Fleabane (Conyza spp.)
Fleabane also known as Horseweed is a tall growing dicot with a tough, hairy stem. 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, it can sometimes be seen over the Winter months.
Fleabane has alternate dark green leaves that have toothed margins. These leaves have a coating of fine hairs on the surface, and these hairs coupled with its woody stem, make it very difficult to control.
In Australia there are three main species of Fleabane:
- Flaxleaf Fleabane (Conyza bonariensis). This is the most common.
- Tall Fleabane (C. sumatrensis) and
- Canadian Fleabane
For the sake of brevity and their management we are going to term all of these as Fleabane or Horseweed.
Why is Fleabane a Major Weed?
- Fleabane is a prolific seed producer.
- It is very competitive.
- Fleabane is very difficult to control with herbicides.
- Fleabane can develop herbicide resistance.
- In areas where there is limited competition, Fleabane invades and establishes quickly.
- Fleabane can build a strong root system over Winter and grow rapidly in the Spring. Over-wintering Fleabanes are very difficult
to control.
At first Horseweed, when young or in closely mown turf, 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 seeds form.
After you read this, you will be able to:
- Identify Fleabane or Horseweed.
- Know the habitat of Fleabane.
- Know the best options to control Fleabane or Horseweed.
How to Identify Fleabane.
During its vegetative stages Fleabane can be 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: Flowers are white to yellowish, and 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 covered in soft hairs.
Reproduction: Fleabane seeds are spread by wind, and mature Flaxleaf Fleabane produces an average of 110,000 seeds. These can be 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 buried seeds maintain their viability.
The optimum temperature for germination of Fleabane is 20°C with a base temperature of 4.2°C and a maximum temperature for germination of 35°C. 50% emergence occurs 4 to 5 days after moisture is supplied, at 20°C
Comments: Fleabane leaves are grey-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. The leaves that are near the base of the plant often wither early. Fleabane tolerates drought and frost, but is not competitive in shade.
Habitat: Fleabane is mainly 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:
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 during the germination period.
- Spray weeds when they are a small rosette and before stem elongation starts.
- Consider using an adjuvant to improve herbicide uptake.
- Moisture stressed plants are much more difficult to control with herbicides.
- Rotate herbicide mode of action groups.
- Control plants in surrounding areas so they don’t flower and act as a source of seed to infest turf sites. You must manage the seed bank for long term Fleabane management.
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.
Trying to mow this weed out is not effective and will encourange this weed.
If the soil is not compact then hand pulling after stem elongation works, but on heavier, compact soils you will need a fork to prevent the plant breaking and regrowing from the base.
Chemical Control of Fleabane:
The first point is that Horseweed has been confirmed as glyphosate resistant. However, you get the best Fleabane control when it emerges in the Spring, and it is still small. Small Fleabane plants are relatively 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.
Gallery.
Post-Emergent Control of Fleabane.
It is vital to use herbicides when the weed is a small rosette only 5 cm in diameter or less. This is because as the weed matures control efficacy declines. A spray adjuvant will produce better results, improve rainfastness and overall herbicide performance.
Selective post emergent herbicides for Fleabane include:
- 2,4-D Amine. Apply when weeds are young and actively growing. Repeat applications may be necessary.
- Dicamba. Apply to rapidly growing plants. Smaller plants are more easily controlled. Use higher rates for larger plants. Do not apply when outside temperatures exceed 26°C. Do not use on Bufffalo.
- Contra M. Do not use on Buffalo.
- Casper Turf Herbicide. Add a non ionic surfactant at 0.25 to 0.5 v/v. We recommend OptiSpread 1000. Do not use on Buffalo.
- Recondo Herbicide. Do not use on Buffalo.
- Pylex.
- Weed Blast MA.
- Warhead Trio.
You will get the best results when you:
- Only apply herbicides to actively growing weeds.
- Allow adequate time between spraying and mowing 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 we recommend the use of ProForce Manta Ray.
Other options include:
