Weeds!

Skyrail Nature Diary: July 2013


This month we will be looking at weeds occurring along the cableway. This may sound like a strange topic but let’s face it, weeds make up a significant amount of the vegetation and total eradication seems to be impossible. Of course, we do our best to control the weeds in areas which we have easy access. So, what is a weed exactly? Simply stated, a weed is a plant that has been imported, accidentally or intentionally, to an area where it doesn’t belong. How are weeds bad? This one isn’t always as obvious as you would think. Three things that all weeds have in common are that they are fast growth, have huge amounts of seeds that are often hard to kill, and a lack of animals that normally control the weed populations. This means that weeds have the potential to take over large areas of land on which native plants would normally grow. The native plants are often suffocated under a barrage of short-lived and fast growing weeds and/or have to compete for limited resources. In the past we would often introduce animals that we believed could control the weeds only to be faced with an even worse pest on our hands. The Cane Toad is a classic example. That doesn’t mean that introducing animals from abroad never works. Dung Beetles have been introduced to take care of our cattle’s cowpats while the Cactoblastis Caterpillar was introduced to get rid of the Prickly Pear Cactus. Both introductions worked because the introduced animals were specialists that were unable to eat anything else.

Here in the Wet Tropics, weeds will not only pester us on land but in water as well. Salvinia and Water Hyacinth are by far the two worst. Salvinia is a floating fern from South America that produces which completely choke stagnant water like pond and lakes. The leaves are egg shaped and water repellant. A weevil has been shown to be effective against these pests. Water Hyacinth is originally from the wetlands of South America and produce pretty purple flowers with a blue centre. The have large gas filled floats under their leaves and long trailing roots. Poisoning and manual removal seem to be the most efficient means of control.

Some of the more noticeable weeds in the Wet Tropics are trees. The two most commonly encountered ones are the African Tulip and the Camphor Laurel. African Tulip has many medium egg shaped leaves covered in light fur and has an incredibly soft timber. The flowers are large red ewers. The fruits are green boat shaped pods loaded with hairy seeds. These trees grow incredibly fast and sucker readily from the long roots, giving the impression of a whole forest of them and are particularly common in swampy areas, often completely dominating the vegetation. The Camphor Laurel is more common in upland areas and is quite fast growing. These trees are related to the local Pepperwoods and Cinnamon Laurels and like them, they have poisonous leaves and fruits. The leaves have a distinct pitchfork pattern on them. Since no local leaf eating animals have encountered these trees before, they have no antidotes against the plant’s toxins. This makes these trees, resembling the above mentioned locals, quite dangerous for the local wildlife.

Quite a few of our worst weeds started out as popular garden plants, which makes sense since fast growing plants that produce lots of flowers as often as possible are what most people want in their gardens. Three very prominent ones in the Cairns region are Lantana, Singapore Daisy and Sensitive Weed. Lantana is an amalgam of several varieties of the same species devised to flower continuously throughout the year. This means that biological control is nearly impossible. Lantana makes up massive impenetrable thickets of lanky bushes that easily take over any land with enough sunlight. They are thankfully easily shaded out in well developed rainforests. Manual removal has been shown to be the best control method. Singapore Daisy has beautiful yellow flowers and produces numerous thick runners with deep roots. They seem to prefer creek and river edges, often literally choking our waterways. They are very difficult to eradicate. Sensitive Weed is often known as Prickly Grass due to its spiny stalks. The leaves are mimosa like and close when touched. Sensitive Weed prefers nitrogen depleted soils and often out-competes grass on lawns in the dry season. Fertiliser is often the best way to control these nasties. Ironically they are grown in nurseries in Victoria!

It may come as a surprise that some Australian plants can be weeds. The Umbrella Tree and the Pink Evodia are trees native to the Wet Tropics and are quite impressive looking when flowering and fruiting. However, when transplanted to areas where they don’t occur naturally, they often become pesky weeds. Both trees are fast growing and used to moderately wet conditions. The Umbrella Tree often has multiple trunks and the leaves consist of whorls of very large oval leaflets that resemble umbrellas. The fruits are borne on large whorls of spikes and resemble large raspberries that are toxic. When introduced to Brisbane a while back as a street tree, it didn’t take long before they became a problem. Even in the Wet Tropics these trees grow quite fast. In Brisbane their normal sources of water during the wet season were no longer available and they sought water from water pipes, sewerage lines and local drains with their incredibly invasive roots, causing all sorts of problems. The Pink Evodia is a coastal species and was introduced to the Atherton Tableland so it could attract Ulysses Butterflies to the area. The result is that they started taking over, pushing out local trees wherever it was wet enough for them.

 

By Tore Linde