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Washington State University

WSU Snohomish County Extension office has moved!

Our office is now located in the Snohomish County Parks & Recreation Administration Office at Willis Tucker Park, 6705 Puget Park Drive, Snohomish, 98296.

What Is a Weed?

(425) 338-2400

What is a Weed? | Integrated Weed Management | Understanding Weed Types | Weed Control

What is a Weed?

A weed is any undesirable plant. Weeds can:

  • Be a nuisance
  • Be a hazard
  • Cause injury to man and animals, such as poison ivy or poison hemlock
  • Compete with garden plants for soil nutrients, light and water
  • Harbor insect and disease pests

Weeds tend to be very competitive and are capable of taking advantage of disturbed areas. They often produce large amounts of seeds or are capable of quick reproduction. Weeds are generally a problem where the desired crop is doing poorly or the soil has been disturbed.

Integrated Weed Management

Weed management requires an integrated approach which is referred to as Integrated Weed Management. This requires the incorporation of all appropriate management techniques including chemical, mechanical, and cultural practices in a weed control program. Integrated Weed Management involves:

  • The proper identification of the weed and knowledge of the weed’s biology. Most WSU Extension offices are able to identify most common weeds.
  • Determine why the weed is a pest.
  • Evaluate the suitability of various control measures which may be available.
  • Evaluate the results. This will evaluate how well the control measures worked.

Understanding Weed Types

Weeds may be classified in at least two ways. The most common are: Appearance or Life Cycle.


  • Broadleaves: These plants are also called dicots. Broadleaves have veins that radiate from a larger vein. Broadleaf weeds typically have tap roots, such as a dandelion, or fibrous roots, such as lambsquarters.
  • Grasses: These plants are also called monocots. These plants have long, narrow leaves with veins that are parallel to each other. Grasses have a fibrous root system.

Life Cycle

  • Annual Weeds: Annual plants germinate, grow, flower and set seed in one year or less. Temperature, light, and moisture trigger annual plants to germinate. Winter annuals, such as the mustards and chickweed, germinate in late summer or fall, overwinter, flower the following spring, set seed, and die before the heat of summer. Summer annuals, such as redroot pigweed and common lambsquarters, germinate in late spring, grow during the heat of summer, flower, set seed and die from cold temperatures. Annuals spread only by seed.
  • Biennial Weeds: Biennial plants complete their life cycle in two growing seasons. They germinate and form a rosette the first year. During the second growing season they form a stem, flower, set seed and die. Biennials, such as bull thistle, spread only by seed. Control them by hoeing or digging when they are small.
  • Perennial Weeds: Perennial plants grow for many years. Simple perennials, such as dandelions, spread only by seed. Spreading perennials spread by seed and underground reproductive structures. Quackgrass spreads by rhizomes, underground, horizontal stems. Canada thistle and field bindweed spreads by a spreading root system. Yellow nutsedge produces small tubers. Control of spreading perennials may be very difficult because of this extensive underground root system.

Weed Control

There are five basic weed control strategies that may be used:

Prevention | Mechanical | Cultural | Chemical | Weed Control Tips


The easiest way to control weeds is by preventing them from getting started in the garden. Do not allow weeds to enter with garden seeds, in manure, by sharing perennial plants that have weeds seeds or vegetative parts with the roots, using contaminated compost, or allowing weeds in or bordering the garden from going to seed.

Mechanical Control

This is the most common method of controlling weeds. The gardener destroys weeds by hand pulling, hoeing, tilling, or digging and removing the underground portions of weeds. Mechanical control takes time and energy, but is usually quite effective for most weeds. Be careful not to dig or till too close to desirable plants and damage their root systems.

Cultural Control

  • Competitive Crops. All plants in the garden compete for light, water and nutrients. Turf, ground covers, and some vegetable crops, such as potatoes, can compete well with weeds because of quick growth and dense foliage.
  • Mowing. Clipping weeds slows their growth and prevents weed plants from going to seed. Use a lawn mower, clippers, or string trimmer to prevent weeds from going to seed. Mowing will not eliminate perennial weeds.
  • Mulching. Mulching is the practice of covering the soil with a material to prevent light from reaching weeds seeds. Mulch may be an organic material, such as straw, sawdust, compost, or grass clippings, or synthetic, such as black plastic. If weed seedings are able to penetrate the mulch layer, they are easily pulled. Mulch will not control spreading perennial weeds, such as Canada thistle or horsetail emerging from rootstock. Organic mulch will also keep the soil cooler while a synthetic mulch will warm the soil faster, retain soil moisture, and keep fruit cleaner. The mulch layer should be two or more inches thick, depending on the type of mulch.
  • Cover Crops. A cover crop is capable of choking out many germinating weeds. Consider using a summer cover such as buckwheat or an overwinter cover such as oats, barley, rye, or triticale mixed with winter peas.

Chemical Control

Herbicides are chemicals used to kill weeds. They are generally the last resort for home gardeners. Herbicides have several disadvantages:

  • They are expensive.
  • They are difficult to apply with accuracy.
  • Drifting or leaching may occur and damage desirable plants.
  • Proper storage and handling may be a problem.
  • Many herbicides are labeled for specific crops and are not suitable for a garden with a wide variety of crops.

Weed Control Tips

  • Eliminate weeds early in the growing season, before they develop a spreading root system or set seeds.
  • Prevent weeds from entering the garden by avoiding the use of fresh manure.
  • Cultivate on a sunny, warm day so weeds that have been pulled or tilled will dry out and die.
  • Apply an adequate layer of mulch to prevent seedling weed emergence.
  • Remove weed clippings from the property if there is a chance that seeds have been set.