Dieter Kadereit: Cultivation of Tuberous Drosera

All photographs: Dieter Kadereit
weblink http://www.tuberous-drosera.net/

A Czech translation of this article was published in the journal Trifid 2015/2.

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Dieter Kadereit
the author of the article

Another summer has ended and many plants or showing signs of fall having arrived, let it be e.g. the trees outside getting their yellowish or brown coloured leaves which will eventually cover the lawn or let it be or seasonal growing carnivorous plant like some Sarracenia species showing their full beauty. Everything seems to be getting ready outside for the winter with its short and often grey days.
This is my preferred season. More and more winter growing Drosera species are starting to appear, the flowering season is just about to start and it will continue until approximately April. I really enjoy checking the pots for signs of new plants breaking through the substrate surface. They will increase in size each day. Very fast and very dynamic!

By now I am growing tuberous Drosera for nearly 15 years and it seems to get better every year. This is quite a bit of time to make lots of experiences, both good and bad. Meanwhile I come to think that I have good growing conditions for many species and over the years my collections has grown a lot in size. One of the things I learned over time is that conditions which work well in one place may not be as good in another one, even though I often do not have an explanation for those differences. You may have made similar experience e.g. when moving to a new home. The purpose of this text is to explain my experiences and conditions so that – in case you are interested – you can use that as a starting point to optimize your growing conditions for these beautiful plants.

Let’s start with the conditions these plants have adapted to. Most of the species can be found in the southern part of Western Australia. The climate there is quite extreme as the summers are hot and come with very little, if any, rainfall whereas the winter is the rainy season. All plants need to adapt somehow to those conditions. Some are growing as short living plants like D. glanduligera which grows into a flowering plant within very few months to produce many seeds and finally dying at the end of the wet season. Another option is to reduce the loss of water during the summer by reducing the plant surface and covering all remaining surfaces to minimize evaporation. Many pygmy Drosera behave like this when they form the stipule bud.
Finally, a third option is to let all plant parts above the surface die back and wait for the next rainy season deep in the soil, well protected from the intense heat in the burning sun and probably even being close to some residual moisture. Obviously, the tuberous Drosera have adapted in that fashion just like some terrestrial orchids and many other plants. Furthermore, this is not unique for Australia. A similar behavior can be observed for some Drosera species in South Africa which are growing in similar habitats and are forming dormant roots instead of tubers.

Just let me continue with this a little further. If the seeds would germinate directly after ripening, the plants would be still tiny when the rainy season ends. Therefore, just to survive in those habitats in the long term, the plants somehow need to make sure the seeds germinate primarily when the rainy season is just starting so that the youngsters have as much time as possible to gain enough strength to survive the first summer.
Furthermore, wild fires often ravage through the habitats during summer clearing out most of the plants at the surface. What a great time for using this occasion to colonize the free soil. Nevertheless, you need to have some way to know when it might be a good time to germinate. All those things will become important again when we get to the topic of seed germination even though I am not sure at the moment whether these Drosera need wild fires for germination.

There are many ways to grow tuberous Drosera. Even though I concentrated on the Western Australian species when describing the conditions, we should not forget that there are quite a few growing in other parts of Australia or New Zealand, even in some areas of south-east Asia or Japan. This paper will focus on the general growing conditions which may not be fully applicable for species growing outside of Australia.

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Drosera pallida

As already mentioned, the plants start to appear at the surface sometime around fall. I am somewhat unclear with the timing as it very much depends on the conditions as well as the species. I keep the plants potted during summer in a cool spot. The temperatures there usually do not exceed 20-25 °C even during those weeks with temperatures of more than 30-35 °C or even higher. Just thinking of the situation in the habitat this may not be too different depending on the depth in which the tuber is. Those tubers close to the surface will most likely get much more heat exposure during the hot months in habitat whereas the tubers much deeper in the soil will be in cooler and more stable conditions. My growing conditions in this respect are probably closer to those of a tuber deep in the soil. It will be very different if you keep you pots in a greenhouse (even if shaded under the bench) or even somewhere exposed to sunshine. Those will see a more intense variation of the temperature and therefore may start to grow somewhat later.

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Drosera pallida in flower

When being exposed to the stable and cool conditions in summer, the first plants often start to appear in August or even late July. The first species to appear are usually D. (aff.) pallida, D. aberrans, D. macrantha and a few others, but often one of my D. pallida wins the race. Nevertheless, it can take until December or even January until the late species appear. The latest for me often are some D. gigantea or D. bicolor, which will also grow much longer into the summer.
Once I find the first plant on the surface, the pot will be made ready for the growing season. It will get a little bit of long-lasting fertilizer, an additional layer of sand on the substrate – I will explain this later -, and finally a nice place in a tray either with natural light or under T5 fluorescent bulbs. The pots get watered by the tray method. If the weather is ok during fall, I may place the pot outside where the occasional rain will do the watering.
During winter I need to protect the plants from the occasional frost outside. I have two main growing spaces for this. One is the same place I use for dormancy. Despite an direct air exchange to the outside, the temperatures are quite stable there and stay above 0 °C unless we get an extreme cold spell as we had in February 2012 when the temperature dropped to -10 to -15 °C for two weeks. The plants will get a photoperiod of 10-12 h and the only sources are the T5 bulbs. The second growing space is my wintergarden, which has less protection from the outside conditions. Therefore, I mainly keep Dionaea and Sarracenia there, but also all adult pygmy Drosera and some tuberous Drosera. The plants growing there will get the natural light (which is not that much during those many overcast winter days) plus some extra light once again using T5 bulbs for the Drosera. However, the distance from the light is much higher so that I preferably use those conditions for the climbing species.

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Tubers of Drosera pallida

Even though the plants can withstand mild frosts, I prefer to protect them from frost. Ideally, the temperature during the day should be above 5 °C or a little higher. When being exposed to lower temperatures, the plants will stop growing, which would be no general problem as they will continue to grow once the temperature rise. However, some species like D. zigzagia will not open their flowers at such low temperatures and may not open them at all if the temperature does not increase again after a few days.
But any winter is going to end and spring will finally arrive. For some species this will already be the time to go dormant again like some D. tubaestylis whereas late species just start to flower. I usually keep the plants in their respective growing space until they have died back above the surface. Please note that many species will not have completed tuber formation at that time. Some species, especially D. platypoda, will need more time until the tubers are formed. In my conditions D. platypoda often still has some stolons in development which will continue to grow in length until they finally produce a tuber at their end. Therefore, it is important that you do not let the pot dry out too early after the plant has died back. In case you are not sure what to do, you can carefully check the substrate, but please be really careful, as you are risking to damage the stolon or the developing tubers. I keep the substrate moist for at least 4 weeks after the plants have died.
If you keep the substrate moist but not soaking wet, there is not much which can go wrong even if the first tubers are dormant.

During dormancy I keep the pots relatively dry, but they will get a short „shower“ once or twice during the summer. The main reason for this is that often smaller tubers are found closer to the surface and they may desiccate more easily than the larger ones deeper in the substrate. The shower is meant to be enough water to moisten the upper substrate layer with the smaller tubers. It will dry out quickly again, usually 2-3 days later there is no sign left that the pots got some water. But the tubers had a chance to regain a little water.

Pots and Substrate

For most species I use pots with a height of 10 or 11 cm. Some species may do well in smaller pots but I prefer these as they keep the moisture during the summer dormancy longer. Some species, however, will place their new tubers at or close to the bottom of those pots, especially when the plants have reached a certain size. For those I use 20 cm deep pots. Typical examples are D. rupicola, D. porrecta, but also some large D. pallida. As a side note, some D. cistiflora are also growing in 20 cm deep pots due to their long roots whereas others form much shorted roots and do well in the 11 cm high pots.
As a substrate I use a mix of white silica sand of approximately 1 mm diameter (sold for aquarium use in some garden centers nearby) and peat. Usually the mixture consists of 3 parts sand and 1 part peat. The high sand content has one main purpose: those mixtures can be easily wetted by adding water. You certainly will have already made the experience that dry peat will swim on water, but it will take days or longer to get it moist. Adding an excess sand to the peat makes the difference as the water will fill the capillaries formed by the sand instantly. The peat will get wetted by this much faster.
There is no specific need to use white sand or this sand size. The reason to use this sand may sound trivial: it is easier to find small tubers. I used more coarse sand (2-3 mm) being a mixture of different colours before and it was a nightmare to find tubers which had approximately the same size.
I did not try other ingredients, but judging from the substrates in the habitats I would expect that many other materials may be added.
I already made the comment that the pots get a top layer of pure sand. In most cases it will be about 1 cm thick. This sand will be removed after the growing season and thrown away. It certainly makes the plant look nice, but this is not the main reason to add the sand layer. Some of the species will produce many seeds during the season and stray seeds may end up in any pot. To make things worse, I often try to pollinate many of those species which do not set seeds without help. Removing this layer of sand helps to keep the pots clean so that only the species listed on the label are growing in the pot.
You do not have to throw it away after the growing season, you may use it as a topping for other pots, e.g. for Dionaea instead. With a little bit of luck you may find some Drosera seedlings appearing in fall.

Pollination

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Drosera planchonii in flower

As already mentioned, some species are self-fertile and will produce seeds either without your help or after self-pollination:

  • D. bicolor (some clones)
  • D. major (some clones)
  • D. peltata complex species
  • D. planchonii
  • D. ramellosa
  • D. salina
  • D. stricticaulis

In addition to the list above I would like to mention also D. lowriei. Some clones can be self-pollinated whereas others do not. The giant plant form seems to be completely sterile.

Those species not listed above will need a genetically different plant (=clone) for successful pollination. In case you have a second clone but it did not open its flowers on the same day, you can carefully collect the pollen using tweezers and store it in the fridge for days or weeks. To ensure its viability, it should be stored dry. Therefore, do not use a ziplock bag. I use paper for this purpose which is sold to pack your sandwich.
Once the second plant is flowering, you can transfer the pollen using the tweezers again or a toothpick. With a little bit of luck you will find the seed pod swelling some days or weeks later.

Germination

I sow the seeds in small pots (5 cm round pots) and repot the seedlings in larger pots once they appear. This saves a lot of space when sowing the seeds and I also made the observation that the seedlings developed faster when I repotted them (compare to sow them directly in the larger pot). This process works well for many winter growing Drosera species as they initially have just short roots and can be placed on fresh substrate easily. Exceptions for this are mainly D. menziesii complex species and D. bicolor. These species produce first an about 1 cm long fine root and only after that their first leaves. Repotting these species requires much patience if there are too many fibers in the top layer of the substrate. Thus, using a higher percentage of sand in the sowing medium for this species is a good idea to speed up the repotting of these species. It sounds complicated but it gets routine quickly.

The seedlings are repotted into 7 cm deep pots. This pot size may actually not be deep enough for some species, even in their first season (e.g. some D. stolonifera may need a deeper pot at the end of the season) but it is sufficient in more than 95% of the cases. If I happen to find a pot which a shoot for tuber production which grows out of the pot, I put some substrate into a (larger+) deeper pot, place the pot with the seedlings inside and stabilize with more substrate. The idea behind this is to avoid tubers being produced outside the pot as the shoots and tubers may get damaged in that situation easily.

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Drosera stricticaulis

Once the seedlings are repotted, I treat them the same way as the adults. They get watered by the tray method, as much light as possible and I try to offer a temperature between 5 and 20 °C which is unfortunately not always possible in my setup. One should avoid significantly higher temperatures and long light periods as both may induce dormancy. As a consequence, the tubers after the first season can be quite small which may increase the risk of desiccation. Therefore, an occasional shower as described for the adult plants above will help them to get through the summer.

Finally, I will come to the most important question: how to convince the seeds to germinate? It is actually in most cases not really difficult. The tuberous and African winter growing Drosera can be divided into two groups. Most require some kind of stratification, but some do not. I will start with the ones which do not need any treatment:
Species without stratification (D. macrantha, D. peltata, D. auriculata and other member of the peltata complex): Sow the seeds in September, keep wet and at natural (shortening) daylength. Seeds will germinate when light period is appropriate, mostly from October to December.

Species needing stratification (most of the others):
Let’s remember the situation in Western Australia. The seed should ideally germinate at the beginning of the growing season which can be achieved if the seeds only germinate after the summer. And this is just what you will observe: any type of heat treatment mimicking a hot summer does the trick for many species.

Option 1: place the seeds in August or earlier in some hot place e.g. in your greenhouse. Sow the seeds then as described above for the species without stratification. This is actually my preferred option. As I have access to a 37 °C warm oven in the lab, I just place the seeds for 2 weeks in that oven and sow them afterwards.

Option 2: sow in August, keep hot (and dry if you want) until mid or end September, then treat as listed above for species without stratification.

Option 3: Give the seeds a hot water bath ~60 °C) for 30-60 seconds. The method works as follows: pack the seeds in some „tea-bags“ (e.g. use filter paper from your coffee machine, do not forget to label the seeds), the seeds are then put into the 60 °C water for 30-60 seconds. Let them dry, sow and treat as described for species without stratification.

The difficult species

Finally, there are some species which even do not germinate after any of the heat treatments mentioned above. Examples are the species in the erythrorhiza complex, D. lowriei, D. ramellosa and probably some other species in the group of fan-leaved Drosera. I have mixed results for D. stolonifera, sometimes germinating in the first season, sometimes preferentially in the second season. D. porrecta and D. humilis most likely also need more time whereas D. rupicola germinates well in the first season.
Although having mentioned the bush fires in habitat above, I do not think that the very difficult to germinate species really need some fire or smoke component for germination. I expect that the main reason for the late germination is the need for the seed shell to erode. In one set of experiments using bleach (sodium hypochlorite solution) I was able to get impressive germination rates for D. ramellosa and D. lowriei seeds, both species which do not germinate easily. That indicates that the seed shell erosion may do the trick. I will continue to investigate this. The next step is to physically weaken the seed shell just by using sand paper or anything similar. I will be able to report results probably next summer.
The easiest way to get these seeds to germinate is actually to be patient. Do not throw the sowing pots away. Just store them somewhere dry and start watering in September again. I had quite a few seeds germinating during the past season which I sowed 3 years ago. The pots by now are overgrown with mosses which die during the summer, but come back after the pots are watered again. Nevertheless, the seedlings will appear among the mosses and repotting will help to give them a good start without the need to outcompete the mosses.

Once your seedlings grow, they will continue to produce new leaves for a while, but most species will stop doing so sometime in the middle of the season. In most cases they produce the stolon now, which will grow deep into the substrate and finally produce a tuber at its end. Just one example: a D. porrecta which germinated last winter formed a rosette of finally approximately 1 cm in diameter. Then it produced a stolon of more than 5 cm in length and finally a tuber of 5-10 mm diameter at its end. Other species may form even longer stolons.

I hope this piece of information will help you to grow you tuberous Drosera or it may create your interest to give them a try. There are some very easy species which can grow under many conditions, even those which a far from those described above. I have some seedlings, especially of species in the peltata complex, in many other pots or containers and I am impressed how well they develop there.