Zantedeschia aethiopica

(Arum Lily)

Not only is the Arum Lily one of the worst weeds of wetter areas, it's also dangerously toxic, and there have been cases of children being fatally poisoned. The south-west corner of Western Australia has been hard hit by this weed, where it has spread over 12,000 hectares. The invasion is so bad that last year Western Australia listed Zantedeschia aethiopica as a declared weed, making it illegal to introduce, sell, or propagate this plant.

Zantedeschia is a weed throughout most of Australia, except the Northern Territory, having escaped from dumped garden waste and from rubbish dumps. The plant is still widely available in nurseries outside WA, and is grown in gardens throughout Australia, so its potential to continue to reek havoc is very high.

All cultivars, including 'Green Goddess' are weeds.

It invades wetter areas, such as coastal and riparian (streamside etc) areas, freshwater wetlands (seasonal and permanent), warm temperate forests, pastures, market gardens, roadsides, irrigation channels and wasteland.

The leaves of Zantedeschia contain water stomata (stomata are like pores, which are usually for the exchange of oxygen and carbon dioxide). In this case the stomata discharge excess water by a process known as "guttation". This prevents water-logging and enables arum lilies to grow in very wet conditions.

It can form large spreading clumps that choke out natural vegetation and impede water flow in streams. It also invades pasture land (where it can poison stock).

It tolerates tropical through to cold climates, doesn't mind sun or shade, and will even tolerate frost down to -10°C. Not only that, it can withstand waterlogging for short periods, wind, salt and most soil types, including sandy.

Death Flower

Most people would be familiar with this old garden plant. In Western culture it is the flower that often symbolises death and is used in funeral arrangements (although it was often used in wedding arrangements too!). It was introduced to Europe in the 1600s from South Africa.

Arum Lilies are not lilies at all, but they do die down in dry conditions, leaving a storage rhizome sitting under the soil waiting for the right conditions to grow again. In wetter environments they do not die back.

Traditionally the plant is boiled and eaten, but the raw plant material is toxic and causes swelling of the throat due to microscopic, sharp calcium oxalate crystals.

Control

The seed of Zantedeschia is attractive to birds, and with up to 500 seeds per flower head, they can be spread far and wide. Small pieces of rhizome are enough to start an infestation too. So, if you are removing this plant, you will need to ensure all plant parts are removed.

In Western Australia, the Department of Agriculture and Food, have published control methods. The first step is that the movement of plants or their seed is prohibited within the State. This of course will prevent further infestation.

Infested areas must be managed so that the spread of seed and plant parts is prevented. Treat to destroy and prevent seed set on all plants:

· Within 100 metres inside of the boundaries of the infested property;
· Within 50 metres of roads and high water marks on waterways;
· Within 50 metres of sheds, stock yards and houses.

Treatment must be done prior to seed set each year. Properties with less than 2 hectares of infestation must treat the entire infestation. Additional areas may be ordered to be treated. The herbicides recommended are listed on the following website but may not be available to home gardeners (and are certainly not low environmental impact chemicals). Glyphosate (as in Roundup) can be used but results are only fair. http://agspsrv34.agric.wa.gov.au/dps/version02/01_plantview.asp?page=1&contentID=7

In the home garden, Zantedeschia is best dug up while actively growing and prior to seed set, bagged, and put in the garbage bin.

Information sources:

Auld, B.A. & Medd R.W., 1987, Weeds - An illustrated botanical guide to the weeds of Australia, published by Inkata Press.
Blood, K., 2001, Environmental Weeds - A field guide for SE Australia, CRC Weed Management Systems, published by C H Jerram Science Publishers.
Conservation & Land Management, Western Australia
Department of Agriculture & Food, Western Australia

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