Vegetable Crops: Introduction to Weed Management

Table of Contents

1. Introduction
2. Preplant Weed and Cover Crop Control
3. Preplanting (PP) Treatments
4. Selective Preemergence (PRE or PPI) Herbicides
5. Selective Postemergence (POST) Herbicides

Introduction

More than 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 herbicides, cultivation, hoeing, mulches, weed prevention, etc.

Perennial weeds are often a more severe problem in perennial horticultural crops such as asparagus. Control quackgrass, bindweed, thistles, horsetail, dandelions before these crops are planted.

Recommendations for snap beans or sweet corn will be found in the field crop section (Chapter 7, Beans, and Chapter 9, Corn, Field & Sweet) because they are similar to those for field corn and field beans. Differences are noted where necessary.

Treatments listed for horticultural crops in the 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 also can 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.

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Preplant Weed and Cover Crop Control

Control cover crops and emerged weeds before seeding or transplanting vegetable crops. Refer tp Table 6-1, Herbicides Available for Preplant Site Preparation.

The Stale Seedbed Technique involves preparing a seedbed early in the growing season, and controlling emerged weeds several weeks later with glyphosate, Gramoxone, Reglone or Ignite. Seeding or transplanting vegetable crops is done trying to disturb the soil as little as possible to delay the emergence of new crop of weeds. This allows the crop to establish well before the next flush of weeds.

Inter-row weeding with a contact herbicide such as paraquat (3 to 5.5 L/ha (1.2 to 2.2 L/ac) (product) in 250-1000 L/ha (100-400 L/ac) water) can be used as a rescue operation where other methods of weed control have failed. The herbicide is directed between the crop rows onto the emerged weeds. Use special low pressure (7-15 kPa) applicators such as dribble bars or vibra jets equipped with shields to prevent wetting the crop. This technique of weeding with paraquat is registered for use on beans, beets, carrots, cole crops, cucumbers, lettuce, onions, potatoes, sweet corn and tomatoes. See Chapter 6, Stale Seedbed and Inter-row Weeding.

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Preplanting (PP) Treatments

Preplanting 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 in the soil by cultivation soon after application. Check the label.

Selective Preemergence (PRE or PPI) Herbicides

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.

Selective Postemergence (POST) Herbicides

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. The weeds should be small for best results.

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

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