Fleabane (Conyza spp.)
Fleabane weed is also known as Horseweed. It is a tall dicot with a tough, hairy stem, and as it matures its stem becomes hard and woody. It germinates in the Spring and becomes a major weed problem in the Summer. Although it is a Summer annual, you can sometimes see Fleabane over the Winter.
Fleabane has alternate dark green leaves that have toothed margins. Fine hairs coat the surface of these leaves, and these hairs coupled with its woody stem, make it difficult to control.
In Australia there are three main species of Fleabane:
- Flaxleaf Fleabane (Conyza bonariensis), which is the most common.
- Tall Fleabane (C. sumatrensis) and
- Horseweed or Canadian Fleabane (C. canadensis).
To avoid confusion we are going to term all of these as Fleabane weed or Horseweed.
Why is Fleabane a Major Weed?
- Fleabane is a prolific seed producer.
- It is very competitive.
- Fleabane is difficult to control with herbicides.
- Fleabane develops herbicide resistance.
- In areas where there is little competition, Fleabane quickly invades and establishes.
- Fleabane builds a strong root system over the Winter, and then grows rapidly in the Spring. Fleabanes that over-winter are very difficult to control.
In closely mown or when it is young, Horseweed appears as a flat, rosette like weed. However as it matures, the stem lengthens and leaves form around the entire stem.
In the Spring and Autumn, large clumps of flowers form around the top of the tall stem, and these form fluffy clusters as its seeds form.
After you read this, you will be able to:
- Identify Fleabane weed or Horseweed.
- Know the habitat of Fleabane.
- Know how to control Fleabane or Horseweed.
How to Identify Fleabane.
Seedlings of Fleabanes can be confused with common winter annual weeds such as Common Chickweed and Shepherd’s-Purse. During its vegetative stages Fleabane is often confused with Capeweed.
Category: An annual or short-lived perennial.
Photosynthetic Pathway: C3 Weed. C3 plants like Horseweed grow best between 15°C and 30°C. Above 30°C, photosynthesis and growth decline due to a lack of CO2.
Flower: The flowers are white to yellow. The flower head is cylindrical to bell shaped, and 5 to 6 mm long.
Height: Fleabane is 1 to 2 m tall.
Leaf length: 2 to 10 cm long
Leaf width: The leaves are 1.5 cm wide, and have a cover of soft hairs.
Reproduction: Fleabane seeds spread by the wind, and mature Flaxleaf Fleabane produces around 110,000 seeds. These remain viable for up to 18 months.
Fleabane seeds only emerge from on or near the soil surface. When they are sown on the surface, 5% of the seed remains viable after 12 months. After burial at 50 mm and 100 mm for 12 months, 10% and 15% of the seeds maintain their viability.
The optimum temperature for germination of Fleabane is 20°C. It has a base temperature of 4.2°C and a maximum germination temperature of 35°C. 50% emergence occurs 4 to 5 days after moisture is supplied, at 20°C
Comments: Fleabane leaves are grey to green, oblong to wedge shaped to oval, and twisted near the base. They are very hairy and the edges are smooth, toothed and often wavy. Fleabane tolerates drought and frost, but does not compete well in shade.
Habitat: Fleabane is a weed of roadsides, especially if bare soil exists where it can germinate. More on lawn and turf grass weeds is in our weed ID chart.
Thanks to The Atlas of Living Australia for the distribution map.
How to control Fleabane.
Cultural control of Fleabane:
An integrated approach will give the best chance to control Horseweed. As the seeds do not seem to survive more than 18 months the aim is to control the weed before it produce new seeds.
- Monitor areas early in the germination period.
- Spray weeds when they are a small rosette, and before stem elongation starts.
- Use an adjuvant to improve herbicide uptake.
- Fleabane plants under moisture stress are more difficult to control with herbicides.
- Rotate herbicide mode of action groups.
- Control plants in nearby areas so they don’t flower and act as a source of seed. You must manage the seed bank to control Fleabane in the long term.
Fleabane is not a great competitor, so a healthy and dense turf cover will outcompete this weed. The key is to improve the competitiveness of your turfgrass.
This means feeding you turf properly, cutting at the right height for the turf species present, and limiting soil compaction.
If you try and mow this weed out it is not effective, and will tend to encourange this weed.
If the soil is not compact then if you hand remove Fleabane after stem elongation works. However, on heavier, compact soils you will need a fork to prevent the plant breaking and then regrowing from the base.
Chemical Control of Fleabane:
The first point is that Horseweed has been confirmed as glyphosate resistant. The second point is that you get the best control of Fleabane when it emerges in the Spring, and it is still small.
Small Fleabane plants are pretty easy to kill, and a late post-emergent application of some phenoxy herbicides during Spring can control them.
Due the hairy nature of Horseweed add an organosilicone surfactant (ProForce Scrubwet) at 200 mL/100 L for optimum results.
The other trick is too make sure that your water rates are not too high. If they are, then the adjuvant will become too dilute to have any benefit.
It can be difficult and expensive to kill mature Fleabane, especially in mid to late Summer. That’s why its important too target small, actively growing weeds. Factors such as weed age can have a negative impact the results that you get.
Pre-Emergent Control of Fleabane.
- The only Pre-emergent herbicide currently registered for turf is BASF Freehand.
- Esplanade is registered in NZ.
Post-Emergent Control of Fleabane.
You must use post emergent herbicides when the Fleabane weed is a rosette up to 5 cm across. This is because as the weed matures, chemicals become less effective. A spray adjuvant will help with the results, improve rainfastness and overall herbicide performance.
Selective post emergent herbicides for Fleabane include:
- 2,4-D Amine. Apply when weeds are young and growing. You may need repeat applications.
- Dicamba. Apply to growing plants. Smaller plants are easier to control, and you need to use high rates for large plants. Do not use when outside temperatures exceed 26°C. Do not use Dicamba on Buffalo grass.
- Contra M. Do not use on Buffalo grass.
- Casper Turf Herbicide. To get the best results add a non ionic surfactant at 0.25 to 0.5 v/v. We suggest OptiSpread 1000. Do not use on Buffalo grass.
- Recondo Herbicide. Do not use on Buffalo grass.
- Pylex. Do not use on Buffalo grass.
- Weed Blast MA.
- Warhead Trio.
You get the best results on Fleabane when you:
- Only use herbicides on actively growing weeds.
- Allow adequate time between when you spray and when you mow. This gives plenty of time for the herbicide to work.
- Do not spray when the weed is under stress due to drought or cold weather.
Non Selective Control of Fleabane.
You can use Glyphosate as a non selective option to control this weed, although herbicide resistance has been reported overseas. If you use Glyphosate and water quality is an issue then use ProForce Manta Ray.
Other options include:
