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Ministry of Agriculture, Food and Rural Affairs

Scouting for Weeds

What are Weeds? Weeds are plants growing in the wrong place. For crops, this usually means plants growing directly in or beside the rows. Some of the plants that are growing on the field edges are also considered weeds, especially those that are prone to spreading into cultivated fields.

Why are weeds a problem? Weeds are of concern because they compete with crops for moisture and nutrients. Some weeds may also be alternate hosts of diseases, nematodes or insects. Interference from weeds may also cause labour inefficiencies. (e.g., tall weeds at harvest, discomfort from allergies, or skin irritations from poison ivy, stinging nettles, or thistles).

Why Scout for Weeds? Although weeds are present in every field, there are wide variations in the species growing and the density of each population. Just as scouting for insects and diseases is well-established in integrated pest management, the need to scout for weeds is the basis for Integrated Weed Management (IWM). The information gathered from weed scouting will allow the grower to:

  • identify weeds present early in the season when they can cause yield losses
  • match herbicides to weeds
  • choose cultivator for weed stage
  • alter cultural practices for different weed life cycles
  • discover weed patches before they spread through field
  • identify areas to avoid cultivation
  • identify areas for spot treatments
  • choose the optimum timing for maximum control

How to Scout for Weeds: Scouting for weeds can be done while looking for insects and diseases, although a separate walk through the field may allow more detailed observations and collections. Here are some basic first steps:

  • Identify type of weed eg. grass or broadleaf
  • Identify life cycle eg. annual, biennial, perennial
  • Identify name of weed if possible
  • Identify source of weed eg. mulch, fence row, weed escapes, irrigation water
  • Take samples for verification, with roots, flowers & shoots (in paper bag inside a plastic bag, with a label)
  • Work a zig-zag pattern through the field - remember that many weeds grow in patches
  • Check field edges for weed invaders

How to Record Weed Scouting: Recording this information on your scouting reports will allow the grower to make decisions in this season, and will provide a long-term record of weed emergence patterns and problems in the orchard. Here are some basic notes that should be written down:

  • Identify the weed, if possible
  • Note the growth stage of the weed and crop
  • Note the number of leaves and their shape
  • Note the stage of flowering
  • Note areas and percentage of the field infested – mark on a field map

Tools Needed for Weed Scouting: A field scout will need a few basic supplies to do a good job of weed scouting. Be sure to bring along:

  • A keen eye
  • A hand lens
  • Field maps
  • Weed ID books
  • Bags for samples and marker
  • Field history

Basic Biology of Weeds for Field Scouting
It is easy to distinguish between broadleaf and grassy weeds. Beyond this, it is important to learn the growth habit of the weed, and target management strategies at susceptible stages.

Weeds have one (or more) of the following life cycles:

  • Annual weeds grow and flower in one year. Some weeds in orchards are winter annuals, i.e., they begin their growth in the fall, forming a rosette, and flower the following spring or summer.
  • Biennial weeds have a 2 year life cycle, producing leaves in the first year and flowering in the second year.

Annual and biennial weeds compete for nutrients and water as they grow near crops. After they flower, they die; however, their seeds may cause recurring problems for several years by forming a soil seed bank.

  • Perennial weeds live for many years, and generally establish from various types of root systems, although many will also spread by seeds. They usually flower every year as well as expand their root system, which allows them to  spread by both methods through fields. Perennial weeds can be very competitive, especially if they grow in thick patches.

The following table lists some common field weeds:

Common Types of Weeds in Fields






Quack grass


Yellow rocket

Canada thistle

Common chickweed

Wild carrot




Poison ivy


Pepper grass


Crab grass


Creeping Charlie

Barnyard grass


Wild grape