Cultivation and describtion of Roridula gorgonias and Roridula dentata

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RORIDULA

When I started growing carnivorous plants I also bought a small German book "fleischfressende Pflanzen" from Thomas Carow and Ruedi Fuerst. This book included a picture of an amazing and extremly sticky plant with assassin bugs running upon it. I fell in love... :-)

The genus Roridula includes two species which are both endemic to South Africa. Roridula gorgonis grows in the cooler and more humid coastal region near the city Hermanus and Roridula dentata is native to the hotter and more arid inland at the mountains of Clanwilliam and Tulbagh' Ceres. Both perenial plants can grow up to 120-150cm big bushes, Roridula dentata sometimes even bigger. The leafes are covered with numerous sticky hairs, which have different lenght. The glue is completly different from carnivorous plants like Drosera, Byblis or Drosophyllum and much more efficient. It cannot be washed away by rain and even dried leafes remain sticky. Roridula (especially Roridula dentata with its more branched leafes) can catch numerous insects and is occasionally even able to trap big prey like wasps or bees.
My plants, standing in an open greehouse during summer, are sometimes covered with thousends of dieing insects and look more black than green. Amazingly no digestive enzymes have been found in Roriula so there seemed to be no sense in killing so many insects. The plants sometimes even have problems if they are covered by so many insect bodies. Photosynthetic rate is significantly reduced and funghi can spread from dead insect corpses to the plant and kill it.
There are two species of "assassin bugs" living on Roridula: Pameridea marlothii on Roridula dentata and Pameridea roridulae on Roridula gorgonias. These bugs can run easily and amazingly fast through the sticky hairs. The bugs have very long legs and are using the upper side of the leafes where are less sticky hairs. Propably they have some kind of "coating" (tiny hairs which will break easily apart from the body when the bug gets accidently in contact with a glue drop). I have noticed that bugs sticked (by myself :-) ) to the tentacles have no problems to free themselfs.
The bugs are feeding on the captured prey and place their droppings on the leafes of Roridula. The plant is possible able to absorb these nutrients through its leafes and can this way benefit from captured insects digestet by the Pameridea bugs. The bugs have an easy source of food and can feed on exhausted prey which would usually be to big and dangerous for the bugs. During danger the bugs are hiding between the plants' leafes where they seem to be quite secure from predators. (which bird would like to land in a sticky plant ?). Juvenile bugs are often found in open flowers of Roridula where the propably feed on nectar. They also help with pollinating the flowers.

Roridula is quite tricky in cultivation. Most people will obtain seed and here the problems beginn to start. Both species are germianting in nature after bushfires have cleared the vegetation. So heat or chemicals (or only more light reaching the seeds?) is necessary for germination. The positive sideeffect is, that the seed of Roridula can be stored for many years.

Roridula gorgonias is relativly easy to germinate and sometimes seed is germinating whereever it had fallen. The best way is to sow it on the surface of a mixture of equal parts peat and sand. Place the pot in a very sunny place with hot daytime temperatures and cool nights (September or March are good months to try in Germany). It is beneficial for germination if you burn some dry grasses on the top of the pot after a few days, but usually it is not necessary. Germination can occure after two weeks, but some seeds can take up to three months.
Roridula denatata has the reputation of beeing very difficult to germinate. I obtained some seed from Silverhill seeds (South Africa) and made some experiments. The result is, that it is not necessary to use gibberlin acid. Seed didn't germinate if placed in a hot and dark room. Seed also didn't germinate when placed in my highland terrarium with warm days and relativly cool nights and medium bright light. But seed germianted (with and without a treatment with gibberelin acid) at a rate of about 60% when placed at a very sunny place at my greenhouse with hot days and cold nights. I treated all of the seeds by burning some dried grasses on the surface of the substrat. Propably it is not the heat and not the chemicals which stimulates germination in natural habitat, but a lots of light reaching the seed after the fire had burned away all other plants.
In spring 2002 I again sowed 6 seeds of Roridula dentata, put them at a sunny place, kept them wet and in August/September (hot days and colder nights) all of them germinated. I didn't use any treatment except burning some grasses on the pot's surface.
In my experience germination of Roridula dentata can take many months so don't throw away your pot to fast.
I have heared about other successfull experiments using liquid smoke, gibberelin acid or freezing the seed but I can get a germination rate of 50% with my method and it is very simple.

After sucessfull germination (the seedlings sometimes have problems to brake the seed shell, be patient !) Roridula gorgonias can stay for some months in the same pot before you have to repot them. Roridula dentata grows much faster and has even at seedling stage a very long root which is penetrating deep into the soil. So my advice is to transplant Roridula dentata seedlings if they are still small. I would highly recommend very deep pots for Roridula dentata.
I'm still searching for the ideal substrat, but a mixture of equal parts coarse sand and peat works quite well. Roridula dentata prefers an even more sandier mix. I also made some experiments with Seramis, vermiculite and perlite but couldn't notice significant differences in growth rate.

Roridula can be grown on the tray system during the hot summer days, if you let the tray dry out before the next watering and use very deep pots. During winter the soil should be kept only moist. As a rule of thumb, keep Roridula dentata as dry as Drosophyllum and water Roridula gorgonias a bit more.
Both species like a lots of sun, and should be placed in full sun (here in Germany !) throughout the year. Roridula dentata has no problems with temperatures around 40°C for short times, but Roridula gorgonias prefers it cooler.
Both species need a lots of air movement! They can die during one day in stagnant hot and humid air!
In greenhouses Roridula should be placed next to the windows or doors. I have seen very nice Roridula gorgonias grown by Klaus Keller outdoors during the German summer. Frequent rain didn't harm the plants.
If you don't have a heated and bright greenhouse (like me), winter can become a big problem. My first year with Roridula gorgonias on a south facing windowsill worked quite well, but in the second winter the plant was to big and I moved it to an east facing window. On that place my Roridula died within one week.
Now I'm growing both species again on the south facing window at 10-15°C and give them also strong additional artificial light from metal halide or high pressure sodium lights. This seems to be enough light for Roridula. Both species don't have a dormany but they are slowing down their rate of growth if days get shorter and temperature lower. In early spring the plants usually starts to flower. My oldest Roridula dentata are still not much older than two years so I have no experience with the flowers of this species. Roridula gorgonis usually flowers around February/March and the seed is ripe at June. Roridula gorgonias usually devides only after flowering while Roridula dentata is much more bushy and grows numerous side shots throughout the year.
One seed capsule of Roridula gorgonias includes 10-25 seeds while Roridula dentata capsules only have three seeds.
Another way to propagate Roridula is through stem cuttings. This works best with Roridula dentata, because this plants divides regularly and a medium sized plant can have up to 50 growing points while Roridula gorgonias usually only divides after flowering and a medium sized plant has only few growing points which can be used for cuttings. If you make a cutting, Roridula will usually divide (like Nepenthes). The cuttings should be taken about 2 cm under the haert of the growing tip where the stem is still green. Place the cutting with the heart at least 5mm above the surface in a bright but cool place and do not! cover it with a plastic bag or something similar. The soil should be only moist not wet. The cutting can stand around for weeks or even months still staying green. But the cutting has only rooted sucessfully if you can notice new growth. After that move the plant next to the other Roridula. Good luck !

During one year Roridula gorgonias can grow about 20-25cm. Roridula dentata can grow twice as fast.

The biggest problem in cultivation of Roridula is that sometimes the plants turn brown on the growing tips and rot from top to bottom within few days. It mostly occurs if plants are grown in stagnant air, high humidity or places not bright enough. To many captured insects can also cause rotting. You can try saving the plants and imidiatly cut away the rotting piece. The remaining piece of stem must be completly healthy otherwise the plant will start rotting again. Often the plants are lost anyway. I don't know what this disease is, but I'm loosing some plants each year, most of them during the winter months.

I got some Pameridea roridulae bugs from a friend during spring 2001. The bugs seems to die after lieing their eggs inside the plants. I have never seen any eggs on the plant. During summer when they have plenty of food lots of Pameridea can live on one Roridula but in autumn and winter, when less prey is captured the bugs become cannibals and their number decreases significantly. They seem to suck also on the plants juices if they can't find any insects but they need to be fed with insects or pieces of meat during winter months. Frozen wasps work quite well, one or two wasps per plant for every two weeks is enough, that some bugs will survive until spring. The bugs will find the dead insects amazingly fast and fighting against each other for the best place at the dinner table.