I used not to think so, but I'm starting to think we do have a problem.
Most people are familiar with the edible prickly pear, which has large paddle-shaped "leaves" (they are actually flattened stems) with small prickles. Many don't realise that there are lots of prickly pear species. Some species have enormous (5cm long) needle sharp prickles, and some have lots of very fine prickles (glochids) that break off if the plant is disturbed and blow around, and if you get them on your skin they can cause nasty sores. If you breathe them in... well... Just dont. Serious injuries can occur through contact with prickly pear.
In Australia twenty-seven species of prickly pear and similar cacti (the "Opuntiod" weeds) are recognised as Weeds of National Significance, and are some of Australia's worst weeds.
I have started mapping prickly pear species on Merri Creek. This has been helped enormously by going to the Opuntioid Weeds Workshop held on 14 May, and learning to recognise the species. Along with other participants in the workshop I was given two fantastic resources - a field identification guide for opuntioid weeds, and a management guide for Opuntioid weeds.
I have to say I have been shocked by the number of species of prickly pear present along Merri Creek, and I have only surveyed from the Yarra River as far upstream as North Coburg. I know there is worse in Fawkner and Reservoir. So far I have found 6 species in 30 occurrences.
Some prickly pear species have effective biological control agents, but some present along the Merri don't. This means control for some species will need to be by careful manual removal or repeated herbicide application. Worse, I found out that Prickly Pears don't mind fertile soil along waterways, and can be very effectively spread along waterways by floodwaters. Both seed and plant fragments can be distributed downstream to create new infestations. So after a big flood we might suddenly find prickly pears coming up everywhere along the Creek.
Also many prickly pear species are spread by birds eating the fruit, and pooing out the seed elsewhere. Unfortunately it is legal for many of the species to be grown, so birds eating the fruit in a back yard can poo the seed out along the Creek. And of course what do you do with the prunings? Chuck them over the back fence! Fortunately, in the Port Phillip and Westernport CMA area which includes Merri Creek, it is not legal to sell Opuntias, except for the cultivated one Opuntia ficus-indica (Indian Fig). Unfortunately the most common Opuntia, with the biggest infestations along the Creek that I have seen so far, is this species. Most of these infestations appear to have originated from gardens nearby. It is regarded as an environmental weed in Victoria. I haven't seen any signs of any biological control agents on this species, although there is a Cochineal insect (Dactylopius opuntiae) which at least in Queensland can infect it and at least reduce its growth. Also the famous Cactoblastis Moth can set the growth of Indian Fig back, but apparently not eliminate it.
Opuntia monacantha (Drooping Prickly Pear) is a nasty thorny number. It is the second most common species along Merri Creek and is a real threat. It is a regionally controlled weed in the Port Phillip Catchment and landowners must take all reasonable steps to prevent the growth and spread of this species. Along Merri Creek there is a Cochineal insect variety which does a lot of damage to this species. Apparently the Cochineal Insects have varieties which are very adapted to specific species of Opuntia, so what thrives on Opuntia monacantha won't affect any of the other Opuntia species along the Creek. This species is showing some signs of spreading in floodwaters, and is quite common in Coburg North in the riparian shrubby weed zone.
Opuntia stricta (Erect Prickly Pear) is also regionally controlled weed in the Port Phillip Catchment, meaning landowners must take all reasonable steps to prevent the growth and spread of this species. I have only found this species so far in 2 places along Merri Creek. Apparently this species is very susceptible to Cactoblastis, but with such small numbers of plants, manual removal or herbicide control would be most appropriate.
Opuntia schickendantzii (Chicken Dance Cactus) [Someone has a sense of humour] is present in one infestation along Merri Creek in Clifton Hill. It is easily recogniseable by its narrow strappy pads, covered in short spreading spines. I am sure it got there in dumped garden rubbish, but now needs some serious herbicide treatment. It is a restricted weed in the whole of Victoria, meaning it is illegal to sell the species.
There is at least another couple of species present in small numbers along the lower Merri Creek, possibly Opuntia puberula and O. elata. These are both restricted weeds too.
Opuntia robusta (Wheel cactus) is regionally prohibited in the Port Phillip Catchment which I haven't found yet and hopefully won't. It is a dreadful weed in drier parts of the state.
Opuntia aurantiaca (Tiger pear) appears to be regionally controlled in the Port Phillip Catchment, but is another species that I haven't and hopefully won't find on Merri Creek. It breaks into bits easily and hitches a ride on passing animals, shoes, people, or even car tyres! Awful. There is a picture in the management guide of it growing through a dead wallaby, and quite likely it killed the wallaby in the first place.
You can find my map on Google mymaps. I will continue to update the map as I continue my survey work.
Of the infestations I have mapped so far I am heartened that the City of Yarra is onto the problem. There are signs that they have recently started spraying Opuntia species on the Merri Creek escarpment north of Knott and Coulson Reserves. My plan is to be in touch with the various Councils to encourage them to act on their infestations. A future article will deal with this.