This chapter gives general information on the cultivation requirements of carnivorous plants; for further details, see the individual descriptions on pp.32-55.
Commercially available composts are not recommended for carnivorous plants, but it is easy and cheap to mix suitable composts at home and does not require precise measurement or many ingredients. Care must be taken to ensure that the mixture is made up in the correct proportions, as carnivorous plants vary in their preferences, from those needing free drainage to those wanting permanently wet bog. Most plants fall into one of four categories, for which appropriate mixtures are shown in table 3.
Table 3: types of compost Ingredients
Equal parts by volume of moss peat and washed sharp sand
Equal parts by volume of moss peat, washed sharp sand and perlite or vermiculite
Live sphagnum moss with some charcoal
All these composts are without nutrients. This is important, because carnivorous plants always grow in infertile soils or water. Addition of fertilizer to the compost may cause root damage, malformation of the trapping leaves or death of the plant and therefore should not be attempted.
Opposite above: Drosera capensis, one of the easiest sundews to grow and ideal for a windowsill
Below: The beautifully marked Sarracenia leucophylia will enhance any collection
Sharp sand is used to give the compost an open texture. The grains should measure 1/16 to 1/8 in. (1.6-3 mm) and should be washed first to remove as many chemicals as possible. To do this, partly fill a bucket or bowl with the sand and add ordinary tap water. Agitate the sand with your hands or a stick and then pour off the water, repeating until the water runs clear. For larger amounts, fill one third of a deep container with the sand and flush it out with water from a garden hose, pushing this well into the sand to stir it up. If the sand is not washed, there is a tendency for chemical impurities to leach on to the soil surface and leave ugly orange-brown deposits, which severely detract from the beauty of the plants. The sharp sand should be acid or neutral, as not all carnivorous plants are lime-tolerant. River sand of the same grain size, when available, is the best kind to use.
Sphagnum moss is less common as a growing medium than is often supposed. It retains high volumes of water and can damage some plants if left around the plant neck, particularly in winter; in the right conditions, it also grows very quickly and may smother small plants. Therefore, it should only be used where specifically recommended. Sphagnum itself is now rare in the wild, many sites being protected by conservation orders. However, it is easy to buy or grow. Garden centres usually offer sphagnum for sale in the spring and summer months and any spare moss can be stored moist in a plastic bag kept in a shady place. For longer storage, pour off excess water and refrigerate or freeze it. To grow even a small amount of sphagnum, a large shallow container is necessary and plenty of growing space. The shredded sphagnum is scattered on to the container and rain or distilled water added (not hard tap water as it is intolerant of lime). If placed in the light and kept wet, the moss will grow.
Plastic containers are normally used for carnivorous plants and have several benefits, being cheap, easy to sterilize, light and less fragile than clay. In the very wet conditions required by most plants, clay pots can also become saturated and produce unattractive algae or slime on the outside.
The size of the container is usually unimportant. However, most carnivorous plants may be grown in half-pots, that is, pots in which the height is approximately two thirds of the maximum width. These have two distinct advantages: they are economical because of the smaller amount of growing medium they hold; and they need less water because of their reduced volume, which can be helpful during prolonged dry spells, especially where water
storage facilities are limited. The relatively small pot size does not matter, since the root systems of most carnivorous plants take up remarkably little space. (For the few exceptions, where larger pots are required, see pp.36, 40 and 41).
Watering is generally very simple. Each pot should stand in a plastic saucer, about 1 in. (2.5 cm) deep and at least 1 in. (2.5 cm) wider than the pot, which is kept almost full of water throughout the growing season. In the cooler conditions of winter, when the majority of plants experience a period of dormancy, the soil should remain just damp. They may then require water once or twice a month. However, the few tropical species which continue in growth, such as tender Pinguicula, should be stood in a saucer of water all year.
Watering from above is usually best avoided. It can initiate rotting, especially where moss is growing round the plant, and water droplets left on the leaves may cause scorch, particularly with sticky-leaved plants. In most cases, the wide saucer full of water provides sufficient humidity and misting or syringing is not required. Nor is it advisable to fill the pitchers of Sarracenia and similar plants with water. If the soil, water and humidity are satisfactory, the pitchers will control their own water content.
On the whole, rainwater or other soft water is the most suitable for carnivorous plants. The majority of them grow in the wild in acid or neutral soils and will not tolerate lime, and certainly few plants appreciate the chlorides and fluorides added to household tap water. Rainwater, usually plentiful in Britain, is easy to collect in barrels fed by downpipes from house or greenhouse roofs. Water from a local river or lake, so long as it is acid or neutral, is equally good. Expensive alternatives, which are feasible only for the smallest collections, are distilled and deionized waters. Distilled water is absolutely safe. However, prolonged use of deionized water (which has been treated to replace certain chemicals, including the unwanted calcium, with others) could lead to a build up of chemicals in the soil; to avoid any risk of damage, the soil should be changed within six months.
In the event of lime-free water being temporarily unobtainable, the simplest solution is to use household tap water, which in the short term will harm very few plants. If at all possible, it should be first boiled and cooled before use. In hard water areas, tap water should be resorted to for the minimum amount of time and, if this does unavoidably run to several weeks, the compost should be changed as soon as regular supplies of rainwater become available again.
There are some plants which are lime-tolerant and may be given tap water, again preferably boiled and cooled before use. They include many of the hardy Pinguicula and the tropical Nepenthes.
There are carnivorous plants to suit all the various temperature ranges found in nature and mimicked in greenhouses (see table 4). Although most gardeners are surprised to learn that any of them are hardy, there is indeed a choice of plants for growing outside in temperate climates. Many species which are not considered hardy will also survive long periods of frost, but because growth is severely set back, there is little point in growing them outdoors. Perhaps surprisingly, most of the hardy species will grow as well under glass as when fully exposed (except during the coldest winters when some protection may prove beneficial). Normally, protection only serves to advance the growth period. However, hardy species of Pinguicula, like P. grandi/lora, produce fewer off-
Table 4: minimum winter temperatures for carnivorous plants
Temperature range outdoors (hardy)
320F (Q0C) minimum
410F (50C) minimum
7Q0F (210C) minimum
640F (180C) minimum (days) 520F (110C) minimum (nights)
Sarracenia purpurea purpurea, S.flava; Drosera rotundijblia, D. angJica, D. /ili/ormis fiJi/ormis; Pinguicula grandrflora, P. vulgaris; (Jtricuiaria vulgaris, U. australis, U. intermedia, U. minor all Sarracenia; Drosera binata, D.filiformis traceyi, D.peltata, D. whittakeri; Dionaea muscipula; Pinguicula grandi/Iora; Darlingtonia cali/ornica; (Jtricuiaria praelonga, Li. intermedia all tropical Pinguicula, especially P. agnata, P. gypsicola and the P. moranensis group; most [Jtricuiaria; tuberous, pygmy and tropical Drosera, especially D. pygmaea, D. aliciae, D. capensis and D. adelae; highland Nepenthes; all Sarracenia except S. purpurea purpurea; Cephalotus follicularis lowland Nepenthes highland Nepenthes sets when given winter protection, while P. vulgaris definitely prefers to be grown outdoors and attempts to maintain it in a greenhouse are likely to fail.
For a 32°F (0°C) winter minimum, some of the Australian tuberous Drosera such as D. peltata and D. whittakeri are worth experimenting with. Although the plants rest as tubers for about eight months of the year, they add tremendous variety to the genus and are quite easy to grow, even tolerating short periods of sub-zero temperatures.
All Sarracenia do well when cool and frost-protected, especially S. purpurea purpurea and S.flava, which do not like warmth in winter. Some Pinguicula are also hardier than expected and certainly many of the P. moranensis group will survive for a few days at 32 to 50°F (0-10°C), with the exception of P. moranensis caudata.
Above: The tuberous sundew, Drosera whittakeri, a rosetted plant with large white flowers
Below: A whole group of beautiful and varied dwarf sundews is represented by the appropriately named Drosera pygmaea
For the avid enthusiast, the 41°F (5°C) minimum will allow the greatest range of carnivorous plants to be grown. Many of the hardier plants, including Sarracenia, will still perform well in these temperatures and all the tender ones will survive and grow happily, apart from a few of the truly tropical species. This is the safest temperature at which to maintain new plants if their preferences are unknown. Highland Nepenthes, previously thought to need higher temperatures, also succeed in these conditions, providing the humidity is very high.
Among plants which are readily available at present, only the lowland Nepenthes require higher temperatures than these.
Two factors will contribute to the health of your plants - light intensity and the number of hours of light received out of 24 hours, referred to as day length.
Most carnivorous plants do best in a position which receives the maximum amount of light and too little will cause them to grow long and spindly, making very poor specimens. In the home they may be placed in a south-facing window, or a north-facing one in the case of Pinguicula. In the greenhouse, some precaution should be taken against scorching from the sun. The toughest of plants, for example Sarracenia, will tend to burn around the trap lip, especially where nectar droplets magnify the rays of the sun. Light shading in summer is quite sufficient to protect them and several materials are available from garden centres for the purpose. A tinted or white plastic sheet or a single layer of coarse-meshed netting can be fixed inside the greenhouse roof or windows, or a proprietary shading paint can be applied to the glass. These methods will also do for Drosera (apart from the pygmy and tuberous species, which appreciate any degree of strong sunlight).
Among the more specialist plants, Utricularia present a different problem. Although many withstand direct sunlight, too much will promote the growth of moss and algae, which quickly smother the leaves and may kill the plants. Nepenthes are far more sensitive and prefer to be grown in shade. They can be considered to be similar to many of the tropical orchids.
Pinguicula need some care. The thousands of leaf glands produce a coating of mucus, which invariably leads to damage from scorching, and it is advisable to allow no direct sunlight to fall on them. This has the advantage that the leaves often develop a generalized pink to rose hue or reddish veining. These plants show their true splendour only when grown out of direct sunlight, in shade or, for best results, under artificial lighting.
Artificial lighting can be used with most carnivorous plants and is particularly suitable for Pinguicula, L/tricularia and Nepenthes. It eliminates the risk of scorching and guarantees that light will be available for the required time each day, irrespective of weather conditions. On the other hand, the lighting must normally be placed near the plants, which detracts from their beauty, and it can be expensive to run.
Fluorescent tubes designed for plants (such as Grolux) are cheap and easy to obtain. They remain cold and so can be placed very close to the plants. This is essential, as the light they emit is not effective with carnivorous plants at distances beyond about 1 to 2 ft (30-60 cm). For very small or rosetted plants, the distance should be 6 in. to 1ft (15-30 cm). However, large upright plants will be difficult to light in this way and Sarracenia often become limp and straggly, since the base tends to be shaded by the upright pitchers.
For large collections, tall plants and Nepenthes, there are more powerful light sources, which not only light a small room with a single bulb, but also radiate heat and help to maintain the required temperature range. These systems (for instance, Sunlighter) can be bought through advertisements in plant magazines. All artificial lighting is best controlled by a compatible automatic switch or clock. During the growing season, usually in summer, provide nine to twelve hours of light; in winter provide seven hours.
Day length is important to most carnivorous plants. It may influence flowering and can trigger or interrupt growth in those which have a natural dormancy period. A summer day will give eight or more hours of light and in these conditions, plants produce their summer foliage and may flower. With shorter days, many plants become dormant. This is particularly true of the hardy sundews, Drosera, while the pygmy sundews enter a reproductive stage at the same time. Short days also cause Sarracenia and Cephalotus to produce only non-carnivorous leaves and other plants, such as Darlingtonia, cease growth until the days lengthen.
Generally speaking, nature should be left to its own devices. Only when a plant fails to flower or stubbornly refuses to end its dormancy should alteration of day length be considered.
Various propagation techniques work for carnivorous plants, but whatever the method used, new plants are best potted on only
after a root system can be seen. If plants are transplanted before this occurs, it is necessary to maintain high humidity, keep them well watered and ensure that they are warm and receiving plenty of light. Direct sunlight should be avoided for very young plants, especially those with no root system. It is usually advisable to begin propagating in spring or early summer to produce strong plants in time for the winter.
Almost all the plants available will set viable seed, with a few notable and annoying exceptions, for instance, the various pygmy Drosera and L/tricularia. Seed can also be purchased from various sources, including some commercial suppliers and specialist plant societies.
All seed can be sown on the surface of the growing medium suitable for mature plants [see under the descriptions, pp.32-55) and germinated in the light, but out of direct sunlight. Most seed will germinate at 55 to 65°F (12.5-18°C) and need not have any bottom heat or other special treatment. Nepenthes require higher temperatures and higher humidity.
Germination can take from two to three weeks, to as much as two years with some tuberous sundews. As long as seedlings are well protected from direct sunlight, all can be treated as adult plants. Seed of some carnivorous plants will germinate only if fresh and it is generally advisable to sow seed as soon as possible after collection.
Seed of hardy plants should be exposed to frost first and will germinate in spring as temperatures climb above 50°F (10°C). In most cases the seeds can be sprinkled on to a bed or tray of sphagnum, placed outdoors and covered with a sheet of glass to prevent them being washed away by heavy rain. If sown in a bog garden, the seed should be protected from birds with chicken wire.
The horizontal stems or rhizomes of Sarracenia may be cut into sections about 1 in. (2.5 cm) long, with a clean sharp knife, taking care not to damage the roots. Each section is then planted and treated as an adult plant. Make sure it lies horizontally with any large roots pointing downwards and do not allow soil to cover the top of the rhizome. Take rhizome cuttings in spring, during pitcher growth. The remainder of the original plant should be left with several good roots. Sarracenia can rot if allowed to form thick clumps and rhizome cuttings should be taken every three or four years.
Several other plants respond well to the taking of root cuttings, including Drosera, especially those with thick fleshy roots such as D. capensis, and CephaJotus. The method is basically the same as for Sarracenia, except that the cut sections should be covered to their own depth with soil. Stand the pot in water to keep the soil moist and, when the leaves appear, the plants can be repotted and treated as adults.
Leaf cuttings are suitable for a variety of plants. With Drosera, entire leaves are removed from the stem, making sure that the leaf base, where it attaches to the stem, is also taken, and laid sticky side up on a bed of growing medium, shredded live sphagnum moss or perlite. The medium should be watered and then a layer of live shredded sphagnum moss placed over the leaf and sprayed lightly. The pot or container is covered with a piece of glass, a clear plastic lid or clear plastic film to maintain humidity and placed in a light warm position, but out of direct sunlight. Watering or spraying will be required until new plants are seen to emerge from the leaf cuttings. These can be safely potted on when a root system has developed. (If these daughter plants are removed at a very early stage, the original leaf cutting will often continue to produce new plants. As the daughters are still relatively fragile, they should be kept warm, humid and out of direct
sunlight until a root system has developed.) Use only the healthiest-looking leaves and ensure maximum contact with the growing medium for best results, by shaping the soil to fit the leaves or by carefully pinning them down with thin wire. Check daily for any movement of the leaves and reposition them if necessary. Different species of Drosera react in different ways and it is worth experimenting, by cutting the leaves into sections or using parts of the leaf blade rather than the stalk.
Leaf cuttings of Dionaea can be treated in exactly the same way, although it is essential that the entire leaf is taken as new plants will develop from a bud at the base. Use new leaves as soon as they mature and discard the traps, which often move and interfere with development of the new plants.
The leaves of tender species of Pinguicula are ideal for leaf cuttings. Although summer trapping leaves can be used, most growers prefer to take the dormant winter leaves, which are more plentiful, are not sticky and will not detract from the beauty of the plant in summer by their absence. Up to three quarters of the total number of winter leaves on each plant can be safely removed. Place these on the surface of the growing medium or perlite. (Perlite has the distinct advantage of being free of moss spores, which allows the new plants to grow without competition.) Water the soil to ensure it is damp, cover the container to maintain humidity and keep it in a warm light place out of direct sunlight. After a few weeks, the leaves will begin to show the growth of new plants as tiny buds. They should be potted on as soon as a root system has started to develop or when they are beginning to overcrowd each other. To increase the yield, simply cut each leaf into smaller sections before placing it on the medium. New plants grow from any point of damage. As if this method were not simple enough, some species, such as P. primulrflora, actually grow daughter plants on their leaf ends without assistance. These new plants can be left to root or potted on as soon as they are big enough to show leaves. As with young plants from Drosera leaf cuttings, maintain a high humidity for your Pinguicula plants.
Nepenthes can be propagated from stem cuttings. Take off the top of the plant as a length bearing three or four pairs of leaves and remove the base pair. Cut off half of each remaining leaf. Dip the cut end of the stem in rooting hormone and plant this in live sphagnum. The plant and pot should then be wrapped in a clear plastic bag and kept warm, as for adult plants, checking regularly that the sphagnum does not dry out.
Gemmae are highly modified leaves designed to protect the plant during dormancy or to increase natural propagation. The most dramatic example is found in the pygmy Drosera, which during dormancy produce an abundance of gemmae from the centre of the plant. If left unchecked, they will often smother and kill the parent plant and a toothpick or small blunt piece of plastic can be used to dislodge them. Keep the plant away from others when doing this, as the gemmae part from the parent at some speed and will grow where they land! Collected gemmae can be sown as seeds, with the small dent in the surface facing up. The growing bud develops from the base of this dent. Gemmae can be stored for several weeks on damp paper, sealed in plastic and kept in a refrigerator.
Darlingtonia and Cephalotus can be increased by dividing between clumps on the rhizome. Always ensure that divisions are free from decay by cutting out any tissue which is brown rather than white.
Other plants divide naturally and can slowly be increased in this way. Tuberous species like Drosera whittakeri may produce new underground daughter tubers, but these are best left untouched for one or two years. Stoloniferous species, including Drosera proli/era and most L/tricularia, will spread laterally and can be simply divided from the parent with a knife if the growth remains permanent. Hardy Pinguicula will produce offsets or daughter buds during the winter, which can be removed from the base of the parent in late winter and treated as mature plants.
All carnivorous plants are adapted to catch their own food because they live in areas where the soil lacks nutrients. If they are planted in soil that contains much in the way of chemical nutrient, one of two things usually happens. At best, the plants are oversupplied with food and do not form traps. Such plants are of little value to the collector. At worst, the chemical concentration (even though weak for normal plants) is so high that it damages the roots and kills the plant. Similarly, foliar sprays can injure the plant through overfeeding.
However, there are some cases when the very careful use of a weak foliar feed can be beneficial. Seedlings and immature plants can be encouraged to grow by applying a foliar feed suitable for houseplants (such as Baby Bio), at about one quarter the recommended concentration. Plants should be sprayed no more than once a month and always in the early morning or evening to avoid any risk of leaf burn. This treatment can be given to Sarracenia and Darlingtonia seedlings, Nepenthes and Pinguicula (applying very light mistings only).
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