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 is an annual or biennial weed that germinates in the Spring and becomes a major problem in the Summer. Although it is a Summer annual, you sometimes see Fleabane weed in the Winter.
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.
In Australia there are three main species of Fleabane:
- Flaxleaf Fleabane (Conyza bonariensis), which is the most common.
- The annual Tall Fleabane (C. sumatrensis) and
- Horseweed or Canadian Fleabane (C. canadensis) which is also an annual.
Thanks to The Atlas of Living Australia for the distribution map. 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 produces a high percentage of viable seed.
- 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.
- Dense stands smother native vegetation.
- Fleabane builds a strong root system over the Winter, and in the Spring grows quickly. Plants that over-winter are very difficult to control.
Growth of Fleabane.
Fleabane has alternate dark green leaves with toothed edges. Fine hairs on the leaf surface, and these hairs together with its woody stem, make it difficult to control.
There is only limited emergence of Fleabane in the mid Winter. However, young Autumn or early Winter seedlings do grow over the Winter even in cold or dry conditions.
While you can’t see much above ground growth, strong root growth continues over this period. The tap roots of Fleabane are able to grow more than 35 cm down into the soil, and this allows them to survive severe drought conditions.
The other advantage of growing a strong root system over the Winter, is that Fleabane builds up food reserves. This means that in the Spring it is ready to start to grow immediately.
In close mown turf or when it is young, Horseweed looks like 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.
You can often confuse Fleabane seedlings with other winter annual weeds like Common Chickweed and Shepherd’s-Purse. In its vegetative stages Fleabane is often confused with Capeweed.
How to Identify Fleabane.
Photosynthetic Pathway: Fleabane is a C3 Weed. Being a C3 plant, Horseweed grows best between 15°C and 30°C. Above 30°C, photosynthesis and growth decline due to a lack of CO2 to sustain any further increase in growth.
Flower: The flowers of Fleabane are white to yellow. The flower head has a cylinder to bell shape, and is 5 to 6 mm long.
Height: Fleabane is 1 to 2 m tall. Flaxleaf Fleabane is up to 1 m tall whilst Tall Fleabane can be up to 2 m.
Leaf length: 2 to 10 cm long
Leaf width: The leaves are 1.5 cm wide, and have a cover of soft hairs.
Reproduction of Fleabane.
Fleabane only reproduces by seed, and wind spreads the small, light seeds. Mature Horseweed plants produce around 110,000 seeds, and these are viable for up to 18 months.
Fleabane seeds only emerge when they are on or near the soil surface, and none emerge below a depth of 2 cm.
When the seeds are on the surface, 5% 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: The leaves are grey to green, oblong to wedge shaped to oval, and twisted near the base. Fleabane tolerates drought and frost, but does not compete well in shade.
Habitat: Fleabane infests farm land, orchards, vineyards, roadsides, abandoned fields, and industrial sites. This is especially the case if bare soil exists where it can germinate.
Fleabane weed is common in light soils but you also sometimes find it in heavy soils.
More on lawn and turf grass weeds is in our weed ID chart.
Fleabane Control in Lawns.
Prevention of Fleabane:
An integrated approach will give the best chance to prevent 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 when Fleabane germinates.
- 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.
Cultural Control of Fleabane.
Fleabane is a poor competitor and does not like shade. This means that a healthy and dense turf cover will outcompete this weed. The key is to improve the competitiveness of your turfgrass.
So feed you turf properly, cut at the right height for the turf species present, and limit soil compaction.
If you try and mow this weed out it is not effective, and tends to encourange this weed. Mowing encourages lateral branching of Fleabane from
the base of the plants, and hardens them off.
Mowing also reduces the leaf area for herbicide coverage, and makes post-emergent herbicides ineffective.
If the soil is not compact you can hand remove Fleabane once stem elongation occurs. However, on heavier, and compact soils you will need a fork to prevent the plant breaking, and regrowth from the base.
Biological Control of Fleabane.
Currently there is no effective biological control in Australia to control Fleabane. However, work is ongoing looking at potential agents and one of these is a rust fungus (Puccinia cnici-oleracei) that only infects Flaxleaf Fleabane.
When this rust pathogen infects Flaxleaf Fleabane it produces 50 to 60 % fewer flower heads and, therefore, less seed.
There is also ongoing work into three insect candidates. A stem-boring weevil, a leaf-galling midge, and a stem-gall-forming tephritid fly.
Chemical Control of Fleabane:
Some Horseweed plants have been confirmed as glyphosate resistant, and mature Fleabane seems to have a natural tolerance to high levels of Glyphosate due to its leaf struture.
A successful control program for Fleabane depends on the when you treat it, the use of herbicide mixes, and the use of sequential applications.
The most important thing to do is make sure that you apply herbicides when the plants are small, actively growing and are not under any stress.
When young Fleabane seedlings are at the rosette stage, and less than 10 cm across they are easy to control. However, be aware that if you spray very young seedlings (up to the two leaf stage) there is a small leaf area for uptake, and control is often not successful.
Small Fleabane plants are pretty easy to kill, and a late Spring post-emergent application of phenoxy herbicides controls them. Due the hairy nature of Horseweed use an organosilicone surfactant (ProForce Scrubwet) at 200 mL/100 L with these.
The other trick is too make sure that you don’t use too high a water rate. If water rates are too high, then the adjuvant becomes 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, as factors such as weed age have a negative impact on the results that you get.
Pre-Emergent Control of Fleabane.
The results from pre-emergents often depend on the application timing, and the amount of rainfall or irrigation.
- The only Pre-emergent herbicide currently registered for turf is BASF Freehand.
- Esplanade is registered for Fleabane in NZ.
Post-Emergent Control of Fleabane.
- You must use post emergent herbicides when Fleabane weed is a rosette up to 5 cm across. This is because as the weed matures, chemicals become less effective.
- A spray adjuvant helps with the results, by improving its rainfastness and overall herbicide performance.
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.
Selective post emergent herbicides for Fleabane include:
- 2,4-D Amine. Apply when Fleabane weeds are young and growing. You may need several applications.
- Dicamba. Apply this to growing plants. Small plants are easier to control, and you need to use high rates for large plants. Do not use Dicamba 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 from Casper, add a non ionic surfactant at 0.25 to 0.5 v/v. We suggest OptiSpread 1000. Do not use this on Buffalo grass.
- Recondo Herbicide. Do not use on Buffalo grass.
- Pylex. Do not use on Buffalo grass.
- Weed Blast MA.
- Warhead Trio.
Table of post Emergent Fleabane Herbicides.
Product | Active | Chemical Group | Rate/Ha | Comments | ||||
2,4-D | 2,4-D | 4 | 1.8-3.2 | Wet foliage thoroughly. DO NOT mow lawn for 1 week before and at least 1 weed after application. DO NOT use on Buffalo grass (WA only). | ||||
Casper | Prosulfuron + Dicamba | 2 + 4 | 800g-1Kg | Apply from Autumn to Spring. Use high rates in cool months or if high weed pressure. Control takes 4 to 6 weeks. Use an NIS at a rate of 0.25 to 0.5% v/v. | ||||
Contra M. | Dicamba + MCPA | 4 | 6.5 L | Apply in 250-400L water. DO NOT use on Buffalo grass. After use do not mow for 2 days before or after application or fertilize within two weeks. | ||||
Dicamba | Dicamba | 4 | 1.2L + 3.2L of 2,4-D Amine 625g/L | Use a minimum of 1000L/Ha water. Do not spray on Buffalo or Bent Grass. | ||||
Recondo | Trifloxysulfuron | 2 | 225g | Use an NIS at 0.25% v/v (1000 g ai/L), 0.42% v/v (600 g ai/L) or Overtake Oil at 1%v/v. Ensure uniform placement onto leaves & into crowns. Water volume is 400 to 800 L/ha. You may need a repeat application in 4 to 6 weeks. Allow at least 6 weeks before overseeding. You may see discolouration on Qld Blue and Zoysia. | ||||
Pylex | Topramezone | 2 | 110g | Apply to growing weeds and not to stressed weeds. | ||||
Weed Blast MA | Bromoxynil + MCPA | 27 | 0.375 mL/ 100 m2 in 4-6 L water + 0.5% MSO | Make two applications 21-28 days apart. You may see bleaching of Bentgrass after 7-14 days. Do not water for 24 hrs post application. | ||||
Warhead | MCPA + Clopyralid + Diflufenican | 4 + 12 | 5 L | You may see discolouration on kikuyu, carpet grass and Queensland blue. Avoid overlapping. Use an NIS. | ||||
Non Selective Control of Fleabane.
You cannot use any of these on lawns or turf areas to selectively remove Fleabane.
- Glufosinate-ammonium controls Fleabane for 4 to 6 weeks, but it regrows and recovers due to the limited movement of glufosinate.
- Glyphosate. You can use Glyphosate but if water quality is an issue then use ProForce Manta Ray.
The folllowing are non-selective but also have a long term residual and stop any re-growth of Fleabane weed.
- Renegade. Renegade stops weed germination for up to 12 months. This reduces the need for multiple herbicide applications.
- Numchuk Quad. This gives post and pre emergent control for up to 12 months.
- Cortex Duo. Cortex Duo gives a rapid knockdown of Fleabane, and residual control for up to 3 months. It is also safe to use around trees.
