Chilean Mayten - A Ticking Time-Bomb?
Close up of foliage and female flowers.
Experts have estimated that about 20 new plants ‘jump the garden fence’ each year and become naturalised in New Zealand. One such plant that appears to be of concern, particularly around Canterbury, is Chilean mayten (Maytenus boaria). “This evergreen tree appears to be a ticking time-bomb with real potential to become a major environmental weed throughout New Zealand, hence this article to raise awareness,” explained botanist Murray Dawson.
There are more than 50 species of Maytenus and most are tropical in origin. Several have been cultivated in New Zealand, but M. boaria is by far the most widely grown due to its cold tolerance and horticultural qualities. Chilean mayten belongs to the Celastraceae family and is native to South America,naturally occurring from about 30 to 50°S in Chile and Argentina.
Chilean mayten is long-lived, fairly hardy and drought resistant. Saplings are very shade tolerant but also grow in full sun. A small to medium-sized tree, Chilean mayten typically reaches 6–8 m after several decades, but under optimal conditions and given enough time it can eventually grow up to 20–30 m tall. When mature this graceful tree develops a straight trunk and pendulous branchlets that sway in the wind, similar in effect to a weeping willow (Salix babylonica). The leaves are relatively small, with finely serrated margins. In New Zealand there appear to be separate male and female plants, although in their native range flower gender may be more variable. Flowers are greenish-yellow, relatively inconspicuous, and appear (in New Zealand) from late August to early October. The seeds are contained within berry-like red fleshy arils and mature from March to June. Plants produce flowers and fruits from an early age (3–5 years, 2 m tall).
“At first glance Chilean mayten looks a lot like a native New Zealand plant, resembling a small-leaved māhoe (Melicytus) or perhaps Hoheria,” explained Murray. When it’s still a shrub it looks rather nondescript, with its small, evergreen leaves and (unless in fruit) few distinguishing features. Because it blends in so well, Chilean mayten can easily be overlooked, especially since its shade tolerance allows it to establish among other plants and later out-compete them.
The plant is recorded as introduced in 1881 as part of the Christchurch Domain Board’s plant exchange programme, and a large specimen was highlighted as growing in the Christchurch Botanic Gardens in a 1945 visitors guide. By 1929 Chilean mayten was being sold commercially by Duncan Davies nursery, New Plymouth. Based on herbarium specimens, it was first thought to be naturalised in 1986 at Church Bay, Canterbury, followed soon after by similar records from Gisborne (near Eastwoodhill Arboretum) and more recently in 2012 from Whanganui (Bason Botanic Gardens).
So why is this previously benign tree now going rogue more than 130 years after its first introduction into New Zealand? “Full credit for unlocking this puzzle goes to Joe Cartman, Christchurch City Council Nursery Supervisor and a renowned plantsman,” revealed Murray. Joe realised that up until the mid-1980s only male plants were sold by nurseries. This was based on observations he made of dozens of trees around Christchurch, and elsewhere, that were all males. Joe commented that these male plants appeared to be uniform and probably from the same clone. Material would have been easy to propagate vegetatively by pulling up suckering shoots and growing them on through root cuttings. However, from the mid-1980s seed-grown plants started to appear on the market, and inevitably some of these were female. Birds love to eat the fleshy fruits, and thus Chilean mayten has literally now gained wings in New Zealand. This has allowed the plant to disperse well beyond the original (all-male) plantings and colonise new areas.
Chilean mayten suckers readily, and in some growing conditions these suckers can form dense thickets more than 10 m away from the parent tree. Suckers can be controlled in some situations by mowing or stock grazing. However, it takes a lot to stop them from re-emerging. Even when trees are cut down, and the stumps are repeatedly poisoned and regrowth removed, suckers can be persistent and difficult to control.
“Those removing trees can expect to have to deal with a legacy of suckers and seedlings for decades afterwards,” cautioned Murray. “At the Canterbury Agriculture and Science Centre at Lincoln, where I work, landscaping planted in the early 1990s by the former Ministry of Works included a female Chilean mayten tree in the South American courtyard garden. Today saplings are rampant throughout our native and exotic plantings on campus, some male and others female,” said Murray. A similar situation is occurring at the nearby Lincoln University campus, along with evidence of spread via seed dispersal.
“The future horror of Chilean mayten was reinforced when I heard a story from a landowner near Waimate, and his efforts to control it,” said Murray. From 2001 John Stevens planted more than 10,000 native plants on 4 ha of his land. This revegetated area gained a QEII covenant in 2007. Unfortunately, around 2002/03 his neighbour on the opposite side of the road planted a shelterbelt that included some 50 plants of Chilean mayten, recommended for that purpose by a local plant nursery. Some of those trees must have been females, as 13–14 years after the shelterbelt was planted, thousands of Chilean mayten seedlings became apparent in John’s native plantings across the road. These appeared as isolated plants and also thick patches of seedlings (particularly under native trees and shrubs favoured by the birds to roost on).
John’s neighbour readily agreed to kill the Chilean mayten from the shelterbelt, and together they trialled different treatments in 2016. They found spraying with Starane® (60 mL per 10 litres) to be the most effective, killing off all adult trees along with their suckers that were growing up to 6 m away from the parent trees. Drilling and filling trunks with neat Tordon® was the next most effective treatment, and neat Roundup® the least effective of their trials.
As a result of its weedy characteristics, Chilean mayten was added to the National Pest Plant Accord (NPPA) in 2012, banning its sale, distribution and propagation in New Zealand. “I was therefore surprised to spot the plant listed in a landscaping and tree planting appendix to the proposed 2014 Christchurch Replacement District Plan, and tendered a written submission pointing out that it cannot now be considered for planting under any circumstances,” said Murray.
In April, Murray spoke on this emerging threat at an event organised by the Canterbury Branch of the New Zealand Biosecurity Institute. “Ironically, after speaking I found a Chilean mayten tree in fruit during the field trip in Governors Bay on Banks Peninsula, and saplings spreading into the park next door,” said Murray.
So what can be done about this ticking time-bomb? “We need sharp eyes looking out for this new pest plant. Saplings and trees – of females in particular – should be hunted down and actively removed as soon as possible to prevent further spread of seedlings,” Murray suggests. The cost of inaction could well be a new environmental weed becoming established throughout New Zealand, and perhaps capable of invading native forest. Let’s hope it’s not already too late.
Activities to raise awareness of Chilean mayten have been partially funded by the Ministry of Business, Innovation and Employment as part of Landcare Research’s Beating Weeds programme.