Feeding and fertilizing carnivorous plants
When carnivorous plants are discussed it seems that most people agree that they are a strange breed of plants grown by a strange breed of people. Difficult, weird, freakish are only a few of the epithets applied to the the plants that eat animals.
The dogma is that finding themselves in an environment very poor in nutrients they have (in addition to the photosynthesis) evolved adaptations which enable them to capture and digest animal prey through which they get all the nutrients they need. For this reason they should never be fertilized because this will kill them.
However while it is much more easier and safe to give the novice the instruction "Do not fertilize!" this also causes much grief to our beloved meat-eating plants. Think about it - there is no place on earth where only carnivorous plants live. Always alongside with them normal plants grow, in the same soil and under the same sun. They do not starve, they do not suffer. Carnivory is not the only way to survive there. Then why our carnivorous favourites have developed such special adaptations? In order to eat more than normal plants and to utilize an alternative source of food that provides more than the sun and the soil could. Even more they wouldn't evolve such complex traps for the occasional bug now and then. The possible access to a constant and virtually endless supply of huge amount of prey was driving the evolution of these structures in the first place. And this abundance of prey was the normal state during most of the time of their evolution. If it was for the scarce amount of bugs they catch in today's collections these plants wouldn't become carnivorous at all! From this viewpoint it turns out that their nutritional needs are not lower than those of their ordinary neighbours and that in cultivation they should be fed no less than the "normal" plants! It is also important to understand that in a more natural setting the carnivorous plants catch much more prey than they have available in the city. Their diet consists not only of insects, but a broad spectrum of organic matter, starting with the invisible pollen that sticks to the leaves of the sundews and end with rodents and their poo and even small birds that have been found in the pitchers of some Nepenthes.
So in order to be healthy and happy we must feed and fertilize them, but how to do it when it is empirically proven that putting them in normal soil and fertilizing them like a regular house plant will instantly kill them?
Same as any other plant we must strive to provide them with conditions as similar as possible to those in their natural habitat. When growing carnivorous plant the starting point is usually acidic, nutrient poor substrate and clean, limestone-free water with minimum dissolved minerals. These are the basics but on their own they do not provide conditions for growth. On top of them we start to add. First we add light - it is the main energy source and providing an adequate quantity and quality of light solves 80% of the issues of the cultivation. Second - air - it is self explanatory, but the carbon from the air is the main building material for the plants. Third - minerals in the water and substrate.
Substrate and water
The substrate really shouldn't be ordinary garden soil. The substrate most carnivorous plans are accustomed to is one that holds low level of nutrients because they are constantly leached and slower replenished by the soil micro flora and fauna. In cultivation however the opposite happens - in time salts accumulate in considerable quantities. For this reason not only should they be planted in poor substrate, but also the water that is used for watering should be super clean and low in dissolved salts.
In cultivation carnivorous plants generally do best with rain water. Distilled, reverse osmosis or deionized water can also be used. For a collection of more than just a couple of small plants it is not environmentally conscious or generally reasonable to use distilled water (distillation is a very resource-intensive process). Same goes for buying any bottled water for similar reasons (transportation, packaging, etc). If rain water is unavailable or with poor quality the next best option is a reverse osmosis system, provided that you will utilize the waste water too.
Of course the first thing to check is the bulletin of your local tap water supplier and look for the parameters as general mineral content, hardiness, electrical conductivity and specific pollutants (like fluoride, etc). You may discover that your tap water will work well too.
A TDS meter does not tell you anything about the quality of the water. It measures just the electrical conductivity of the water which is proportional to the total quantity of dissolved solids. Some units show µS/cm, others convert this value to PPM. Normal drinking water with conductivity of 50 µS/cm is good, distilled water with added table salt that also reads 50 µS/cm is not good, you get the idea. Use the TDS meter it only to get a general idea about the total mineral content after you know for certain that there are no specific pollutants in the water you are testing.
So rain water is usually the best option unless you live in a heavily polluted industrial area. You also have to pay attention to collect it in a way that doesn't add pollutants to it (water flowing from the roofs in the first minutes of the rainfall for example carries a lot of dirt in it). If you can't collect rain water a reverse osmosis system is better than distilled or store-bought water, especially if you find a good way to utilize also the waste water that it generates, otherwise you will waste four times more water. Tap water can be good enough in some regions and totally unfit for carnivorous plants in others. The water supplier companies normally publish bulletins with the local tap water parameters - find it and read it. For most carnivores good is water that doesn't contain any specific pollutants and has electro conductivity of 50 micro Siemens per centimetre (µS/cm) or lower. Some species, most notably Sarracenia can be ok with up to 150 µS/cm. Using any store bought water for me is wrong for many reasons, unless maybe if you have just one or two plants and even then the amount of natural resources used up to produce and distribute a bottle of water is unacceptable for me. The substrate should also be acidic in most cases, with pH between 4.5 and 5.5. Some species however prefer neutral to alkaline substrate. The main ingredient is oftens pure acidic peat. A good environmentally friendly alternative is milled pine bark. The rest of the ingredients are live or dried sphagnum moss, washed river sand and perlite. Many minerals and some organic and inorganic compounds which the plants can't create must come from the environment. In nature they come from three places - the substrate, the water and the prey. The substrate in nature may be nutrient poor but it is never so poor as it may become in a small pot watered with distilled water! Distilled water is a strong solvent and dissolves and carries away most minerals from the substrate (that's why distilled water is very good for emergency flushing in case of overfertilization). Thanks to the activity of the whole ecosystem in nature there is a low but constant influx of nutrients in the substrate. There is one more important factor - in most habitats there is a constant influx of oxygen in the substrate with the constantly seeping through the substrate fresh water. In contrast in cultivation the water is often stagnant and creates anaerobic conditions which are totally unfit for carnivorous plants! The flowing through the substrate water in nature carries some nutrients and at the same time washes them away - this is what we should aim to accomplish in cultivation too. Instead in cultivation the minerals that the water carries are left behind after its evaporation and in time start to poison the plants. That's why oftens it is better to top water generously and the water leaching through the pot and the one in the tray to be discarded and refreshed oftens and not left to evaporate and refilled. I think that this is more important than to water with water with zero dissolved minerals. Especially important is to flush the substrate like this a couple of days after fertilizing in order to prevent mineral build-up. When I flush the substrate I usually use nearly the same volume of clean water as the volume of the pot by slowly pouring on top of the substrate and letting it freely drain through the drainage openings of the pot and discarding it entirely (actually I use it for other, non carnivorous plants).
The best food is of course the natural one - insects: flies, ants, grasshoppers, also spiders and every similar prey suitable in size for the corresponding trap. Avoid prey that the plant could never catch in nature - like ham for example ;) Also beware that some may manage to escape or even damage the trap - like wasps cutting through Sarracenia pitchers. Or some are too fat (like some insect larvae and caterpillars) and slow do digest and absorb and may cause the trap to rot. It is not fatal, just don't be surprised. As for the amount of prey - what the plants catch in nature is incomparably more in quantity and more diverse compared to what they catch in the city. It is normal that a trap doesn't stay empty for more than a few days. And because this doesn't happen on the windowsill we should supplement the diets of our pet plants.
Fertilizer (artificial food)
There are cases when there is no way for you to feed the plant with natural food, imagine for example a nepenthes with not a single pitcher. Or you simply find it unacceptable to hunt for grasshoppers behind the apartment building or to keep a bucket with rotting fruits on the balcony in order to multiply fruit flies.
Then at help come the synthetic or organic fertilizers. And here is the tricky moment because important is not only the composition (it should not contain urea), but also the concentration and way of application. With the fertilizers the risk of poisoning or overfeeding the plant is real, so if you experiment be cautious. Some options are discussed below.
As a start the rule that about 80% of the feeding up should be foliar and no more than 20% through the roots is helpful because with the root feeding the risk of overdosing is much higher.
Toxic mineral levels can be reached in two ways: acute overfertilization when using too high concentration and slow, chronic - when salts build up in the substrate little by little over a long period without being leached out - this may happen even without applying fertilizer, just because of incorrect watering.
How much food should they get?
Natural food - as much the plant can take. There is no danger of overfeeding when using natural food! Every trap has a set capacity. The sundew has a limited number of sticky tentacles, the Sarracenia has a set volume of the pitcher, the Dionaea trap can't hold a prey too large and strong. Stick the whole leaves of the sundew with flies, fill to the top the pitchers of the Sarracenia with grasshoppers and they will thank you with larger and more colourful leaves.
When you start to fill up the pitchers of the Nepenthes with fertilizer you will notice how quickly the plants absorb the solution and the level drops. The plants secrete their own fluids in the pitchers, but in nature the rain and condensation may refill them too. Nepenthes are able to selectively absorb water and nutrients from the fluid in their pitchers while retaining the balance and concentration of digestive enzymes and bacterial colonies (same goes for Cephalotus). This property can also be used to alleviate water stress during acclimatization. So the principle with Nepenthes pitchers is to keep them at all times full to their natural water line marked by a change in the surface appearance inside the pitcher or approximately up to the half. It is different with the Sarracenia. With the exception of Sarracenia purpurea the rest don't keep a visible high level of fluids in their pitchers and the lid keeps most of the rain out. Even so they react good when you fill them with fertilizer and absorb it quickly. With them the principle is to fill as much liquid so the pitcher can stand upright.
On the upper photo you see a well fed Drosera binata. For this feast I don't have any service, it has captured them alone, but when the season isn't that good I have to feed it. And this is the contents of a Sarracenia pitcher, at the end of the summer they are full to the brim.
When and how often should the plants be fed and fertilized?
Always when the plant is capable of absorbing the food. This is easily recognised with the sundews and Drosophyllums - when their leaves are covered with dew they are ready to eat. For all species the rule is to fertilize only plants that are not dormant and are not sick. You should strive to provide a constant supply. When you fertilize in most cases a weekly foliar application during the growing time is adequate, but the frequency should be adapted to the needs of the plant. More frequent application of a weaker solution (lower dose) is better than rare application of higher dose.
Do not underestimate the fertilizing at the end of the summer and during the autumn before the dormancy because this is the time when the plants which go dormant should be able to accumulate enough food to live through the winter (like bears, you know). Early spring is also a time for generous feeding, which should sustain the intensive growth. During the summer if the plants have access to natural food the fertilization can be done less frequently.
Other factors for optimal nutrition
Alongside with the physical availability of nutrients there is one more critically important factor that is often misunderstood and overlooked. This is the presence of oxygen in the root zone. Oxygen is critically important and without it the transport of nutrients from substrate in the roots is not possible. In nature many carnivorous plants live in high water levels, sometimes a part, most or even the whole root system is underwater, but thanks to many factors there is always some influx of fresh water and with it of oxygen. In cultivation on the contrary the plants are most often grown in stagnant water, which soon leads to anaerobic conditions in the substrate and with this to poor nutrition and health. Aside from nutrient uptake the oxygen is necessary for the process of breathing and the synthesis of many compounds.
Adequate light is another absolutely critical factor. Sunlight is best, but if you must use artificial light use full spectrum, UV included.
There are many options for fertilization, but a commercially available fertilizer for carnivorous plants doesn't exist as far as I know. The things that I have experimented are outlined below.
What works and is recommended:
- MaxSea is wonderful. I use it in concentration of 0.5-1.0 g/l or 400-800 µS/cm. This is a very low and conservative concentration compared to what some other growers use. Most plants can take more, however I mist them with the lower concentration very frequently, during some periods almost daily. I do it this way because lower, steadier supply of nutrients is better for the plants than rare sudden bursts. 125 PPM / 195 µS/cm in the tray water works well for Sarracenia. 600 PPM / 940 µS/cm works well for filling the pitchers of Sarracenia.
- Micronutrients only (a fertilizer without N-P-K, but only the micro elements) - at a strength of 50-100 µS/cm for foliar application and 30-50 µS/cm for soil application. This greatly improves the health and colour of the plants!
- Kelpak, 1:500 (and up to 1:250) solution - works great, but it is not exactly food as it is a hormone rich extract of sea plants so you need just a couple of applications per season.
- Osmocote slow release fertilizer - I wasn't happy with the results when I first used it on Drosera regia, it was not possible to achieve precise dosage and the effect is unpredictable, it seemed easy to overdo it. I used Osmocote Bloom, because this was what I had. It seems however that a lot depends on the type used - the longer lasting balanced types are better.
Recently my Nepenthes enjoyed about one granule per litre of substrate of Osmocote Pro 5-6M and showed good response, they could take even more.
For Drosera regia aim at 5-10% of the dosage that is recommended on the label. Reapply after the time interval stated on the label or a bit earlier in lowland conditions because minerals leach faster through the granule coating at higher humidity and higher temperature. Take care not to damage or abrade the granule coating as this may cause instant release of all the nutrients stored in the granule. Combine with natural food if possible.
One thing that I am not sure about is whether the different granules contain different minerals or or all the minerals are first mixed and granulated and then coated. I suspect it is the first. This has implications because when you apply just a couple of granules you may end up applying just one mineral, that's why I try to select and use from all types of granules present in the mix which differ by color and size.
What didn't work so well for me:
- Food for aquarium fish, beta food - it works, but I didn't use it long term because of concerns about high salt contents and because the way of application isn't practical for a large collection. It is spread/dusted on the sticky leaves of sundews. The particles that are not absorbed under high air humidity turn into balls of mould which threatens the pants health. Blood worms (Chironomus sp.) are a good option in this category. Here it is sold frozen or dry in aquaristic stores.
- Carnivorous plants are capable of digesting plant food too. A mixture of chlorella, spirulina, bee pollen, kelp, deactivated brewers yeast in dried powder form mixed with some water is something that I have come up with and that works with Drosera, Nepenthes and Sarracenia. However I have abandoned this too for practical and financial reasons.
How are fertilizers applied?
- Through the misting of the foliage.
Works with all species. When I apply the fertilizer as a foliar spray I use a spray bottle that produces a very fine mist. All the aboveground parts of the plant (the leaves) are capable of absorbing dissolved nutrients. The mist is applied to the point of run-off - aim for the leaves and stop when droplets start to merge and roll. A few drops on the substrate will do no harm, but don't soak it with the solution. This works for all species of carnivorous plants. There is no need to trigger the traps of VFTs.
- Through filling the pitchers.
This works with the species that produce pitchers of course - Nepenthes, Sarracenia, Heliamphora, Cephalotus. However I don't have personal experience with Heliamphora and Cephalotus.
- Through top watering.
This is self explanatory. Works with all species. The important thing is to be careful not to overdo it. In order to avoid this always use a safe concentration and between the applications always top water generously with clean water in order to flush the unabsorbed salts from the substrate.
- Through fertilizing the substrate with granulated slow release fertilizer. The main thing here is that it is best if the granules are burried and mixed in the substrate and not just placed on top of it. It is important to add the granules after the substrate is thoroughly mixed and ready and not to mix too much afterwards in order to avoid damaging the coating of the granules. If the granules are abraded, cracked or otherwise damaged they will release their contents at once and not in a controlled fashion which won't be good.
For me this fertilization program had the most striking effect on the Nepenthes because for the first time after many years during which I have kept them now I have plants with pitchers on every leaf throughout the whole year, despite the otherwise not perfect conditions. Neither the humidity is high enough, nor the light and temperatures are entirely adequate, but even so they are constantly pitchering since I started to regularly feed them. Now I mostly do the root feeding method with my Nepenthes and mist everything else - Droseras, Pinguiculas, Dionaeas, Utricularias and Drosophyllums and Sarracenia.
I hope that through sharing my experience I'll help you make your carnivorous plants more healthy, strong, beautiful and happy. The carnivorous plants are greedy, do not forget to feed them!
Author of the text and photos: Petar Kostov, 20170920. Updated June 2021.