Botany of Auckland Pest Plants

There are over 40,000 exotic plant species in Aotearoa, a number that completely swamps the 2,400 native species that were here first [1]. Most exotic plants arrive in Aotearoa intentionally for cultivation. Many of these species then ‘jump the garden fence’ and become naturalised in their own wild populations, some becoming invasive species that outcompete native plants [2]. As the country's most populated city, Tāmaki Makaurau is absolutely packed with invasive plant species. They travel by animal and wind to every nook and cranny of our parks, maunga, street-sides, and gardens. But who are these plants? What is their ancestry and where have they come from? How different are they to what is already here? This article will explore the botany of some common invasive plant species and also give advice on removal should readers be inspired to do some weeding of their own.

To form sustainable populations in any place, a plant must be able to survive and reproduce under the conditions presented to them. In pre-human times, Auckland provided diverse substrates on which many different plants could grow. For example, the volcanic ash and rock that is abundant across Auckland has allowed fertile, granular soil to develop [3]. Additionally, sediment deposition along river floodplains, such as near the Manukau Harbour, has allowed fertile silt soils to develop [3]. In contrast, greywacke rock uplifted in the Hunua Ranges has created steep, weathered infertile slopes [4]. These different landscapes allow all sorts of plant species to find a place that suits them and develop diverse communities.

In more recent times, settlers' arrival to Auckland and the subsequent development of agriculture has made soils more suitable for plant growth. Fertilisers, lime application, irrigation and drainage activities involved with farming reduce the limitations that soil conditions can have on plant growth, and create a more homogenous landscape [5]. This results in far less opportunities for plants with tolerances for difficult conditions to colonise space. Consequently, introduced plants that can more efficiently exploit the nutrient and water resources available to them, are fastgrowing, and spread quickly, are excellent competitors in these conditions.

Beyond agriculture, Auckland is now heavily urbanised, and has become the largest city in the country. There has subsequently been an increase in introduced and exotic species as people bring all kinds of plants from all over the world to plant in their private gardens [6].

With such intense changes in water, nutrient, and light availability in Auckland landscapes, native plants have plenty on their plate. The addition of exotic competitors makes the conservation of native forest communities in the urban world incredibly challenging. It's important for Aucklanders who want to reduce invasive plant populations in the city to ‘know their enemy’, and understand as much about the origins and ecological function of these invasive plants as possible.

As a refresher, plants are classified in increasingly smaller groups of genetic relatedness: Division, Class, Order, Family, Genus, Species.

Climbing Asparagus Fern Asparagus scandens

Climbing asparagus fern. Image from weedaction.org.nz

Climbing asparagus fern is one of the most common invasive species in Auckland. It is a monocot, which are not woody plants, and as a result they don’t get incredibly large like trees. Monocots diverged earlier on the evolutionary tree than the Eudicots, which comprise most other flowering plants, and are characterised by a lack of secondary growth. This means that their shoots cannot get wider as they get taller, and so limits the structural stability and subsequently the height that these plants can achieve. The climbing asparagus fern, for example, can grow prolifically but all of its stems remain as delicate, thin tendrils, which achieve height by growing on other plants. Although it is described as a ‘fern’ in its common name, climbing asparagus has been historically considered part of the Lily family (Liliaceae), related to lilies and tulips. It is now placed in the family Asparagaceae, and neither of these families are remotely related to ferns1 . The Asparagaceae family is home to many popular houseplant species, and climbing asparagus fern is one of these. Its genus, the Asparagus genus, is made up of around 300 species that grow mostly as vines in the forest understorey, and this species, Asparagus scandens, is native to the understorey of coastal South African forests [7]. In Auckland, climbing asparagus smothers understorey plants and grows all along the ground in shady areas, preventing seedling germination of other species [8]. If you see it around, you can get rid of this plant by spraying it with common glyphosate herbicide repeatedly.

Tradescantia Tradescantia flumenensis

Tradescantia. Image from forestflora.co.nz.

This species is one of the hardest weeds to get rid of. Like the climbing asparagus fern, it is a member of the monocots and related to the Lily family, although rather than being from the asparagus family it is from the Commelinaceae family [12]. Commelinaceae are mostly tropical and subtropical herbs, and are mainly used by people for ornamental value. Native to South America, Tradescantia is a well known houseplant, and although the green variety is no longer allowed to be sold in Aotearoa, the purple and white variegated variety is still very popular with Auckland residents [13]. If you have this houseplant, make sure you don’t throw it outside in a compost or rubbish bin. It needs to be burned or sprayed with herbicide to kill it. This is because Tradescantia can regrow from cut stems, allowing fragments to wash up along rivers and waterways [14]. From these fragments, Tradescantia creates large mats of groundcover in forest understoreys that prevent native seedling germination [15]. An alternative form of weed control is chickens — they love eating the leaves! Interestingly, although Tradescantia has detrimental effects on forest understorey regeneration, it is used as shelter by endangered native skinks [16]. In ecosystems there are always multiple ecological functions of each organism, and it can be difficult for conservationists to evaluate the net effect that each species has on communities with so many different needs.

Woolly Nightshade Solanum mauritianum

Woolly nightshade. Image from weedbusters.org.nz.

Woolly nightshade is a small tree with large, fluffy oval leaves. It germinates and grows to reproductive age easily, and its fruits are eaten by native and exotic birds, promoting its spread. Woolly nightshade is part of the very large clade of the Asterids, which is the most recently diverged group in plant evolution, and also the largest clade with around 80,000 species [9]. Unlike their earlier diverging sister group of Rosids, which comprise the majority of other flowering plants, Asterids usually have fused petals, so that the flower forms a tube around the stamens and carpel [10]. The purple flowers of woolly nightshade follow this trend, with a fused base and five pointed lobes which would have once been five separate petals in an ancestral species. Woolly nightshade is part of the Solanaceae (nightshade) family, which is a famous family with many toxic plants. This species is no exception, and it produces toxins which prevent other plants from colonising the soil around it, known as allelopathy [11]. Plants of the Solanaceae family are found all over the world, but are most diverse in South America, which is where woolly nightshade is from. Its genus, Solanum, includes globally popular cultivated species that millions of people rely on for food, including potatoes, eggplant, tomatoes, and chillies. The woolly nightshade plant has a faint smell that I personally think is nauseating. If you see it in your area, you can handpull small plants and cut the trunks of large plants (the wood is very soft) and paste the stump with tree killer.

Tree Privet Ligustrum lucidum

Tree Privet. Image from weedaction.co.nz

This tree is a member of the Olive family, Oleaceae. Unlike the other invasive species mentioned in this article, privet is from the northern hemisphere, from temperate regions in East Asia [17]. Like the well-known olive, tree privet has small, dark purple berries. These berries are poisonous and are thought to have negative effects on native insects [18]. Another famous exotic species from the Oleaceae family that has invasive characteristics in Auckland is jasmine, which has very sweet smelling flowers. These species, like most other Asterids, have petals that are fused at the base, and members of the Oleaceae family have four petals per flower [19]. Tree privet is an invasive species in Auckland because it produces high quantities of viable seed and is long-lived, surviving as a small tree species for around 100 years [20]. Like woolly nightshade, tree privet forms a subcanopy that outcompetes other native species, preventing native regeneration and succession processes [21]. This species is much harder to pull out than woolly nightshade and also has much harder wood, making it more difficult to saw through. Most of the time removing it involves cutting the base of the trunk and painting the stump with strong stump-killer — tree privet will reshoot from a cut stump so it is very necessary to apply herbicide.

Moth plant Araujia hortorum

Moth plant. Image from weedaction.co.nz

Moth plant is a vine from the Apocynaceae family. The common name of this family are the milkweeds, and this is because these plants release white latex when their stems are broken. Moth plant is no exception, and its milky latex is irritating on skin and stains clothes. Most of the species of the Apocynaceae family are endemic to the tropics, including the moth plant, which is native to South America [22]. As another Asterid, its five white petals are fused at the base and can trap and kill insects [23]. The fruits of the moth plant are very distinctive: massive pods that are about the size of a hand and shaped like a rugby ball. Inside these pods are hundreds of seeds, each with a feathery plume attached to one end to help them disperse by wind. Moth plants germinate in massive numbers, and smother and strangle other plants. Small moth plant seedlings can be pulled out, and large vines should be cut at the base and painted with weed killer. It’s also important to remove the pods (as once they open they cause further infestation), and destroy them so that they don’t open.

These five weeds are arguably the most common in Auckland, but there are thousands more invasive plant species growing happily in the city. Once you start looking for weeds, it can be overwhelming just how many there are, particularly those growing along roadsides and on unmanaged land. Conservation is a strongly value-driven science and activity, and is the result of human conceptions of nature and wilderness being projected onto the nonhuman world. Consequently, significant conservation outcomes require huge amounts of effort and intervention. Exotic plant control is a central component of conservation, and understanding where such plants come from, and how they grow and spread, is important for achieving plant conservation goals.