Polygala myrtifolia, a beauty and a beast!

By Sue Olsson, Coordinator,
Friends of Tomaree National Park

Commonly known as Myrtle-leaf Milkwort, also as Butterfly Bush, Parrot Bush, or simply Polygala, the purple flowering pea from South Africa can flower variously during the year, making it popular as a hardy and showy garden plant.

However it has a beastly side.

Having escaped from gardens, it’s become a widespread and invasive weed, smothering native bush, particularly low coastal scrub, creating an almost monoculture.

In places like Cape Schanck in Victoria, Polygala myrtifolia has almost completely replaced the bushland.

At Tomaree NP, we’re working hard to not let that happen.

More locally on the Tomaree Peninsula – the myrtle-leaf milkwort, Polygala myrtifolia, seems to have been planted in Nelson Head’s gardens. It’s prevalent throughout Shoal Bay’s coastal dune, particularly the western strip, neighbouring Anzac Park, and in Tomaree Lodge’s bushland.

It has a long history in Tomaree National Park – possibly emanating from Tomaree Lodge. For us, in the Friends of Tomaree National Park, our association with Polygala goes back to our beginnings – our first workbee in October 2020, just 4 years ago!

At Tomaree’s Zenith Beach – we sighted an unfamiliar pea plant. Fortunately one of our team members, Barb, in our newly established team had seen National Parks and Wildlife Service rangers working elsewhere on what looked like this pea and alerted us to its potential as a problem plant.

Learning about Polygala from John
at our second workbee, October 2020

We checked with John, our mentor, who confirmed the pea was Polygala myrtifolia. He’d been controlling it at Stoney Ridge Reserve and Polygala virgata informally on Wanda Headland for some time.

This became our priority task in these early days at Zenith Beach, as it was flowering when we started and easily seen — a great task for a new group learning the ropes!

Alan up-ending Polygala bushes, while Lisa dives deep to find Polygala’s base and delighted at having exposed the banksia behind.

Returning two weeks later, dead Polygala over the dune demonstrated its pervasiveness.

We cheered at the sight of death and destruction, yet felt strangely satisfied.

We’ve removed the numerous Polygala on the windward side of the frontal dune, and scattered between the bushes on the ridge and down the dune’s leeward side. And we’ll keep maintaining this area.

Managing Polygala myrtifolia is a constant for about 5 years at each site

Birds, ants, water, soil movement and garden refuse can disperse Polygala seeds, which remain viable in the soil for up to 5 years.

Removing a Polygala big enough to seed provides an opportunity for a multitude of seeds to germinate.

Seeds tend to germinate in autumn but can germinate at any time of year moisture is available.

Increased germination follows fire, disturbance and denudation.

Germination can occur in heavy shade and under light mulch.

Seedlings grow quickly, flowering when less than 50 cm tall. It grows mainly over the cooler and wetter winter and spring periods. One reference indicates time to first flowering is 3 years (WA), though we feel it’s been faster in our wet weather these recent years, and we work on about 12-18 month cycles to check. Seedlings thrive in shade – so we’re extra vigilant looking under bushes in areas where Polygala occurred.

The mature plant flowers mainly in spring to summer to produce seed for an autumn germination. High levels of seed production and seed dormancy results in significant soil seed banks.

Once adult plants are removed, for several years new seedlings need to be removed preferably before flowering and seeding. Seedlings are hand pulled. Larger plants can be cut at the base from which they rarely regrow, preferably before flowering and seed set. This creates less soil disturbance and less seed germination than pulling the larger plants. If you’d like to find out more about seed germination and its implications, this research abstract is a good summary.

Polygala is seemingly only in northern Tomaree National Park

We started working on Polygala at Zenith Beach, and fortunately it was almost restricted to the centre block between the two access paths to the beach. Concurrently we also managed bitou.

Since late 2020 when we started, we’ve had three checking cycles and hopefully none have seeded in that time. However we have found flowering Polygala, hopefully none seeded, where we missed young plants on earlier checks.

Removing larger Polygala from the leeward side of Zenith’s frontal dune where it also thrives

We’ve since managed Polygala on Tomaree summit, and believe in three years none have seeded above the yellow brick path to the bitumen path of the gun emplacements. Below that area we are gradually bringing Polygala under control. However Polygala (and bitou) around Tomaree Lodge is a constant source of recruitment into the national park until it can be controlled also.

At Box Beach we’re removing Polygala

We are yet to address Polygala at Wreck Beach.

Our priority is to maintain the areas already managed and ensure no seeding occurs, rather than spread ourselves too thinly over too many sites. But we have it on our horizon.

Several plants in Tomaree look similar.

In flower there’s little to confuse with Polygala – though some have suggested the threatened mint bush Prostanthera densa in flower could be confusing to the novice.

However seedlings are easily confused. Seedlings of Coast Teatree, Leptospermum laevigatum, the native Coast Beardheath, Leucopogon parviflorus, and the pea Pultenaea daphnoides also look similar to Polygala seedlings. We allow Polygala seedlings to grow sufficiently for us to confidently distinguish from native seedlings.

Young native Monotoca or Leucopogon, pointy leaves

Polygala seedling, rounded ends on leaves

Mixture of native seedlings left, and Polygala centre right

Tea tree or Leptospermum seedling, rounded ends on leaves, opposite along the stem

Polygala seedling rounded ends on leaves, “spiral’ or random along stem

A single Monotoca elliptica plant with dark green mature leaves and larger light/bright green juvenile leaves (which will eventually turn dark)

Polygala is banned in New Zealand ~ why not Australia?

With so many coastal ecosystems under threat from urbanisation, stopping new sources of potential weed invasion, such as from plant sales, should be a high priority. Polygla can thrive inland also, so a ban on Polygala sales has the potential to minimise new invasive sources across a broad spectrum of Australia’s landscape.

CSIRO research concluded: “There is high risk of P. myrtifolia further invading sensitive ecosystems because of its high germination success. Preventing seed dispersal and ensuring seedlings are controlled on emergence are critical to reducing its impact. We recommend that the cultivation and sale of this species should be prohibited across Australia to prevent further spread.”

Polygala is banned in New Zealand, so why is this not happening in Australia too?

If you or neighbours grow Polygala myrtifolia

Consult Grow me instead to find an alternative native plant, though best if the alternative is a local native from your area. There are so many beautiful hardy local Tomaree shrubs there’s really no need to grow Polygala.

Mountain devil Lambertia formosa bears fascinating red flowers.

Broad-leaved Drumsticks Isopogon anemonifolius with eye-catching yellow flowers and intricate foliage

And there’s two native Polygala species in NSW, though not in Tomaree Peninsula

The Dwarf Milkwort Polygala japonica is largely in grasslands and grassy understory of open forests along eastern NSW, and the threatened Polygala linariifolia is found in north eastern NSW.

And there’s additional Polygala weed species, Polygala paniculata and Polygala virgata.

Tit bits of info – fact or fancy?

Name comes from the Greek “Polus” meaning much and “Gala” meaning milk and refers to the ancient belief that animals eating these plants produce more milk. The species name myrtifolia means myrtle-like leaves.
In its native South African habitat, it’s described as a pioneer species — readily establishing in disturbed areas hence its invasive nature here.

Polygala myrtifolia, a beauty and a beast!