Praxelis (Praxelis clematidea).

Praxelis (Praxelis clematidea) is also known as Giant Bluetop. It is an annual or short-lived perennial herb and has a fibrous root system. It is a weed of the tropics and sub-tropics, and although it tolerates partial to full sun, it does not grow well in fully shade.

Praxelis is on the Alert List for Environmental Weeds. This is a list of 28 non-native plants that threaten biodiversity and cause other environmental damage.

The distribution map is courtesy of The Atlas of Living Australia.

After you read this, you will be able to:

• Identify Praxelis.
• Know the habitat of Praxelis clematidea.
• Know the best cultural and chemical controls for Praxelis clematidea or Giant Bluetop.

Praxelis contains pyrrolizidine alkaloids which are poisonous to stock that eat it, and has allelopathic properties.

Praxelis clematidea is an upright herb that grows from seed and also grows vegetatively.

Photosynthetic Pathway: Praxelis is a C3 weed.

Category: It is a broadleaf (Dicot) weed.

Flower: The flowers of Praxelis clematideaare blue or lavender, and are in clusters of about 35 to 40 tiny flowers at the ends of hairy stems.

Height: Praxelis grows to a height of 40 to 50 cm

Leaf length: The leaves are hairy underneath, oppositely arranged, and between 25 to 60 mm long.

Leaf width: The leaves are 25 to 37mm wide.

Reproduction: After it germinates, Praxelis produces hundreds or even thousands of small black seeds in as soon as three or four months. Most germination occurs after rainfall, but you can see this weed all year round in gardens or irrigated pasture.

The seeds germinate over a temperature range of 20 to 30 °C. High (45 °C) and low (10 °C) temperatures reduce germination. Maximum germination occurs when seeds are at the soil surface, and no seeds germinate at a depth of more than 1 cm from the surface.

Seeds have a “pappus” or a cluster of barbed bristles that help their spread. The seeds spread by wind or water, or attach to animal fur and feathers, clothes, or machinery.

Praxelis is also capable of vegetative growth. This occurs when roots and new plantlets form along branches in contact with the soil.

Habitat: It is usually found in disturbed areas along roadsides, in pastures, along railway lines, in open woods, and on fence lines.

For more information on weeds check out our weed ID Chart.

You can control Praxelis by cultural and chemical means. However, due to the high seed production of praxelis, you will achieve long-term control with a combination of pre and post emergent herbicides to prevent re-growth after it germinates.

Prevention is the most cost-effective form of weed control, so keep uninfested areas free of Praxelis, and do not attempt control on your own.

Hand pulling of small areas of Praxelis is not recommended. This is because the seed falls off and actually increases the area of infestation.

The fact that Praxelis does not grow in shade is also an indirect means of control. A thick turf cover with very few thin or bare areas stops germination. This means:

• Mow at the right height of cut for your turf variety.
• Fertilise as needed. Take into account shade and the time of year.
• Don’t over irrigate or let your turf dry out.
• Keep on top of any disease.

Chemical control options are limited for Giant Bluetop. For example, BASF Freehand gives poor control as a pre-emergent herbicide. The University of Florida suggests the use of Envu Specticle herbicide but this is not registered in Australia for this.

Non Selective Control of Praxelis.

• Glufosinate-ammonium provides control for 4 to 6 weeks, but it regrows and recovers due to the limited movement of glufosinate.
• Glyphosate. You can use Glyphosate as to control Giant Bluetop under APVMA Permit 11463. If you use Glyphosate, and water quality is an issue then use ProForce Manta Ray.

The following are non-selective but also have a long term residual and stop re-growth of Praxelis.

• • Renegade. Renegade stops the germination of Giant Bluetop for up to 12 months, and reduces the need for multiple herbicide applications.
  • Numchuk Quad. This gives effective post and pre emergent Praxelis control for up to 12 months.
  • Cortex Duo. Cortex Duo gives a rapid knockdown of Giant Bluetop, and residual control for up to 3 months. It is also safe to use around trees.