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.

After you read this, you will be able to:

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

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. Praxelis has never been found in NSW. It is present in Queensland.

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

Praxelis clematidea is an upright herb that grows from seed and also grows vegetatively. Its stems have a cover of soft downy hairs.

Photosynthetic Pathway: It is a C3 weed.

Category: Praxelis is a broadleaf (Dicot) weed.

Flower: The tiny flowers of Praxelis clematidea are blue or lavender. There 35 to 40 of these flowers in clusters at the ends of hairy stems. You tend too see these from January to May, but they can be present the year-round.

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

Leaf length: The leaves of Giant Bluetop are hairy underneath, oppositely arranged, and 25 to 60 mm long. They are sharply toothed along their edges with 5 to 8 teeth on each side.

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

Reproduction: After it germinates, Praxelis produces up to 1,400 small black seeds per plant 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.

Praxelis seeds germinate over a temperature range of 20 to 30 °C. Both high (>45°C) and low (<10°C) temperatures tend to reduce germination. The maximum amount of germination occurs when the seeds are at the soil surface, and no Praxelis seeds germinate at a depth of more than 1 cm below the soil surface.

The seeds have a “pappus” or a cluster of barbed bristles that help them to spread. Praxelis spreads 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.

Comments: If you crush the leaves they smell like cat’s urine.

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 its high seed production, you will tend to only achieve long-term control if you use a combination of pre and post emergent herbicides to prevent any re-growth once it germinates.

Preventing Praxelis is the most cost-effective way to manage this weed. So aim to keep uninfested areas free of Praxelis.

• Hand pulling Praxelis is not recommended, even if it’s only in small areas. This is because the seed falls off and will actually increase the area of infestation.
• The fact that Giant Bluetop does not grow in shade is also a control option. A thick turf cover with very few thin or bare areas stops germination.

This means:

• Mow at the right height of cut.
• Fertilise as needed. Take into account shade, the turf type 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 Praxelis or 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 does not have a registration in Australia for Praxelis.

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 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 any 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 Praxelis, and residual control for up to 3 months. It is also safe to use around trees.