Carnivorous Plants
Carnivorous plants are a specialized group of plants that grow in wet, boggy, acidic soils. These bogs are typically comprised of peat soils which are low in the mineral salts and other nutrients vital for the plants survival. One of the most critical plant nutrients is nitrogen which is usually taken up by plants as nitrates. Nitrogen is a nutrient that is easily leached out of even ordinary soils. For this reason the plants that live in these soils have evolved into carnivorous plants that capture and digest insects as a means of obtaining nitrates.
While these plants can obtain nutrients from gases (carbon dioxide which breaks down into carbon and oxygen creating carbohydrates) and what little nutrients they can obtain from the soil, they are healthier and more vigorous when their diet is supplemented with the nutrients obtained from insects. Here at the University of Illinois five of the most common genera of carnivorous plants are grown with goals of adding more genera in the future.
The Venus Flytrap (Dionaea muscipula)
Obviously, the most well know carnivorous plant is the Venus Fly Trap. Carl Linnaeus named the VFT after Diana, the Greek goddess of love and beauty.
The Venus Fly Trap is probably one of the most fascinating of the carnivorous plants. It is native to the coastal plains of North Carolina and the northeast coast of South Carolina. The Venus fly trap captures its prey by modified leaves that have developed traps at the end of the leaf. The trap of the VFT is classified as an active trap. This means that there is actual movement of the plant in the capture of its prey.
There are numerous theories on how the trap actually moves. These theories are too long to add to this page, but are quite interesting. If you are interested in learning more here are a couple of web pages that explains some of the theories of the fly trap movement:
Wikipedia: Venus Flytrap and Live Science: "Venus Flytrap's Speed Secret Revealed" by By Robin Lloyd
Now for those who are not interested in the theory behind how the traps close but are interested in how they close, here is the simple version. There are four hairs on the inside of the leaf of the VFT called trigger hairs. Sweet smelling nectar is produced by glands on the inside of the trap attracting insects. When an unsuspecting insect walks across the leaf and touches the hairs in quick succession, the trap closes. The trap does not close tightly at first, but within seconds it will be tightly closed trapping the insect. It is believed the reason the trap does not close tight immediately, is so that small insects can get out of the trap and the plant does not expend energy and time on digesting a insufficient meal. The plant than releases digestive enzymes much like those we produce in our stomach and digests the soft body parts of the insect. Insects have an outer skeleton called an exoskeleton which is not digestible by the plant. After digestion is complete, which can take from five to ten days, the trap reopens and the exoskeleton is than washed away or blown away. A single trap will capture insects one to three times depending on the size of the insect. Then the trap will blacken and die.
Keep in mind that the purpose of the trap is to provide food and energy to the plant. Even though the leaf dies after capturing an insect, energy is provided to the plant to produce more leaves and traps.
American Pitcher Plant (Sarracenia sp.)
There are believed to be eight or possibly nine Sarracenia sp. All but one of them are native to the Coastal States of the Southeast, ranging from Alabama to the Carolinas. The one species that is not restricted to the south is Sarracenia purpurea ssp. purpurea. This pitcher plant can be found along the
eastern seaboard into the upper Midwest and into much of Canada. You can find this pitcher plant in the Volo Bog north of Chicago almost at the Illinois/Wisconsin border. The American Pitcher plants capture their prey by means of passive traps called pitfall traps. The traps are specialize leaves that have developed into tubes. Once again there is sweet nectar at the base of the cap that attracts the insects.
(The picture to the left is of Sarracenia flava var. rugelli and the picture to the right is Sarracenia purpurea ssp. purpurea)
This nectar intoxicates the insect. The insect then falls into the trap. The interior of the traps are either smooth or have hairs pointing downwards preventing the insects from climbing out of the trap. At the base of the trap the digestive enzymes are found. This liquid will rise higher in the tube as the tube fills with insects. With one particular trap, Sarracenia psittacina, the top of the tube balloons out and turns back down towards the tube leaving a small opening for the insects to enter. Any insect unfortunate enough to enter this particular plant is doomed to a torturous death. The balloon like chamber is pitted with chlorophyll free areas that act like windows letting light into the tube. The insect tries to exit the trap via these windows only to find itself trapped.
(The picture to the left is of Sarracenia psittacina and the picture to the right is pointing out the base of the cap on a Sarracenia flava var. rugelli)
Additionally, this particular plant has stiff needle-like hairs on the inside of the tube forcing the insect down the tube. If the insect should try to retreat, the insect will be pierced by these hairs. By the end of summer, the traps of the pitcher plants are full of insects. If you cut open the trap it is interesting to see the succession of insects caught from flies to moths to wasps and ladybug beetles.
(Picture is of a Sarracenia tube full of insects)
Tropical Pitcher Plants (Nepenthes sp.)
Nepenthes are the pitcher plants of the Old World Tropics with most of them native to the Asian rainforests. These pitcher plants differ from the American pitcher plants in that many of them are lianas, vines growing up into trees, rosettes, or epiphytes growing in the leafy debris caught in tree branches.
There are two classifications of Nepenthes based on the climate in which they grow. Lowland Nepenthes grow in areas below 3000 feet where the days are hot and nights are warm and the humidity is high. Highland Nepenthes grow higher in the mountains (above 3000 feet) where the days are warm and the nights are cool and the rainfall is high.
While there are many lowland Nepenthes, the majority of Nepenthes are highland Nepenthes.
This plant has a very unique structure. The pitcher is actually the leaf modified as a trap. The part that looks like a leaf is actually the leaf petiole. The pitcher holds liquid used to drown the insect. The plant actually produces the liquid in the pitchers. This is impressive considering there is one species that will hold up to three quarters of a gallon of liquid in it's pitcher. The pitchers of nepenthes vary in size from 4-5 inches to 12 inches (Nepenthes truncate) to 24 inches (Nepenthes rajah). It has been recorded that a rat was found in the pitcher of Nepenthes rajah.
The nepenthes are also unique in that they are similar to bromeliads where some animals, insects and spiders have learned to survive and live in the liquid of the nepenthes. There are mosquitoes and species of frogs that lay their eggs in the liquid of nepenthes and are not affected by the digestive enzymes.
Sundews (Drosera sp.)
Sundews are found world wide. They are tropical or temperate and grow on every continent of the world. Sundews are dainty, little plants glistening in the sun and much to the dismay of any unsuspecting insect another notorious carnivorous plant. The leaves of the sundew are lined with hair-like structures or tentacles and on the tips of the tentacles are dewy drops that are thick and sticky. At the base of the tentacle is the gland that produces digestive enzymes. The insect is attracted to the glistening dew drops believing it to be flower nectar. Unfortunately for the insect, the dew drops are sticky and the insect is easily captured, very similar to a spiders web.
As the insect struggles to free itself, it just becomes more enmeshed by the plant. The more the insect is stuck to the plant the better for the plant. The plant can only digest body parts of the insect that are actually touching the glands. However, sundews are a second group of plants that use active traps to capture their prey.
Once an insect is caught, the tentacles of the sundew will wrap around the insect holding it tighter to prevent escape. There are a few sundews that as the insect struggles more, the leaves will wrap around the insect holding it even tighter.
Bladderworts (Utricularia sp.)
Bladderworts are the largest group of carnivorous plants encompassing over 200 species around the world. They are terrestrial growing in waterlogged, wet soils or aquatic growing in ponds. Several species are epiphytes growing in mossy trees. The plants are admired mostly for their flowers as that is the part of the plant that is visible. The true nature of the plant lies beneath the surface of the substrate in which they grow. An anomaly among plants, bladderworts are rootless. The majority of the plant grows beneath the soil surface or submerged in water.
The plant is mostly stem material. The plants capture small microorganisms and insects with bladder-like traps, hence the common name bladderwort. The aquatic bladderworts trap small insects, such as water fleas, Daphnia, and mosquito larvae, or in the case of the terrestrial bladderworts, protoza or paramecia. The traps are one of the most complex structures of the plant world. The traps are specialized leaves attached to the stems. They can range in size from as small as a pinhead to as large as one-eight of an inch wide. The trap has a door which only opens inwardly. Outside the door are several trigger hairs. When the insect touches one of these hairs the door is opened creating a vacuum sucking in both water and the insect. These traps may appear as active traps like the Venus Flytrap, but they are actually passive traps called suction traps. The trigger hairs do not induce movement by the plant but act as a lever opening the door of the trap. Digestion occurs within 15 minutes at which time the trap is ready for another unsuspecting insect.
Growing Carnivorous Plants in Illinois (Carnivorous Plants Group.)
Growing carnivorous plants in Illinois is a challenge even
for the best of gardeners. It takes patience to learn which plants will survive
our climate and that is only accomplished through trial and error. The
University of Illinois garden is in Champaign, Illinois, or Central Illinois.
Any gardener south of Central Illinois should have luck with a little care and
learning how to set up a proper bed for carnivorous plants. Anything that grows
in our garden will grow south of here. Unfortunately, I cannot say what
carnivorous plants will grow north of Central Illinois except for the purple
pitcher plant. If you are in town during the summer months come and see our
gardens.
The Garden Bed
When establishing a carnivorous plant garden, a special bed
needs to be created and prepared. These plants grow in native bogs throughout
the United States. The plants cannot be planted directly into our soils. There
are two ways to create a bed for carnivorous plants. One method is to buy a tub
used for aquatic ponds. Make sure the tub is at least 18 inches deep. The
second method is to dig your own "pond" making sure it is at least 18 inches
deep and then lining the hole with the liners used to make water gardens. If
using a plastic tub for aquatic pools, you will have to drill holes about 5
inches from the top for drainage. The carnivorous plants should not be sitting
in water.
Soil
Do not use any local soil. Use only milled or baled
sphagnum peat as your soil medium. This can be bought at most garden centers.
Make sure the bag says sphagnum peat. If it does not, do not use it. Some
additives that can be used to mix into the peat are perlite, washed play sand or
lava rock also called kaolite or expanded clay. While some people like
including these additives for aeration, they are not necessary. If you should
use sand, make sure you buy play sand not sand used in construction. Also, wash
the sand until the water runs clear. Sand has salts or minerals in it that are
detrimental to carnivorous plants.
Water
Never use tap water of any kind. Well water is usually
safe as long as it does not go through a water softener. The best water to use
would be rain water or if you need to buy water use only distilled water. Do
not use bottles labeled as spring water or water like Hinckly Schmidt. These
types of water have salts and minerals added back to the water to make it taste
better.
Humidity
Most of the carnivorous plants that you will try growing
come from the coastal states of the United States from Alabama up through North
Carolina. These states have humidity levels of close to 100% year around. Most
of Illinois has enough humidity during the summer months that lack of humidity
should not be a problem.
Light
Carnivorous plants grow best in direct sunlight for most of
the day. While some light shade is beneficial during the hottest part of the
afternoon, it is not necessary.
Dormancy
Again, most of the carnivorous plants that you will be
growing will require a dormancy period. Here at the University of Illinois, the
gardens are left uncovered until November, sometimes December. The plants are
than covered with straw and than the straw is covered with some kind of porous
material like a thin layer of landscape cloth, burlap, some kind of material
that will hold up during the winter and allow rain water to soak through to the
plants. The material is held in place with landscape cloth stakes. This
protection is kept in place at least until mid March. Timing for lying the
protection down in the Fall and removing it in the Spring will be determined by
where you live. The dates provided are what are used here in Central Illinois.
Pests
The majority of the pests you will encounter will be
animals. Squirrels and raccoons like to dig in the soft soil burying their
treasures or looking for snails to eat. Rabbits may try nibbling on the
trumpets when they first appear in Spring, but usually leave them alone after
they find they are not very appetizing. Lastly, watch for snails and slugs.
They can become a problem in the bog gardens.
Plants grown outside at the University of Illinois
garden:
Drosera binata – the only sundew that seems to really
survive our winters.
Drosera filiformis ssp. filiformis -- may survive in milder
climates
Sarracenia flava
Sarracenia flava ssp. Rugelli
Sarracenia flave 'red tube'
Sarracenia oreophila x leucophylla
Sarracenia 'Dixie Lace'
Sarracenia purpurea ssp. purpurea
Sarracenia psittacina
Sarracenia x
catesbaei X alata
References
These two books should be in every gardener's library if
they are interested in growing carnivorous plants:
The Savage Garden by Peter D'Amato
Carnivorous Plants of the United States and Canada by
Donald Schnell