Regional Gardening Reports :: National Gardening Association

In the Garden:
New England
February, 2007
Regional Report

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Glass enclosures of all types can be made into terrariums.

Tropics Under Glass

There's nothing like a cold, dry New England winter to zap the moisture from our indoor air so that everything we touch delivers a static charge. We set up humidifiers, slather on the skin lotion, and dream of moist, tropical climes. You know, the kinds of places where many of our houseplants originate. They, too, suffer in hot, dry indoor air, showing brown and crinkly leaf edges, dropping flower buds, and generally looking unhappy. Ferns especially. So recently I decided to treat some of them to a trip to the tropics ... under glass.

Terrariums can take many forms; all you really need is a clear glass container without drainage holes that's large enough to fit the plant or plants without them hugging the sides, which can lead to disease. You don't need an aquarium-sized home; a small round vase or candle holder can accommodate one special plant. You can find some unique containers by searching antique shops and second-hand stores. Glass bell jars are beautiful but can be pricey. I opted for a glass pedestal cake dish with a tall lid, which was on sale for $3.

For a rainforest atmosphere, you'll need a jar with a lid. Since the air will remain more moist, you'll need to check the plants frequently for sign of disease and remove any affected leaves or plants. This environment is suitable for plants that require high humidity.

A container with high sides and an open top will have its own microclimate that's more humid than the surrounding air but not quite as moist as a closed container. Many indoor plants are adaptable to this kind of terrarium.

Cacti and succulent dish gardens are sometimes called terrariums but that's a topic for a humid summer day.

Compatible Plants
Plants that appreciate the atmosphere in a terrarium are those that thrive on humidity and indirect light. Choose plants that will not grow large or quickly. Some good options are ferns, bromeliads, Swedish ivy, baby tears, creeping fig, oxalis, iresine, fittonia, pilea, miniature begonias and anthuriums, and carnivorous plants such as pitcher plants, venus fly traps, and sundews, which can be very temperamental. African violets, too, can be tricky in a closed system because their leaves are susceptible to rotting, but in an open terrarium with good air circulation, they might be fine. Combine plants of different heights, colors, and leaf shapes just like you would in a garden bed.

Creating the Miniature Environment
The base layers of a terrarium consist of a bottom drainage layer, an activated charcoal layer, and a soil layer. Without drainage holes, water can accumulate in the bottom, so a 1- to 3-inch layer of pea gravel or coarse sand will help keep the soil from staying too wet.

On top of the drainage layer, place about a 1/2-inch layer of activated charcoal (as is recommended for aquariums) to act as a filter and purify the water and air as the organic materials gradually decompose.

Spread a thin layer of sphagnum moss on top of the charcoal to keep the soil layer from sifting down into the gravel and clogging it. Then spread a thick layer (depending on container size) of a soil mix consisting of one part coarse builder's sand to two parts sterilized potting mix. Moisten the mix beforehand. If space allows, sculpt some hills and valleys.

If you have some indoor plant tools, they are handy for working with terrariums. Otherwise use a fork to rake the surface and a spoon or chopsticks to make holes for plants. If you're using a tall jar, you may need something like long-handled barbecue tongs. Remove plants from their pots and remove excess soil. Some people recommend disentangling the roots before planting, others just slip plants into place. I do a little of both, depending on the plant.

Add rocks, or shells, or even miniature pink flamingoes if that tickles your fancy. Mist the plants and leave the lid off for a couple of days until the foliage dries. Then close the lid.

In a completely closed system, the terrarium should need very little water. Moisture will collect on the insides of the glass and drip back down onto the soil and plants. If condensation becomes excessive, just open the lid to air it out for a day or two. When the plants need water, do so gently and sparingly.

Even in a terrarium without a lid, err on the side of too little water rather than too much because there's no place for the water to drain.

Keep the terrarium in bright light but out of direct sunlight, which can cause the temperature to build up inside the glass.

Don't fertilize the plants regularly because you don't want to encourage rapid growth and fertilizer build-up, but if plants seem to need a boost, use a very dilute (1/4 strength) solution.

Snip any leaves that touch the glass, and pinch the tips and trim plants as needed to keep them in bounds. You may need to replace a plant now and then.

Now, if only we could transport ourselves so easily to a rainforest!

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