Carnivorous plants

For some reason many aquarists also have an interest in carnivorous plants (and often other unusual plants and/or animals). Hopefully this article will give an insight into how these plants can be cultivated and enjoyed at home.

Why?

Carnivory in plants usually means that the plant traps and digests insects and other small invertebrates to obtain nitrogen due to there being a lack of this nutrient in the plants natural environment. Nitrogen deficient soils tend to either occur up mountains, where rain has washed away much of the soils goodness over many years or else in bogs and swamps where nutrients are unavailable due to the acid conditions which can cause a lack of decomposition and nutrient recycling. Often carnivorous plants occur in association with peaty soils or sphagnum moss. This should give you an idea the kinds of places to look for New Zealand’s several native species. You can start looking pretty much anywhere any outdoor scenes for Lord of the Rings were shot – around National Park and Mt. Ruapehu are good areas for Drosera and Utricularia, as are the far north such as Lake Ohia and around Ahipara.

The plants

Several families of plants have developed a carnivorous lifestyle and have used different methods to trap their prey. Perhaps the most common technique is to have some sort of sticky substance on the leaves which traps the insect. Insects may be attracted to a sweet smell released by the leaf or by the glue glistening in the sun, looking like dew. A large group of plants that practice this technique are in fact called ‘Sundews’ for this reason. These members of the Drosera genus are widespread around the world with many species occurring in both the Northern and Southern Hemispheres, with Australia being a particularly rich source. There are a couple of species native to NZ though and in fact there is one that is not found anywhere else in the world, Drosera stenopetala, which typically occurs in cool wet bogs either quite high in the hills in the north island or else lower down further south. The other common ‘sticky’ plant are members of the genus Pinguicula, none of these naturally occur in NZ or Australia although there are many to be found in Central America (especially Mexico), North America (especially in the South Eastern states) and Europe.
The other common way to trap insects is for the plant to have some sort of reservoir of digestive liquid in a tube that the insect falls into, drowns and is digested. Typically these ‘pitfall’ or ‘pitcher’ type traps are divided into two main groups, the tropical and temperate species. Tropical pitcher plants of the genus Nepenthes are distributed though South East Asia mostly, with occasional species being found in Australia, Madagascar, India and so on. By far the most are to be found in Indonesia and Malaysia. Nepenthes are typically vine-like plants that either scramble along the ground (or down cliffs) or climb trees. They come in a wide range of shapes and colour patterns as well as sizes, with the largest holding up to a couple of litres of liquid (and being reported to trap small rodents/lizards etc) down to tiny thimble sized pitchers which can only trap the smallest of insects. The temperate pitcher plants belong to three genera with the largest and most common being the North American Sarracenia, the others are the South American Heliamphora and the north western US Darlingtonia. The North American species generally occur in boggy lowland areas while the Heliamphora of South America only occur on peculiar flat top mountains called Tepuis.
Using a typical Sarracenia as an example, the plant consists of several tall thin tubes arranged in a rosette or along a rhizome. The tubes generally have lids held above the tops to stop rain from washing any prey away. Insects are attracted to a sweet smelling nectar that occurs around the mouth of the tube. Waxy surfaces and downward pointing hairs ensure that any insect that falls into the tube is unable to climb out and is eventually digested.
Active traps, which exhibit some sort of motion are less common and with the exception of the commonly seen Venus Fly Trap (Dionea) generally occur in very damp or aquatic habitats. The bladderworts, Utricularia species, have tiny depressurised traps that when triggered suck in small aquatic invertebrates such as cyclops, daphnia or mosquito larvae. These plants often have attractive flowers and this is why they are sometimes cultivated. New Zealand has a couple of native species that usually occur in peaty bogs or acid pools.
The ‘poster child’ for carnivorous plants, and the one most people think of when such things are mentioned is of course the above-mentioned Venus Fly Trap. Most people will have seen pictures of this North American species so I won’t describe it further here.
There are a few other miscellaneous, although equally interesting, plants that exhibit variations on the above techniques for their own specific uses and situations.

Cultivation

Most carnivorous plants are relatively easy to grow providing certain requirements are met. In Palmerston North I grow many Sarracenia, some Drosera, Pinguicula, Utricularia and the Venus Fly Trap outdoors all year round. Most are actively growing in the spring and summer months and tend towards dormancy in the winter. Most of these plants, along with other more tender species can also be grown on windowsills in a typical house. Glasshouses or conservatories also provide other options if they’re available and will increase the number and variety of plants able to be grown. A very few species (mostly lowland tropical species such as some Nepenthes) require the constant hot and humid conditions of their jungle homeland and only really do well in heated terraria (fishtanks have other uses) but these species are not commonly available in NZ and so should not put people off growing the others.
If there are any simple tricks to growing carnivorous plants then they can be summarised as follows. Always use rain water only. These plants are adapted to low nutrient environments with very few dissolved salts. Too much of either will cause the plants to fail in the long term. Most plants (with the exception of Nepenthes mainly) like to have their pot sitting in an inch of water at all times, this mimics their boggy natural habitat. Keeping with the ‘low nutrient’ theme the best potting mix used for the majority of species is a 50:50 mixture of peat and washed pumice sand (or other inert coarse material). Some live sphagnum can be grown on the top of the pots for larger plants such as Sarracenia and you can be sure that if the sphagnum is doing well then the conditions are also good for the plants. As far as light goes, most of the commonly available species appreciate good bright light but perhaps not full sun all day, they’re not desert plants. Some can be grown under fluorescent lights but this often produces green leggy plants unless proper care is taken. Never fertilise your plants, they’ll catch more than enough insects themselves and don’t really need very much to keep them going if all other conditions are met. Excessive nutrients will often cause the death of these plants.

Propagation

Like all plants the carnivores typically reproduce - after flowering - by seed and this can be sown in similar media to that which the adults grow in. Several types of Drosera etc are very prolific seed produces and spare plants are easily generated over time. Many species can also be propagated by cuttings or division and this is a quick way to obtain mature plants. Cuttings, once they’ve got roots, can be treated much like older plants.

Further information

As with many hobbies, there are specialist organisations dedicated to enthusiasts of these plants. The New Zealand Carnivorous Plant Society produces a quarterly colour journal and holds monthly meetings in Auckland although meeting are also held in other centres at various times and locations. There are several good books available which illustrate a wide range of species and their cultivation requirements, including one written by Bruce Salmon exclusively about the NZ native species. The internet also has plenty of informative sites where more information, pictures, advice etc can be found. A good place to start is the FAQ (Frequently Asked Questions) page at www.sarracenia.com/faq.html.

Andrew Broome, January 2004
NZKA & Palmerston Nth Aquarium Soc.