Blue Heliotrope (Heliotropium amplexicaule).

Blue Heliotrope is also known as Purpletop, and is a Summer, biennial weed. It is a spreading prostrate herb with a deep root system that grows to a maximum height of 60 cm.

Purpletop has blue flowers with a yellow centre, a coiled stem, with hairy soft green foliage, and a pungent odour.

Blue heliotrope (Heliotropium amplexicaule) is of South American origin, and has become a serious weed problem in grazing lands of northern NSW, and southern Queensland.

Purpletop is a declared noxious weed in 14 local government areas of New South Wales. It is a problem because:

• It is very competitive, and results in significant yield losses.
• It is very drought tolerant.
• Blue heliotrope can set seed with minimal moisture.
• Purpletop is toxic to stock.
• The prostrate shoot growth of Blue heliotrope’s allows it to cover a large area to access sunlight.
• Herbicides have had limited success to reduce the impact of Purpletop, and have not stopped its spread.

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.

All parts of Blue Heliotrope contain high levels of alkaloids and are toxic to humans and livestock. They cause 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.

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

Category: Broadleaf (Dicot).

Photosynthetic Pathway: Purpletop is a C3 Weed.

Flower: The flowers of Blue Heliotrope are blue to purple with yellow centres, and up to 6 mm in diameter. The small flowers are at the coiled tips of the flowering stems are are held above the plant.

Rainfall triggers Purpletop to flower, but flowering tends to start in November, and then continue through the Summer until March. It can flower several times over this period.

In frost free areas it grows and flowers throughout the year after rain.

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 is 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 of Blue Heliotrope.

Blue Heliotrope reproduces from both seed and root fragments. It spreads aggressively, and produces many sticky seeds that adhere to animals and machinery. This seed can also wash along the soil surface and into waterways.

The number of flowering flushes per season reduces in drought but increases in wet Summers.

Buried seeds remains viable for more than a decade, and if you don’t control Blue Heliotrope, it develops vast soil seedbanks that can be 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.

Vegetative reproduction of Blue Heliotrope, occurs via shoots growing from buds on the lateral roots from the central taproot.

Root fragments spread by:

• Machinery.
• Flooding.
• Cultivation.

Comments:

• It is in the same family as Paterson’s curse, and as it matures it develops woody root stock.
• The tap root of Blue Heliotrope is up to 1 m long, and is the key reason it is so drought tolerant.
• Existing plants produce a flush of new growth in the Spring and Autumn, and then flower. In warmer areas, plants may flower, and set seed even earlier than this.
• Blue Heliotrope does not like frost, and in areas like the ACT it tends to die off in Winter. It then regrows from its root system in the Spring.

Habitat: You find Blue Heliotrope in a wide range of habitats, and you often see this weed along roadsides or in lawns, parks and gardens. Blue heliotrope colonises disturbed areas and favours thin or bare areas or overgrazed pastures. It tolerates shade but not waterlogging.

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

• Blue Heliotrope tends to not invade dense and well-managed turf.
• So one of the best ways to prevent Purpletop is if you maintain a vigorous, dense turf cover. Adopt good practices such as mowing at the right height and fertilizing you turf to help prevent this weed.
• Prevent thin and bare areas, so you reduce the opportunity for this weed to germinate and establish. Bare ground, favours Blue Heliotrope growth.
• Hand removal is difficult and not a good idea unless you are very thorough, and there are only a few localised plants. Its ability to regrow from small root fragments means that you must ensure that you remove all vegetative material.

The best time to hand remove plants is when soil is moist. For best results:

• Grasp the stem as close to the base of the plant as possible.
• Rock the plant from side to side and gently tug. This loosens the soil and allow the roots free up.
• If the stems are break off from the taproot, use a sharp blade to help free the roots as you pull them out. You must remove the entire root system.
• Remove the plant, and shake off any soil.
• Bag off the plant and take off site for disposal.

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 root system.

Systemic herbicides do however work well against Blue Heliotrope as they destroy its root system. As with all herbicides don’t spray this weed when it is under stress. The best time to spray is when the weed is actively growing and starting to flower.

Pre-Emergent Blue Heliotrope Herbicides.

We recommend Pre-Emergent Herbicides such as: