Venus Flytraps & Planting Containers
The main issues to be aware of when considering, choosing and preparing a planting container for a Venus Flytrap are the differences between the conditions experienced by a plant growing in natural soil in the earth, and a plant growing in a container. Containers introduce problems that plants do not experience in their natural environment, or experience only to a much lesser degree.
Insulating pots help to prevent plants' roots from overheating in hot weather and direct sunlight.
When growing in the earth rather than in a pot, plants benefit from the temperature and moisture buffering properties of the huge mass of soil around them. The Earth provides a thermal mass that provides and radiates heat long after the air temperature above it becomes colder than the surface of the Earth. Natural soil heats and cools much more slowly than air, allowing plants to be more comfortable, preventing too quick or too drastic temperature changes.
In pots, above the level of the natural soil surface, the relatively small amount of soil in the container does not hold much heat, and is strongly affected by temperature variations of the surrounding air. The soil in a pot can cool much more quickly, or heat to a dangerous level during hot weather and in direct sunlight.
For the same reason that natural soil retains heat and buffers the temperature, it also provides a large reservoir from which a plant can draw moisture, drying much more slowly than soil or growing medium in a planter or other container. Whereas the soil around a plant when growing in the earth will change from wet to moist to fairly dry over a longer period, the limited amount of soil in a pot can dry out much more quickly, or can retain water for too long if it is very moisture retentive because there is not a mass of soil around it to absorb the extra water. In nature, very wet portions of the soil yield some of their moisture by capillary action and osmosis to drier soil around it, causing the soil to be generally moist rather than very dry or very wet.
Choosing & Preparing a Container
The main objectives in choosing or preparing a container in which to plant Venus Flytraps (and a lot of other plants that one wishes to grow in containers) are to provide sufficient temperature and moisture buffering characteristics to compensate for those lost by not growing the plants directly in the earth. In addition, containers made of some particular types of material should be avoided, for reasons explained below.
One approach to providing temperature and moisture buffering for a Venus Flytrap is to use a planting container that is more generous-sized, rather than small and cramped, with more soil around the plants' roots and rhizome to slow both temperature variation and moisture loss. Venus Flytrap roots can often grow to 8 or 10 inches (20-25 centimeters) or more in depth, so a container that has an inside depth of at least 4 inches is helpful, and up to about 6, 8, 10 or 12 inches is more ideal.
Color and Insulation
To help buffer the temperature of the soil and prevent it from becoming too hot or too cold too quickly, there are several characteristics that are important to consider when choosing a planting container. Dark colored containers can heat up very quickly when exposed to direct sunlight. Lighter colors which reflect sunlight rather than absorb it are generally better than darker colors, especially in climates with hot summers and intense sun (such as at higher altitudes with less intervening atmosphere to dissipate and diffuse the sunlight). In hot weather and intense direct sunlight, the soil in a small pot in which a Venus Flytrap (or many other types of plants) is potted in can heat up so quickly and to such a high degree that it is possible to literally bake and kill the roots and the entire plant, sometimes within mere hours.
Insulating containers are ideal for Venus Flytraps and many other plants. They keep the soil from heating too quickly or from overheating, and they diminish the rate of heat loss of the soil when the surrounding air becomes colder. Insulating containers are of several different types, depending upon the material(s) of which they are made and their design and construction. Containers made of a plastic foam (such as polyurethane or polystyrene foam ("styrofoam")) are exceptionally insulating and are excellent choices as a planting container for Venus Flytraps. Other insulating containers are of multi-wall construction, often made of plastic, with an inner and outer wall separated by space filled with air or some light, insulating material. These work well too, but in all cases a planting container for Venus Flytraps should have one or more drainage holes to allow excess water to escape when the plant is watered.
Unconventional insulating items can make excellent planters for Venus Flytraps, such as cheap styrofoam beverage coolers or large, insulating plastic mugs such as are sold at convenience stores for cold soft drinks or hot coffee, and very cheap (but very effective) styrofoam disposable picnic beverage cups or tumblers (16-ounce styrofoam beverage cups work very well as Venus Flytrap planters).
Container Types & Materials
Planting containers exist in a wide variety of materials: glass, ceramic, plastic, concrete, natural rock and various artificial rock-like substances, and wood for example. For Venus Flytraps, it's important to choose a container that is inert. That is to say, the container should add almost nothing to the water or soil in which a Venus Flytrap grows. This is because the roots of a Venus Flytrap are very sensitive to dissolved minerals and other soluble material and easily subject to salt burn (a chemical, non-thermal burn-like effect caused by dissolved mineral salts)
Containers that are OK
Containers made of plastic (including plastic foam such as styrofoam), glazed or unglazed high-fired ceramic (stoneware or porcelain), glass or resin composite (a hardening resin with a powder such as crushed stone added as filler) are all inert and fine to use as planting containers for Venus Flytraps. These containers are not soluble in water nor in the mildly acidic growing mediums often used for Venus Flytraps, so they add almost nothing to the water or soil, which is important as mentioned above.
Most types of wood are also fine to use as planting containers, although one might wish to avoid a wood that might shed some soluble material (such as resin) into somewhat acidic water and soil. The woods to question or doubt would generally be those that have a strong smell (such as cedar), while woods with only a mild aroma are generally fine to use, for as long as they last. Wood when used as a planting container is wet or moist much of the time, which subjects it to decomposition. To help prevent this, wood can be brushed with a liquid silicone wood-sealer product or painted with clear acrylic medium (used by artists as a medium and thinner for acrylic colors and also sold as a sealer for the grout between ceramic floor tiles). Both silicone and acrylic are inert and fine to use to prepare a wooden container or porous red clay pot (allow the silicone or acrylic to dry and cure for a day or two before use).
Common Red Clay Pots
You might wonder if cheap red clay pots, which are low-fired, are alright to use as planting containers for Venus Flytraps. The answer is surprisingly yes (with the caveat below), because natural red clay when fired to maturity is inert. The reason that red clay is low-fired (matures at a low temperature in a kiln) is because red clay contains a substantial amount of iron oxide (which gives it the red color) that acts as a flux. The major problem with red clay containers is that there are many red-clay-pot look-alikes, containers that appear to be common red clay pots but are actually very cheap low-fired man-made ceramic with whiting or other additives as mentioned above, and are not made of natural red clay. A person experienced with ceramics can tell the difference between natural red clay pots and these artificial red-clay-pot look-alikes. The lookalikes often are red only on the surface (white or some other light color on the inside) and scratch much more easily than natural red clay (which when fired to maturity is difficult to scratch even though it is low-fired). Therefore, if one can't tell whether a pot is really natural red clay or a man-made ceramic body instead that is merely intended to look like red clay, it's better to avoid red clay pots.
If you are sure that an unglazed red clay pot is made of natural clay instead of being an artificial lookalike, it's alright to use that pot as a container for one or more Venus Flytraps. However, there are two things to consider: low-fired red clay is porous, which means that water will evaporate from the outside surface of the pot and the soil will dry more quickly than it would dry in an impervious, vitreous (non-porous) container such as glass, plastic or stoneware clay; and if the red clay pot was previously used, it should be soaked in pure water (rainwater, distilled water or reverse osmosis water) for a day or two before reuse, in order to dissolve and dissipate any dried but soluble material that remains in the pores of the clay from its previous use.
This natural red clay pot has been sealed with artists' clear acrylic medium on the inside, to prevent excess water evaporation through its porous sides. Natural red clay is inert after it is fired in a kiln and is fine to use as a planter for a Venus Flytrap; however, man-made red-clay lookalikes should be avoided (see text).
Containers to Avoid
Many low-fired ceramics are partially soluble in water and are not recommended for planting containers for Venus Flytraps. "Low-fired" means that the ceramic matures (partially melts and fuses) at a fairly low temperature inside a kiln. Low-fired ceramic and clay products are less expensive to make partly because they require less fuel to heat in a kiln. But low-fired ceramics are often composed of a man-made "ceramic body" that contains a fairly high percentage of non-clay material such as whiting (calcium carbonate) which acts as a flux in the kiln, making the clay melt at a lower temperature. However, the calcium carbonate remains partially soluble in water, so although such low-fired ceramic is fine for uses such as bathroom tile and cheap dinnerware, it is not suitable for use as a planting container for Venus Flytraps.
Special Considerations for Containers
There are many ways to make containers less prone to overheating or to create or improve an insulating characteristic.
Pots that are not insulating but which are exposed to direct sunlight can be placed in such a way that their sides are protected from the direct sunlight, such as being shaded by some other object (like a rock or another pot, something heavy or something light (such as cardboard) which is anchored to prevent it from blowing away in the wind).
When insulating plastic-foam or multi-wall plastic pots can't be found, other items made of insulating foam can be adapted for use as a planting container (such as the beverage coolers and cups mentioned above), or an insulating container can be made in other ways, such as by placing a smaller pot inside a larger pot, with a light material between them to act as both spacing (keeping the pots separate) and insulation. For example, a slightly smaller plastic pot can be placed inside a larger plastic or high-fired ceramic pot (both with holes to drain the water, of course) and the space between the bottom of the inside pot and the outside pot, and around the sides, can be filled with perlite (a very light soil amendment (additive) made from volcanic rock expanded by high heat) or lava rock (hardened lava filled with gas pockets). When using perlite, some heavier rock can be placed at the top between the two pots to cover the very light perlite and prevent it from being blown away.
Can I Use a Terrarium?
Terrariums (enclosed containers) introduce problems and challenges to growing Venus Flytraps and other carnivorous plants and require more expert, experienced and attentive care. Terrariums are not recommended for beginners, and even experienced growers often prefer to avoid using terrariums as growing environments. Problems include overheating, water stagnation and concentration of fungal and bacterial microbes that can cause infection and rot. These problems can be addressed, but to do so requires extra work, care and often extra expense.
Heat and Light
The transparent or translucent walls of a terrarium tend to concentrate heat from the light that enters the terrarium. In direct sunlight, if the terrarium is covered or partially covered, the inside temperature can quickly increase from cool or moderately warm to extremely hot in a matter of minutes or hours, and can easily create conditions that actually bake the plants inside and kill them. This is a problem when growing Venus Flytraps, because they love plenty of sunlight.
It's possible to keep a terrarium out of direct sunlight to avoid overheating, and to use relatively cool light instead, such as bright fluorescent light. Flourescent fixtures do produce heat, so care must be taken to ensure that the terrarium does not overheat, but with some experimentation artificial light can be used to provide enough light to the plants without overheating their growing environment.
Water and Drainage
Terrariums are often undrained. That is to say, there are no holes in the bottom of the terrarium from which old water can drain as fresh water is added. A constantly wet and saturated condition near the bottom of the growing medium, or in a drain field of of rough or porous material placed under the growing medium, can cause it to become stagnant, can cause conditions that promote the growth of destructive microorganisms such as anaerobic bacteria. These can infect and rot the roots and rhizome of a Venus Flytrap.
Even in an undrained terrarium water evaporates, and water must be added from time to time. But when water is added but no water is ever removed, the water that is in the terrarium over time contains more and more dissolved material (TDS), mineral salts and other soluble material, which can damage a Venus Flytrap with "salt burn", an overdose of mineral salts that stunts the growth of the plant and is difficult to overcome even with fresh water and fresh medium, once the Venus Flytrap has incorporated the mineral salts into its own tissue, its roots, rhizome and leaves.
There are several methods of providing fresh water to a terrarium growing environment, while removing old water that might be somewhat stagnant or contain too much dissolved material. One method is to create a drain of some sort on the bottom of the container that can be opened or closed as desired. Another is to create a permanent hole in the soil or growing medium to the solid bottom of the terrarium, using a length of plastic pipe for example, into which a smaller diameter plastic tube can be inserted to remove the older water at the bottom of the growing area using either a small water pump or merely by using the small-diameter tube as a siphon.
However one deals with the problem of removing the older water, it's important to change the water, substitute pure, fresh water, and flush the soil of dissolved minerals and other solubles on a regular basis to ensure the continued health of the plants.
Air Movement and Ventilation
Air inside a terrarium is enclosed, at least partially, and with so little movement of the air nor fresh, clean air to replace the air inside a terrarium, fungal spores and other pathogens can accumulate and become concentrated in the space inside a terrarium. If a fungal growth and infection does occur, it can spread very rapidly, and if it is a particularly destructive fungus, it can cause a large amount of harm in a fairly short period. So fresh air is important inside a terrarium.
Fresh air can be introduced to a terrarium by passive or active means, by leaving the top uncovered (passive) or by providing a fan (such as a cheap computer-cooling fan) to draw in fresh air to replace any older, perhaps musty air inside. Regardless of the technique to do so, it is important to introduce fresh, moving air into a terrarium to prevent a buildup over time of fungal spores and to introduce the full range of gasses (such as carbon dioxide, which plants use) back into the growing environment.
The air in a terrarium is likely to become very humid, which can introduce problems as well. A soil surface that never dries out because it is not subject to drier moving air or because the air inside is already near or at 100% humidity, will encourage fungal growth. Terrariums by their nature tend to encourage fungal and sometimes bacterial growth, so extra care must be taken to prevent this, or to treat fungal growth soon after it occurs and is visible. A general purpose fungicide (such as one that contains chlorothalonil) is helpful to address occasional fungal outbreaks.