Heard of a Venus fly trap (Dionaea muscipula)? Of course you have. But do you know how carnivorous plants actually work? Are they a threat to small animals, perhaps, and can they bite your finger? Why do they need to catch prey rather than live off the soil? Charles Darwin apparently described the Venus fly traps as some of the “most wonderful plants in the world”.
There are more than 600 carnivorous plant species spread around the world, most of which grow in places that have poor soil. Even the UK has several native species. Most grow in bogs but some grow on tree branches and even on rocks.
Let’s look at these insect-eating plants to see how they work:
The well-known Venus fly trap is different from most other carnivorous plants in that it has a ‘snap trap’. This is why we seem to love it so much – it captures the imagination as well as flies! When a bug lands on one of the open traps it stimulates the tiny ‘trigger hairs’ on the surface. This causes the two parts of the leaf to quickly close together in order to seal the prey inside. The plant slowly digests the nutrients it extracts from the prey.
Just like a normal perennial plant, it also takes food and water from the boggy soil in and the air in which it grows. As the soil is naturally poor, the insects provide a ‘top-up’ of nutrients.
Here in the UK, when we are growing these beasts in potting compost, there’s less need for flies and the plants can survive for long periods with an empty larder.
Once the trap has closed on a small fly, the chamber is sealed and is airtight. This keeps the digestive fluids in and the bacteria out. There is an antiseptic element to the juice which prevents the soup from decaying. However, if the trap snaps shut on a large fly, there will inevitably be some of the prey sticking out of the trap. This allows mould and bacteria to thrive and the trap generally dies, turning black before falling off.
Inside the trap, the clever plant secretes digestive juices which dissolve the inner parts of the insect, on which it feeds. The process can take from five to 12 days, after which the trap will reopen. The exoskeleton of the dead insect is left and this will blow away in the wind. It’s no wonder that children love this plant! A word of warning, however, don’t attempt to open a trap once it has shut. This is likely to damage the plant. Secondly, if the fly isn’t moving, the trap probably won’t close. So if you are trying to feed the plant manually, maybe using a dead bug, a Lego man or a human finger, you will need to try to mimic the movement of a fly. These plants are clever and they aren't easily fooled!
Next, we have the pitcher plants. These include Nepenses and various forms of Sarracenia. They have tube-like foliage containing digestive enzyme juices. The foliage and secretions smell flowery and irresistible to insects, which land on the lip of the pitcher and then fall into the tube where they drown. They are unable to climb back out due to downward-facing hairs. The plant enjoys a feast of insects thanks to a clever design of nature.
There are other carnivorous plants which have sticky hairs. They include sundew plants (Drosera capensis), found on most continents. They ooze a thick mucus which smells heavenly to a bug. The elongated leaves act like fly paper: the insect lands; becomes ensnared in gloopy fluid and is unable to leave. The nutrients within its body are absorbed by the plant.
Other carnivorous plants have suction traps, such as bladderwort and butterwort including Utricularia. They have trigger hairs near an opening leading to a flattened tube. The semi-aquatic plants await the passing of water fleas and when detected, they suck in nearby water, pulling the bugs with them.
There are mini lobster pot trap plants called Genlisea. These are aquatic plants which have modified leaves that trap microscopic protozoa. The tiny hairs feed the protozoa down a root system which leads to a swollen part which acts as the stomach.
These interesting plants aren’t as difficult to look after as you might imagine. They need to be kept damp, or even wet, throughout the summer. Most people stand them on gravel in a tray of rainwater as the additives within tap water don’t suit them. They like a bright, sunny position. A south facing windowsill is fine. An unheated greenhouse will also be sufficient. They need nothing in the way of fertiliser as they catch their own! If kept inside a house it’s a good idea to place them outside from time to time so they have access to more passing flies. A passing wasp is being enticed down into the Sarracenia tube in the picture below.
If a Venus fly trap produces flowers it’s best to cut these off as they might weaken the plant. You will also notice that the foliage turns brown and starts to die down during the winter. This is perfectly normal. Just keep the plants damp through the winter and cut off any dead foliage. They appreciate a rest period during winter, so any plants kept in the house should be put outside or in an unheated greenhouse.
Check the labels when purchasing carnivorous plants. Not all of them are hardy here in the UK and some will require winter protection. They include Nepenthes 'Bloody Mary' which is also known as the monkey jug plant. It's a spectacular beast with dangling pitchers - an ideal subject as a hanging plant.
Enjoy your carnivorous plants - they are interesting beasts!