Fleabane (Conyza spp.)

Fleabane weed is also known as Horseweed. It is a tall dicot with a tough, hairy stem and leaves. As it matures its stem becomes hard and woody.

It is a Summer annual or biennial weed that germinates in the Spring. It then becomes a problem over the Summer months. Although it is a Summer annual, you can sometimes see it in the Winter.

After you read this, you will be able to:

• Identify Fleabane weed or Horseweed.
• Know its habitat.
• Know how to control this weed.

In Australia there are three main species of Fleabane weed:

• Flaxleaf Fleabane (Conyza bonariensis). This is the most common.
• The annual Tall Fleabane (C. sumatrensis).
• Horseweed or Canadian Fleabane (C. canadensis) which is also an annual.

Growth of Fleabane.

Photosynthetic Pathway: It is a C3 Weed. It grows best between 15°C and 30°C. Above 30°C growth declines. This is due to a lack of CO2 to sustain any further growth.

Flower: The flowers are white to yellow. The flower head has a cylinder to bell shape, and is 5 to 6 mm long.

Height: It is 1 to 2 m tall.

Leaf length: The leaves are 2 to 10 cm long

Leaf width: The leaves are 1.5 cm wide, and have a cover of soft hairs.

Reproduction of Fleabane Weed.

• This weed only reproduces by seed, and wind spreads the small, light seeds.
• Mature plants produce around 110,000 seeds. These are viable for up to 18 months.
• Its seeds only emerge when they are on or near the soil surface. No seeds emerge below a soil depth of 2 cm.
• When the seeds are on the surface, only 5% remain viable after a year.
• Even if the seeds are buried at 50 mm and 100 mm for a year, 10% and 15% of the seeds are still viable.
• The ideal temperature for this weed to germinate is 20°C. It has a base temperature of 4.2°C and a maximum germination temperature of 35°C.
• 50% of the seed emerges in 4 to 5 days after moisture is supplied at 20°C.

Comments: The leaves of fleabane weed are grey to green. They have an oblong, oval or wedge shape, and are twisted near their base. It can cope with drought and frost, but does not compete well in shade.

Habitat: You find this weed in farm land, orchards, vineyards, roadsides, and industrial sites. It thrives if there are bare areas of soil where it can germinate.

It is a common weed in light soils but you also can find it in heavy soils.

More on lawn and turf grass weeds is in our weed ID chart.

An integrated approach gives the best chance to manage Horseweed. The seeds do not seem to survive more than 18 months, so the aim is to carry out control before it forms new seeds.

• Monitor areas. This means that you can see when Horseweed first germinates.
• Spray weeds when they are at the small rosette stage. This is before any stem elongation starts.
• Use an adjuvant to improve chemical uptake.
• When plants are under water stress they are more difficult to control with chemicals.
• Rotate mode of action groups.
• Control plants in nearby areas so they don’t flower. This stops them acting as a seed source.
• You must manage the seed bank to get long-term control of this weed.

This weed is a poor competitor and does not like shade. A healthy, dense turf cover will choke out this weed. The key is to improve the competitiveness of your turfgrass.

• Feed you turf, mow at the right height for the turf species present, and deal with any soil compaction.
• Mowing to control this weed is not effective. In fact it tends to encourage it. When you mow it causes plants to laterally branch from their base, and hardens them off.
• When you mow it also reduces the leaf area for herbicide coverage. This makes post-emergents ineffective.
• If the soil is not compact you can hand remove this weed. This is best done before the stem begins to elongate.
• On heavy, and compact soils you will need to use a fork to prevent the plant breaking. This will stop any re-growth from the base.

Biological Control of Fleabane.

• There is no effective biological control in Australia for this weed. Work is ongoing to look at potential agents. One of these is a rust fungus (Puccinia cnici-oleracei) that only infects Flaxleaf Fleabane.
• When this pathogen infects plants they produce 50 to 60% less flower heads and, so less seed.
• There is also ongoing work into three insects. A stem-boring weevil, a leaf-gall midge, and a tephritid fly that forms stem galls.

• Some Horseweed plants have been seen to have resistance to Glyphosate. Mature plants seem to have a tolerance to high levels of this due to their leaf structure.
• A successful control program depends on when you treat it, the use of the right herbicide mix, and the use of multiple applications.
• The main thing to do is make sure that you spray when the plants are small, actively growing and are not under any stress.
• When young Horseweed plants are at the rosette stage, and less than 10 cm across they are easy to control.
• If you spray very young plants (up to the two leaf stage) there is only a small leaf area for chemical uptake. This can mean that your spray does not work.
• Small plants are easy to kill. The use of a phenoxy herbicide in the late Spring tends to kill them.
• Due the hairs on Horseweed, use a surfactant (ProForce Scrubwet) at 200 mL/100 L with these chemicals.
• Make sure that you don’t use too high a water rate. If the water rates are too high, then the adjuvant will become too dilute to work.
• It is difficult and expensive to kill mature plants. This is especially the case if you try to treat them in the mid to late Summer.
• Factors such as weed age will have a negative impact on the results that you get.

Pre-Emergent Control of Fleabane.

The results from pre-emergents depends on their timing, and the amount of rainfall or irrigation you apply.