The leaves are heart shaped with teeth on the edges, a quilted appearance and a covering of fine hairs. While they can grow to seven metres it is unusual for them to exceed three. All parts of the plant carry the stinging hairs.
The leaves of the Gympie-gympie (Stinging Tree, Dendrocnide moroides) are covered in silica hairs which are like straws of glass. By brushing against the leaves the hairs penetrate the skin. Each is capable of inflicting a nasty sting like a wasp to those unlucky or silly enough to rub shoulders or any other part of their anatomy with this gem of the rainforest. The tubular hairs contain a nasty chemical which excites the nerves and causes pain for two days. Although the pain is intense the agent is not causing any damage to your body; it just feels like it is killing you! The effect is to stimulate the nerves causing a painful sensation. After this there are secondary effects which have lasted for me up to 4.5 months and can vary from person to person. None however are pleasant. They include burning sensations and electric shocks.
I discovered from one of my learned guests that this secondary reaction is a syndrome called, 'reflex sympathetic dystrophy'. Presumably this is caused by nerve damage cause by the chemical in the hairs. It seems that the pain is not reflex nor sympathetic and is not related to dystrophy. And I thought wildlife taxonomists were crazy! If you want to read more about the definitions you might like to go to http://jnnp.bmjjournals.com/cgi/content/full/71/3/291
It is not necessary to actually touch the bush to suffer its impact. Just working near the plants for some time or slashing them with a machine can be enough for the hairs to irritate the membranes of the nose and eyes. After effects vary from person to person and on the area stung but I have had them last for four months. When the area became cold I would get a little electric shock. Others suffer a burning sensation when the area is rubbed.
These plants do not lie in wait to jump out at unsuspecting bush walkers but they do often grow along tracks. For stinging trees to grow, they need good light but protection from wind. Beside roads, tracks and where trees have fallen are the best places to find and avoid stinging trees.
Prevention is always the best medicine. Find out what they look like, stay on the track and don’t touch. If one is stung, the removal of the hairs by hair removal wax or some other method will reduce the pain. Distraction by focusing on something else is often the best pain control. A severe sting will cause the release of lymph and the swelling of the gland in groin or armpit. In the case of unbearable pain or the injury of infants seek medical supervision. Do not try the bush remedies you may have heard of as the best is of negligible use and the worst, dangerous.
Below is the Mulberry -leaved or Shiny-leaved Stinging Tree, Dendrocnide photinophylla. This tree can grow as an emergent of 45 metres but by the time it is more than 5 metres tall it rarely produces stinging cells. Have a look at the hooks rather than hairs of this species. They only sting like a wasp for five to twenty minutes. The good news is there are no secondary effects. You may have noticed the holes in the leaves. These are caused by beetles. The White Nymph butterfly, Mynes geoffroyi guerini, lays its eggs in clusters on both the above species. The brownish coloured larvae are gregarious. Green Ringtail Possums, Pseudochirops archeri, also eats both of these plants. They are particularly fond of the Shiny-leaved Stinging Tree which they will eat through most of the year except the early dry season when the trees lose most of their leaves. After a prolonged dry spell the Gympie-gympie is one of the first plants to respond to a good rainfall. At this time it is possible to disturb a Green Ringtail feeding low during the day on these shrubs.