Horseweed (Erigeron canadensis)

Horseweed, also commonly known as marestail, Canadian horseweed, and Conyza canadensis (its botanical name), is a widespread annual weed found throughout North America and other parts of the world. This adaptable plant can quickly colonize disturbed soil, becoming a significant challenge for gardeners, farmers, and land managers, and at Gardencenterpoint.com, we get a lot of questions about it.

This guide addresses practical solutions, covering essential aspects like identification, effective control methods, and even exploring potential uses of this resilient plant. We will show you how to get rid of this weed.

Horseweed Identification: Recognizing the Enemy

Proper identification is the first crucial step in managing horseweed. Misidentification can lead to ineffective control measures and wasted effort. This section provides a detailed breakdown of horseweed’s characteristics at various growth stages.

Common name	Horseweed, Marestail, Canadian Fleabane, Canadian Horseweed, Hogweed
Botanical name	Conyza canadensis (often seen as Erigeron canadensis)
Family	Asteraceae (Sunflower family)
Genus	Conyza
Species	canadensis
Origin	North America
Life cycle	Annual (occasionally biennial)
Plant type	Herbaceous Forb
Hardiness zone	USDA Zones 3-10 (very adaptable)
Sunlight	Full Sun (tolerates partial shade)
Maintenance	Low (generally considered a weed)
Water	Low to Moderate (drought-tolerant once established)
Drainage	Well-Drained (but adaptable to various soil types)
Spacing	Not applicable (self-seeds readily)
Flowering period	Summer to Fall (typically July – October)
Height	1-6 ft (can reach up to 10 ft in ideal conditions)
Growth rate	Fast
Flower color	White to yellowish-white (small, inconspicuous flowers)
Stem color	Green, often with reddish-brown streaks, hairy
Flower benefit	Attracts some pollinators (though not a major pollinator plant)
Garden style	Not typically cultivated; found in disturbed areas, fields, roadsides
Uses	Traditionally medicinal, potential for essential oil, phytoremediation (research ongoing)
Soil pH	adaptable to a wide range, 5.0 – 8.0
Soil Type	Adaptable to most soil types.
Leaf Type	Simple
Propagation method	Seed

Horseweed, a member of the Asteraceae (daisy) family, exhibits distinct features that differentiate it from other common weeds.

• Seedling Stage:
  • Horseweed seedlings emerge as a basal rosette, a circular cluster of leaves radiating from a central point close to the ground.
  • The initial leaves are typically oval or spatula-shaped, with slightly toothed or wavy margins.
  • The leaves have a slightly hairy texture, particularly on the undersides.
  • The cotyledons (seed leaves) are small and rounded.
  • Seedlings typically appear in late summer or fall, and spring.
• Young Plant Stage:
  • As the plant matures, it develops an erect, central stem that can grow quite tall.
  • The leaves become progressively narrower and more lance-shaped as they ascend the stem.
  • The stem itself is often covered in fine hairs, giving it a rough texture.
  • The rosette leaves may begin to wither as the plant focuses its energy on upward growth.
• Mature Plant Stage:
  • Mature horseweed plants can reach heights of 1 to 6 feet, or even taller under optimal conditions.
  • The stem becomes increasingly branched, especially in the upper portion.
  • The leaves are alternate, meaning they are arranged singly along the stem, not in pairs.
  • The upper leaves are typically smaller and narrower than the lower leaves.
  • Mature plants are easy to identify by their height and bushy appearence.
• Flowering Stage:
  • Horseweed produces numerous small, inconspicuous flower heads.
  • Each flower head is composed of a cluster of tiny, white or pale yellow ray florets surrounding a central disk of yellow florets.
  • The flower heads are arranged in dense, elongated clusters at the tips of the branches.
  • Flowering typically occurs from mid-summer to fall.
• Seed Stage:
  • After flowering, horseweed produces numerous small, dry seeds, each equipped with a pappus.
  • The pappus is a tuft of fine, white hairs that acts like a parachute, allowing the seeds to be dispersed by the wind.
  • A single horseweed plant can produce thousands of seeds, contributing to its rapid spread.
  • The seedhead looks similar to a dandelion, but is much smaller.

Distinguishing Horseweed from Similar Species

Several other plants can be mistaken for horseweed, especially during the early growth stages. Here’s a comparison with some common look-alikes:

Feature	Horseweed (Conyza canadensis)	Common Ragweed (Ambrosia artemisiifolia)	Annual Fleabane (Erigeron annuus)
Leaf Shape	Lance-shaped, becoming narrower up the stem	Deeply divided, fern-like	Oval to lance-shaped, coarsely toothed
Leaf Texture	Slightly hairy	Slightly hairy	Slightly hairy, often with a toothed margin
Stem	Erect, hairy	Erect, branched, often reddish	Erect, hairy, often with a ridged texture
Flower Head	Small, white/yellow, in dense clusters	Small, green, in elongated spikes	Small, white/lavender, with yellow center

Key takeaway: Horseweed’s progressively narrower leaves and numerous small, white/yellow flower heads in dense clusters are key distinguishing features.

Horseweed Control and Removal: Effective Strategies

Controlling horseweed requires a multi-pronged approach, combining preventative measures, cultural practices, and, when necessary, herbicide application. Because horseweed is so persistent, integrated weed management is essential.

• Prevention:
  • Clean Seed: Use certified weed-free seed when establishing new plantings. This minimizes the introduction of horseweed seeds into your garden or field.
  • Sanitation: Clean equipment thoroughly after working in infested areas. This prevents the spread of seeds to uninfested locations.
  • Mulching: Apply a thick layer of organic mulch (such as wood chips, straw, or shredded leaves) to garden beds and other areas. Mulch suppresses weed germination by blocking sunlight.
  • Cover Cropping: Plant cover crops (such as rye, clover, or vetch) during fallow periods. Cover crops compete with weeds for resources and can help prevent horseweed establishment.
• Cultural Practices:
  • Hand-Pulling: For small infestations, hand-pulling can be effective, especially when the soil is moist. Grasp the plant near the base and pull it up, ensuring you remove the entire root system.
  • Hoeing/Cultivation: Regular hoeing or cultivation can disrupt horseweed seedlings and prevent them from maturing. This is most effective when the weeds are small.
  • Tillage: In agricultural settings, tillage can be used to bury horseweed seeds and disrupt established plants. However, be aware that excessive tillage can also contribute to soil erosion and degradation.
  • Crop Rotation: Rotate crops regularly to disrupt the life cycle of horseweed and other weeds. Avoid planting the same crop in the same location year after year.
  • Mowing: Mowing will prevent horseweed from going to seed, but will not kill the plant.
• Chemical Control (Herbicides):
  • Herbicide Resistance: Horseweed has developed resistance to several commonly used herbicides, including glyphosate (Roundup), in many areas. This is a significant challenge for control. The University of California Integrated Pest Management Program reports widespread glyphosate-resistant horseweed populations.
  • Effective Herbicides: The best choices for controlling horseweed will depend on the time of year, resistance to other herbicides, and what other plants are in the area.
  • Pre-Emergent Herbicides: Apply pre-emergent herbicides in the fall or early spring to prevent horseweed seeds from germinating. Effective pre-emergent herbicides for horseweed include:
    • Flumioxazin (e.g., Valor)
    • Sulfentrazone (e.g., Spartan)
    • Metribuzin (e.g., Sencor)
  • Post-Emergent Herbicides: Apply post-emergent herbicides to actively growing horseweed plants. Effective post-emergent herbicides for horseweed include:
    • 2,4-D
    • Dicamba (e.g., Banvel)
    • Clopyralid (e.g., Stinger)
    • Glufosinate (e.g., Liberty) – Note: Glufosinate is a non-selective herbicide and will damage any plants it contacts.
  • Herbicide Application Tips:
    • Always follow the label instructions carefully when using herbicides.
    • Apply herbicides when the weeds are actively growing and not under stress.
    • Use the appropriate application rate and method for the specific herbicide and target weed.
    • Consider using a surfactant to improve herbicide coverage and penetration.
    • Avoid spraying on windy days to prevent herbicide drift.
  • Herbicide Resistance Management: Rotate between different herbicide groups to prevent herbicide resistance from forming.
• Organic Control Options:
  • Vinegar (Acetic Acid): High-concentration (20% or higher) horticultural vinegar can be used as a non-selective herbicide. It works by burning the foliage of the plant. Multiple applications may be needed. Note: Vinegar can also damage desirable plants, so apply carefully.
  • Flame Weeding: A propane torch can be used to kill horseweed seedlings and young plants. This method is most effective on small weeds. Note: Exercise caution when using flame weeders to avoid fire hazards.
  • Organic Herbicides: Several organic herbicides are available, often based on plant oils or other natural compounds. These may have limited effectiveness on mature horseweed plants.

The Impact and Harm of Horseweed

Horseweed’s rapid growth and prolific seed production can have significant negative impacts on agriculture, landscapes, and even human health.

• Agricultural Impacts:
  • Crop Yield Reduction: Horseweed competes aggressively with crops for water, nutrients, and sunlight, leading to reduced yields. Studies have shown that horseweed infestations can significantly reduce yields in soybean, corn, cotton, and other crops. A study by the University of Illinois found that uncontrolled horseweed could reduce soybean yields by up to 50%.
  • Increased Production Costs: Controlling horseweed requires additional herbicide applications, tillage, and other management practices, increasing production costs for farmers.
  • Harvest Interference: Tall horseweed plants can interfere with harvesting equipment, making it more difficult and time-consuming to harvest crops.
  • Allelopathy: Some research suggests that horseweed may exhibit allelopathy, releasing chemicals into the soil that inhibit the growth of other plants.
• Landscape Impacts:
  • Aesthetic Degradation: Horseweed can quickly overrun gardens, lawns, and other landscapes, creating an unsightly appearance.
  • Competition with Desirable Plants: Horseweed competes with desirable plants for resources, potentially weakening or killing them.
• Human Health Impacts:
  • Allergies: Horseweed pollen is a known allergen and can trigger allergic reactions in sensitive individuals, causing symptoms such as sneezing, runny nose, and itchy eyes.
  • Skin Irritation: Contact with horseweed foliage can cause skin irritation in some people.
• Livestock:
  • Horseweed is generally not palatable to livestock, and they will avoid it if other forage is available.
  • There are no reports of toxicity.

Potential Uses of Horseweed

Despite its negative impacts, horseweed does have some potential uses, particularly in traditional medicine and as a source of certain compounds.

• Traditional Medicinal Uses:
  • Native American tribes have traditionally used horseweed for various medicinal purposes.
  • It has been used to treat diarrhea, dysentery, bronchitis, and other ailments.
  • The plant contains compounds with astringent, diuretic, and anti-inflammatory properties.
    • Note: It is important to emphasize that using horseweed for medicinal purposes should only be done under the guidance of a qualified healthcare professional. Self-treating with herbal remedies can be dangerous.
• Essential Oil:
  • Horseweed contains an essential oil that has been investigated for its potential antimicrobial and insecticidal properties.
  • The oil has a distinctive aroma and is sometimes used in perfumery.
• Phytoremediation:
  • Some studies suggest that horseweed may have potential for phytoremediation, the use of plants to remove pollutants from soil or water.
  • It has been shown to accumulate certain heavy metals.
• Other Potential uses:
  • Young leaves can be cooked and used as a potherb.
  • Dried leaves can be used as tinder to start fires.

Frequently Asked Questions (FAQs) about Horseweed

• Q: Is horseweed poisonous to humans or animals?
  • A: Horseweed is not generally considered poisonous to humans or animals. However, it can cause skin irritation in some people, and its pollen can trigger allergic reactions.
• Q: How do I get rid of horseweed in my lawn?
  • A: A combination of strategies is most effective. Hand-pull small infestations, apply a pre-emergent herbicide in the fall or early spring, and use a post-emergent herbicide labeled for horseweed on actively growing plants. Maintain a healthy, dense lawn to outcompete weeds.
• Q: How do I prevent horseweed from coming back?
  • A: Prevention is key. Use clean seed, sanitize equipment, mulch bare soil, and consider cover cropping. Monitor your property regularly for new infestations and address them promptly.
• Q: Is horseweed the same as marestail?
  • A: Yes, horseweed and marestail are common names for the same plant, Conyza canadensis.
• Q: Can I compost horseweed?
  • A: It’s generally not recommended to compost horseweed, especially if it has gone to seed. The seeds can survive the composting process and spread when the compost is used.
• Q: Why is horseweed so hard to kill? *A: Many factors contribute to difficulty controlling horseweed. It produces a large number of seeds that can be spread on the wind. It can emerge in spring and fall, and it has become resistant to several common herbicides.