What Makes a Plant a “Landscape Weed”?
A weed is any plant that becomes undesirable in the landscape because of the following:
- It is growing in a place where it is unwanted (lawn grass in a flowerbed, tree seedlings in a lawn, purslane growing between patio pavers, spearmint invading a raised vegetable bed).
- It is visually unattractive (color, texture, growth habit, growth rate makes it aesthetically unappealing to the eye).
- It poses a health or safety hazard (poisonous plants, thorny plants, fuel for fires).
- It out-competes more desirable plants in the home landscape (competes for water, nutrients, light) or when it escapes into native landscapes (creating biodiversity problems).
- It acts as a host or shelter for other pests (alternate host for rust, attractive to injurious insects, food/shelter for damaging wildlife).
What Characteristics Make Weeds Successful?
Characteristics that make weeds successful where they become a problem include the following:
- Rapid growth rate.
- Prolific seed producer.
- Long longevity of seed.
- Deep roots, stolons, tubers, etc. making them tolerant of adverse growing conditions.
- More “ecologically fit” than other plants in the landscape.
- Adapted to readily spread (wind, animal manure, water, and human activities).
- Often adapted to disturbed soil/sites.
- May not have insects and diseases to keep them in check.
- May be better competitors for light, nutrients, or sun.
Seed Bank
A seed bank builds up as a weed drops seed into the soil over many years –seed can remain viable for years. Persistence and vigilance to keep weeds from going to seed are keys to depleting seed bank. See Tables 1 and 2.
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.
Table 1. Weed Seeds per Plant
| weed | Number of seeds produced per plant |
|---|---|
| Dandelion | 15,000 |
| Canada thistle | 680 |
| Curly dock | 29,500 |
| Lamb’s quarter | 72,450 |
| Mullein | 223,200 |
| Pigweed | 117,400 |
| Purslane | 52,300 |
Table 2. Viability of Buried Seed
| weed | Viability of buried seed (years) |
|---|---|
| Black mustard | 50 years |
| Curly dock | 80 years |
| Foxtail | 30 years |
| Mallow | 20 years |
| Plantain | 40 years |
| Shepherd’s purse | 35 years |
How Do Weeds Get Into Our Landscapes?
Major sources of landscape weeds include the following:
- Weeds going to seed (seed bank).
- Brought into garden in manure and soil amendments or with soils.
- Disseminated from neighboring property’s plants and weeds.
- Deliberate introduction.
Minor sources of landscape weeds include the following:
- Brought into garden with plant materials.
- Brought into garden in irrigation water.
- Brought into garden by humans or animals.
- Using poor quality seed (weed content in seed).
Common Weeds and Noxious Weeds
Common weeds refer to weeds commonly found in various cropping situations, such as the lawn, vegetable garden, flowerbeds, or naturalized areas.
Noxious weeds refer to weed species declared by state or local statues as a threat to agriculture and naturalized areas. Some designations require control under the law.
Legal Designations for Noxious Weeds
List A: All populations of List A species in Colorado are designated for eradication because they are not widespread. (myrtle spurge, purple loosestrife)
List B: These weeds have discrete populations and will be managed to stop their continued spread, or eradicated in certain areas. (Chinese clematis, oxeye daisy)
List C: These weeds are already very widespread, and not required to be controlled; however, education and research continue on these species. (downy brome, field bindweed)
For additional information on Colorado’s noxious weed laws, refer to the Colorado Department of Agriculture Noxious Weed Management Program at https://ag.colorado.gov/conservation/noxious-weeds
Weed Life Cycles
Annuals
Summer annual seeds germinate in the spring, the plant develops and produces seed during the summer, and the plant dies with killing frost in the fall. Examples include crabgrass and puncture vine.
Winter annual seeds germinate in late summer or fall, and plants live over winter as small tufts or rosettes of leaves. It resumes growth in spring, matures seed early in the summer, and dies in summer heat. Examples include downy brome and shepherd’s purse.
Keys to controlling annuals are preventing seed production, depleting the seed bank, and preventing germination:
- Timing is important.
- Winter annuals must be controlled before seed set in early summer.
- Summer annuals must be controlled before seed set in middle to late summer or early fall.
- The use of herbicides at the end of an annual’s life cycle is often ineffective and does not make sense!
- Competition (from other plants and mulch) to prevent seed germination and seedling development.
Biennials
Biennials require two seasons to complete their growth cycle. Seeds germinate in spring. The following season, the plant flowers, matures, and seeds in summer and fall before dying. An example is dame’s rocket.
Keys to control are preventing seed production, depleting the seed bank, and preventing germination and seedling establishment.
Perennials
Simple Perennials have a root crown that produces new shoots every year. It depends upon seed production to spread. Examples include foxtail barley and dandelion.
Creeping Perennials propagate by seed, creeping above ground stems (stolons), and/or creeping underground stems (rhizomes). Examples include quack grass and Canada thistle.
Keys to control are to prevent seed production and to kill the plant. Creeping perennials have a more extensive root system and are harder to control.
IPM: Integrated Weed Management
Colorado State University Extension’s Integrated Pest Management Statement
Colorado State University Extension (CSUE) supports healthy environments for all Coloradoans. CSUE teaches and encourages the use of Integrated Pest Management (IPM) principles – employing a combination of cultural, mechanical, biological, and chemical practices. The use of IPM ensures a holistic approach, encouraging individuals to make informed decisions for minimizing or eliminating the use of pesticides. Any pesticide recommendations (synthetic or organic) made by CSU Extension are based on the best available science, without bias.
- The best weed control is prevention.
- Plant weed-free seed, sod, nursery stock.
- Avoid using plant species known to be invasive.
- Use weed-free amendments, topdressing.
- Use mulch where appropriate.
- Maintain healthy, competitive plants.
- Irrigate and fertilize appropriately.
Methods of Control
Cultural Methods
Irrigation
Irrigation methods and frequency have a direct influence on weeds. Infrequent, deep irrigation droughts out many shallow rooted weeds. Sprinkler irrigation (wetting the entire soil surface) encourages weeds. Drip irrigation (keeping most of the soil surface dry) discourages weeds. Keep non-irrigated areas dry to help suppress weeds.
Lawn Mowing
Many common garden weeds will not survive the frequent mowing of a lawn. However, mowing the lawn too short (less than 2 inches for Kentucky bluegrass) encourages weeds as it reduces vigor of the grass.
Mulching
If maintained at adequate depths, mulching has many benefits, including preventing weed seed germination. For wood/bark chips, a depth of three inches is best for weed control. Less is ineffective. Mulching may not effectively control established perennial weeds growing from the root.
Landscape Fabrics
In landscape management, landscape fabric with wood/bark chips or rock mulch above is common. However, it prevents soil improvement by organic breakdown, decreasing plant vigor. Weed seeds that germinate above the fabric layer will be difficult to pull and may require control using herbicides. Use of landscape fabric should be considered as a deferred maintenance technique rather than a low maintenance technique.
Crop Competition
Competition with the crops and weeds for light, water, nutrients, and growing space is an effective weed management tool. For example, mowing a cool season lawn (like Kentucky bluegrass) gives the lawn a growth advantage, shading out many weeds like crabgrass. Block planting in the vegetable garden and close spacings in a flowerbed, with plants filling the bed space, helps suppress weeds.
Summary: Cultural Methods
- Pros: This is the best long-term control as the gardener increases the conditions for desired plants to grow and at the same time decreases the conditions for weeds.
- Cons: Possibly more expensive and time-consuming; control may be slow.
Mechanical Methods
Tilling/Cultivating
Tilling or cultivating effectively controls 90% of annual and biennial weeds if done before seed set. It also brings a new set of weed seeds to the soil surface ready to germinate. When tilling for weed control, use only shallow cultivation. Deep tilling can damage crop roots. Cultivating/tilling may propagate perennial weeds.
Hand Pulling
Hand pulling is quick when weeds are small, and it is effective for small infestations. A few minutes on a weekly basis to keep the garden weed free will be more effective than a long weed pulling session as the weeds get large. For many gardeners, pulling weeds is a great way to vent stress. With hand pulling, most weed species require that they be pulled out by the roots. The weed will readily regrow if just the tops are removed. It is essential that weeds are removed before they go to seed and add to the seed bank. Some weed species, like purslane, must be removed from the garden bed. It can re-root if left in the garden.
Mowing Naturalized and Low Maintenance Areas
Mowing is a common weed management tool in natural areas and lower maintenance sections of a yard, reducing the unsightly appearance of the yard and fire hazard.
String Trimming (“Weed Whacking”)
Use of a string trimmer is a form of weed management by mowing. It can be effective in preventing weeds from going to seed. However, it can sow seeds if done on weeds with seeds.
Flame (Propane Torch)
Flaming off weeds with a propane torch is a common practice in production agriculture and has limited application in landscape maintenance due to fire hazards. During the flaming process, heat from the flame is transferred to the plant tissues, increasing the thermal energy of the plant cells and resulting in coagulation of cell proteins if the temperature is above 50°C. Exposing plant tissue to a temperature of about 100°C for a split second (0.1 second) can result in cell membrane rupture, resulting in loss of water and plant death.
Thus, the weeds do not need to be burned up, but rather just scorched. Flaming works best on very young weeds. It is rather expensive and may not be cost effective in some production agriculture situations. It presents a fire and explosion hazard; use with caution. Fire prevention measures prohibit the use of flaming in many communities.
Burning
Burning fields and ditch banks is a weed management tool in production agriculture. Generally, a permit is required. Many communities prohibit burning weeds inside city limits.
Solarization
Solarization is a method of heating the soil to kill roots, weed seeds, and soil borne insects and diseases near the soil surface. In regions with hot summer temperatures, it is effective in open areas with full sun. However, do not solarize the soil in the rooting area of trees, shrubs, and other desired plants. Steps include the following:
- Remove vegetation and cultivate the soil to a six-inch depth.
- Sprinkle irrigate the area.
- Cover the area with 4 mil clear plastic. Bury the edges of the plastic all the way around the plot.
- Leave in place for three weeks during the summer heat of July and August.
- After removing the plastic, avoid deep cultivation that would bring up weed seeds, insects, and disease pathogens from deeper soils.
Summary: Mechanical Methods
- Pros: Mechanical methods can be quick, inexpensive, environmentally friendly, and effective on small weed seedlings.
- Cons: Mechanical methods have limited effectiveness on many established perennials and could be detrimental if employed at the wrong time.
Biological Methods
Biological methods include the use of carefully screened insects to attack portions of the weed (i.e., stems, seeds, flowers, etc.). Development of biological methods with insects is rather complex and must be used with caution. The introduced insects must survive and become established in the new ecosystem. The insects need to reduce the weed population but cannot entirely eliminate the weeds as that would eliminate the insect’s food supply. The insects must not attack beneficial plants. The insects must not become insect pest. A great example of biological methods that failed is earwigs. They were intentionally introduced into the United States as a biological control agent and have since become a pest.
Biological methods also include the grazing of sheep, cows, horses, or goats. The purposeful use of grazing animals to control weed patches can be extremely expensive.
Summary: Biological Methods
- Pros: Biological methods can be an inexpensive, long-term control solution. It can be environmentally friendly and require little labor.
- Cons: Biological methods are not always effective, may require a large population of weeds to maintain insect populations (will not work in backyard setting), and does not eradicate weeds. Insects can sometimes attack non-target plants.
Herbicides (Chemical Methods)
he use of herbicides is the use of chemicals that disrupt key physiological processes in plants, leading to plant death. Among the various herbicides, many different modes of action are found.
Be sure to follow the label. It is the law.
Components of the herbicide label include the following:
- Trade Name.
- Common name.
- Chemical name.
- Signal Words (Danger, Warning, Caution).
- Use instructions.
- Weeds controlled.
- Plant tolerances.
- Application rate(s).
- Application timing.
- Application technique.
- Application restrictions.
- Safety.
- Applicator.
- Bystanders, pets.
- Wildlife.
- Non-target plants.
How Herbicides are Applied
Broadcast application refers to a uniform application over a treatment area.
Spot treat refers to application to a specific area, such as directly to individual weeds.
Foliar application refers to application to the leaves.
Soil incorporation refers to tilling or watering the herbicide into the soil after application.
Types of Herbicides
Systemic or Translocated herbicides move internally in the plant. They must be applied during a period of active growth with adequate water. Systemic herbicides are especially good for many perennials. Examples include glyphosate, and 2,4-D.
Contact herbicides only desiccate the portion of the plant that is contacted. Contact herbicides are most effective on annual weeds. Examples include vinegar and diquat.
Pre-emergent herbicides are applied to soil prior to weed seed germination, killing germinating seeds. They will not kill growing weeds. Application timing is critical. For example, to control crabgrass in lawns, pre-emergent herbicides need to be applied late April to early May before the crabgrass germinates, about the time that common lilac blooms. Most require soil incorporation by irrigation. Some desired crops germinating from seeds may also be killed. For example, do not apply pre-emergent herbicides prior to seeding a lawn or laying sod. Uniform application and strict adherence to application rate are essential for attaining good weed control and for preventing injury to landscape plants.
Post-emergent herbicides are applied to foliage of actively growing plants. Examples include 2,4-D, and glyphosate.
Selective herbicides control a limited group of plants, like monocots OR dicots.
Non-selective herbicides are effective on a broad range of plants like, monocots AND dicots.
Summary: Chemical Methods
- Pros: Use of herbicides is generally effective (if the correct herbicide is used), cost-effective, and provides quick control.
- Cons: Use of herbicides can be environmentally problematic when incorrectly applied. Proper use includes proper selection of the specific herbicide for the weeds and for the growing crops in the area, timing of application, correct application rates, correct application procedures, and application safety measure to protect the application and non-target plants. Some require special licensing and may not be used in a home landscape or garden setting.
This publication, reference GardenNotes #351, is developed as part of the Colorado State University Extension Master Gardener Program.