Jennifer Schultz Nelson
Extension Educator, Horticulture
It seems to me that everything old is new again. This fall, I attended a workshop on building terrariums. I vaguely remember my grandma having a terrarium at some point in the 1970's, but really haven't seen many since. People at the workshop (including me!) were so interested and excited about building their terrariums, I think there is some truth to the saying "everything old is new again".
A terrarium is a transparent glass or plastic container in which to grow plants. Depending on the container and plants chosen, the container may be closed, partially or entirely open on top. Terrariums have no drainage holes, so special attention must be paid to water needs of the plants inside.
The invention of terrariums as seen today is credited to English botanist Dr. Nathanial Ward. He observed tiny ferns and grasses growing in a bit of soil in a completely closed bottle which contained a sphinx moth chrysalis he was observing. True to his scientific character, he continued to watch how the ferns and grasses continued to grow for four years, during which time he never opened the bottle to add water.
Dr. Ward experimented with growing different types of plants in glass containers, and published a book in 1842 entitled On the Growth of Plants in Closely Glazed Cases. Large glass cases for growing delicate plants became known as "Wardian Cases". While it became fashionable to have these cases in the home to showcase exotic plant collections, these cases were crucial in transporting plants safely from their native lands over long distances via the sailing ships of the time.
The choice of container to use as a terrarium is only limited by your imagination. While there are containers sold specifically for use as terrariums, there are many options to consider that may be had for minimal investment, or even at no cost, such as: fish tanks or bowls, brandy snifters, clear glass or plastic jars, jugs or bottles. Remember that containers with small openings will be more difficult to plant than ones with larger openings.
To grow a closed terrarium, there must be a way to seal the container. Many terrariums come complete with a lid or stopper to accomplish this. Ideally the lid or stopper, especially if it is covering a large area, should allow light to penetrate. If you are adapting a container for use as a terrarium that does not already have a lid, consider using thin plastic, plexiglass, or plastic wrap to seal the terrarium.
It is critical that the growing medium used for a terrarium be clean, well-drained and high in organic matter. Pre-packaged potting soils and peat-based planting mixes are already sterilized and free of plant pathogens that may affect plants in the terrarium.
There are many possible choices of plants to use in a terrarium. Plants that tend to naturally be small and slow growing will work best. Faster growing and larger plants will work in a terrarium; however they will need more pruning and may at some point need to be removed from the terrarium so that they don't outcompete the other plants present.
When choosing plants for a terrarium, type of terrarium, needs of the plants and location for the terrarium should be considered. For instance, closed terrariums are by nature very high in humidity, so they would not be suitable for cacti and succulents which need much drier conditions to survive. If the terrarium is to be located in a low light area, plants which require high amounts of light will not thrive.
Most tools for planting terrariums are easily found around the house. The size of the terrarium's opening is the only limiting factor.
A long thin bamboo stick or a dowel rod will come in infinitely handy for digging and placing plants, but has other uses as well. Attaching a wire loop at the end of the dowel rod can help steady plants as they are positioned for planting. A cork attached to an end of the dowel rod can help tamp down soil when planting. Wrap a small piece of cloth or paper towel around the end of the dowel rod to clean smudges from the inside of the terrarium. A small blade attached to the end of the dowel rod can be a way to trim dead and dying leaves from plants in an established terrarium.
Before assembling a terrarium, it is important to clean the terrarium container thoroughly. It may be washed by hand in hot soapy water, washed in a dishwasher, or washed with a 10% bleach solution. If using a commercial glass cleaner containing ammonia, allow the container to air out for several days before planting.
To minimize difficult maneuvering and rearranging in the tight space of the terrarium take time to decide on how to arrange your plants before you place them in the container. A rule of thumb is to place taller plants toward the back or center.
About 25% of the terrarium's capacity will be used for drainage material and soil mix. Since terrariums do not have drainage holes, it is important to create a region within the terrarium that will hold a slight bit of overwatering. To do this, add about one to three inches of aquarium or pea gravel to the bottom of the terrarium. On top of this layer add a 1/2 inch layer of horticultural charcoal which will help eliminate foul odors from excess water. Use a small funnel, folded sheet of paper, or a cupped hand to guide the materials into the terrarium.
A layer of sheet moss or nylon fabric may be placed over the gravel and charcoal to prevent the soil mix from moving into the drainage area. Next add enough slightly moist sterilized soil mix so that approximately 25% of the terrarium is filled with drainage material and soil mix. The soil mix should be just slightly moist. If the soil mix is too dry it will be dusty and coat the inside walls of the terrarium.
Before adding plants to the terrarium, carefully inspect them for any signs of insects or disease-- infested or infected plants will not last very long in a terrarium, and may spread their affliction to neighboring plants very easily. Remove any dead or yellowed leaves before placing plants in the terrarium. Trim plants to appropriate size if needed. Prune and loosen roots of potbound plants.
After planting, if you can remove the top of the terrarium, mist the plants and soil lightly. In a terrarium with an opening too small to mist the plants through, carefully add water a tablespoon at a time through the opening. In either case, be careful not to overwater. More than a few drops of water should not be visible in the drainage material. It is far easier to add more water than to remove it!
If in doubt as to whether enough water has been added to a closed terrarium, wait a day before adding more water. If after waiting there is a bit of condensation on the inside walls of the terrarium, the water level is appropriate. If the walls are completely fogged up with condensation, open the terrarium and allow some of the water to evaporate for a day. Then close it and check it again. Open terrariums may also have some condensation on the interior walls when water levels are correct.
If there is too much water in either an open or especially a closed terrarium, the risk of fungal disease and rot is great. If overwatering is suspected, the terrarium should be opened and air circulation increased to help dry it out. A paper towel may be twisted into a "wick" and allowed to touch the soil mixture to draw out some of the water.
A closed terrarium will likely not need watering more than once or twice per year. Water only if no condensation is observed on the interior walls of the terrarium or the plants are wilting. An open terrarium will need more watering than a closed terrarium, but considerably less watering than other houseplants. Water only when the soil is dry and/or plants are wilting.
Fertilizers are typically not necessary in a terrarium unless the plants show clear lack of vigor and yellowing and water level is not the problem. Water soluble houseplant fertilizer may be used at ¼ the label rate. Too much fertilizer may cause plants to grow too big too fast, causing the terrarium to become overgrown
Do not place closed or open terrariums in direct sun. Heat will build up even in an open terrarium enough to damage or kill the plants. Bright indirect light is best for most plants. Remember even "low light" plants need some light.
Most terrariums will need pruning or trimming at some point. Prune plants by pinching out growing tips or using a small blade attached to a stick inserted through the container opening.
Occasionally plants may overtake a terrarium or die, making replacement necessary. It may be necessary to start completely over if plants are extremely overgrown and cannot be removed without disrupting the entire terrarium.