Blue Heliotrope (Heliotropium amplexicaule)

Blue Heliotrope is also known as Purpletop, and is a Summer, biennial weed. It is a spreading prostrate herb that grows to a maximum height of 60 cm, and has blue flowers with a yellow centre. Its stem is coiled, with hairy soft green foliage, and it has a pungent odour.

During Winter in colder regions, plant growth virtually ceases until the temperature increases in the Spring.

The distribution map is courtesy of the Living Atlas of Australia. This shows how this weed is now found over large areas of south-eastern Queensland and northern New South Wales.

The tap root of Blue Heliotrope is up to 1 m long, and is the key reason that this weed is so extremely drought-hardy. It means it can tolerate long periods of drought, and if left un-checked, it rapidly populates bare ground.

Blue Heliotrope reproduces from both seed and root fragments. It spreads aggressively, as it produces many sticky seeds that adhere to animals and machinery.

All parts of Blue Heliotrope contain high levels of alkaloids and are toxic to humans and livestock, causing liver damage and potentially, stock deaths.

Although animals tend to only graze the weed when hungry and alternative pasture is scarce, the weed is toxic both fresh and dried. Horses are the most susceptible.

After you read this, you will be able to:

• Identify Blue Heliotrope or Purpletop .
• Know the habitat of Blue Heliotrope.
• Know the best options to control Blue Heliotrope or Purpletop.

Category: Broadleaf (Dicot).

Photosynthetic Pathway: C3 Weed.

Flower: The flowers of Blue Heliotrope are bluish-purple with yellow centres, and up to 6 mm in diameter.

Height: It grows to a height of 15 to 30 cm.

Leaf length: Purpletop has spear shaped leaves which are a dull green colour. The stem, like the leaves are hairy.

Leaf Width: The leaves are 15 to 20 mm wide, and have a toothed leaf edge.

Leaf ID: Blue heliotrope is hairy along its leaf edges.

Reproduction: Purpletop reproduces from both seed and root fragments. Buried seeds remains viable for more than a decade and if you don’t control Blue Heliotrope, this results in vast soil seedbanks. These can reach up to 50,000 seeds/m2 after eight years.

It takes many years for seedbanks this high to decline, and a field trial in NSW found that it took around 10 years for the seedbank to decline from around 50,000 to 1,500 seeds/ m2.

Comments: It is in the same family as Paterson’s curse. As it matures it develops woody root stock.

Habitat: You find Blue Heliotrope in a wide range of habitats, and you often see this weed along roadsides.

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

One of the best ways to prevent Purpletop is if you maintain a vigorous, dense turf cover. This means adopting good practices such as mowing at the right height and fertilizing you turf properly. These practices ensure that your grass is able to outcompete this weed.

If you reduce thin and bare areas, you reduce the opportunity for this weed to germinate and establish.

Hand removal of this weed is diffilcult and probably not a good idea unless you are very thorough and there are only localised plants. Its ability to regrow from small root fragments means that you must make sure that you remove all vegetative material.

In NSW biological control is being investigated. The blue heliotrope leaf beetle, has shown promise, as has the flea beetle, which is now called the Blue Heliotrope flea beetle.

Blue Heliotrope is difficult to control, and its hairy leaves limit herbicide uptake. Many herbicides only kill the top of the plant, which means it then re-shoots from the remaining root system.

Systemic herbicides do however, work well against Blue Heliotrope as they destroy their root system. As with all herbicides avoid spraying this weed when it is under stress, and aim to apply when the weed is actively growing and beginning to flower.

Pre-Emergent Herbicide.