S-tine cultivator (10–13 km/hr) are required unless otherwise stated. Cultivation-type equipment used for herbicide incorporation is known to spread
perennial weeds to previously uninfested areas. Special attention should be directed toward machinery cleanliness, and/or treating fields with perennial weeds last.

Preemergence (PRE)

Rainfall at 15–20 mm within 7–10 days after application is necessary to activate preemergence treatments. Shallow cultivation, rotary hoeing or harrowing will control weed escapes and improves herbicide activity in the absence of rainfall. These materials prevent emergence of many weed seedlings without reducing crop stand. Apply immediately after seeding or at least before the emergence of the crop. If these materials are applied after weeds have emerged, kill is usually poor. Best results are obtained with this method when conditions for weed seed germination are good.

Postemergence (POST)

These chemicals are applied after the crop plants have sprouted or after the crop has been transplanted. Applied as directed, the weeds can be killed without injury to the crop. Leaf stage of the weeds is critical for good weed control. Smaller weeds are generally easier to kill but there needs to be enough leaf surface to intercept the herbicide. Apply according to recommended leaf stages. Crop stage is important to optimize crop safety. Adjuvants will frequently improve the weed control when used as directed. Weather or other conditions may influence the optimum rate of adjuvant, see the product label for more details. Always use appropriate drift management technology.

Inter-row Weeding

The herbicide is directed between the crop rows onto the emerged weeds. The herbicide is used as a rescue operation where other methods of weed control have failed. Use special low pressure (7–15 kPa) applicators such as dribble bars or vibra jets equipped withshields to prevent wetting the crop. See Stale Seedbed and Inter-Row Weeding, page 82.

Stale Seedbed Technique

The stale seedbed technique is useful when the soil can be worked well before planting and weeds are allowed to emerge for several weeks. Apply one of the nonselective herbicides listed under the Stale Seedbed and Inter-Row Weeding, page 82. Seeding or planting directly into the killed weeds with minimal soil disturbance will allow the crop to establish before the next flush of weed emergence. Follow up with either cultivation, hoeing or postemergent or directed herbicides to control later germinating weeds. Where registered, some herbicides can be applied after seeding but before crop emergence.

Wiper Applicators for Selective Weed Control

Wiper applicators (rope-wick, roller applicator or similar device) have been extensively used with glyphosate (ROUNDUP). Check product labels for use of this application technique with other herbicides.

Related Links

• Weeds in Ontario
• Vegetables in Ontario
• Problem Weed Control in Field Crops
• Integrated Pest Management (IPM)
• Pesticide Application and Safety
• Pesticide Companies and Agents
• OMAFRA Sprayer Calibration Calculator
• Search the PMRA website
• OMAFRA Publication 505: Ontario Weeds
• OMAFRA Publication 75: Guide To Weed Control