University of Vermont Extension 
Department of Plant and Soil Science

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TERRARIUMS: GARDENS IN GLASS

Dr. Leonard Perry, Extension Professor
University of Vermont


What do fish tanks, brandy snifters, and Mason jars have in common?  They are all containers that can be used to make gardens in glass, or terrariums. Whether you put together a terrarium on a rainy summer day or for garden-relief in the dead of winter, this activity is suited for all ages.  Terrariums are once again quite popular, perhaps due in part to the interest in fairy gardens—one possible theme for a terrarium.
  
Growing plants in clear containers dates back to at least 2,500 years ago in Greece.  The terrariums we know today began with a 19th century London physician, N.B. Ward, and were named after him.  These large, enclosed glass containers called “Wardian cases” were the original means of transporting newly discovered plants back to Europe from voyages around the world. 
   
In America, the earliest terrariums were made by women settlers placing the native partridgeberry in handblown glass bowls.  This is a woodland plant with dark green, small evergreen leaves and red berries.  These simplest of terrariums we know as berry bowls.
  
If making a larger terrarium, many plants may be used-- in fact any that are small, or grow slow and can be kept clipped to size.  Keep in mind that, depending on the plants used, terrariums will need to be renovated every year or two as plants grow.  You’ll need to remove any that die, or get too large, and replace with new, smaller plants.  When doing so, take the opportunity to introduce some fresh soil.  If the plants are too out of control, you may just want to start again with new and different small plants. 

Low woodland plants which can be used include mosses, ferns, lichens, foamflower, wintergreen, and partridgeberry.  Some taller plants include other ferns and violets.  Just make sure if collecting such plants during the summer that they are not endangered (check with the state natural resources agency), and that you only collect a few plants from a native population.
   
Some common garden and house plants can be used such as aluminum plant, asparagus fern seedlings, creeping fig, English ivy, strawberry begonia, spider plants, nerve plant, selaginella moss, palm seedlings (which of course as they grow will need to be removed), and peperomias.  There are many low perennial plants you now can find for planting in walks and patios and stepping on, such as the herb thyme, which may be suitable in terrariums.
   
You may even want a theme of a specific ecological area.  For a desert, use some of the many slower growing cactus seedlings and succulent plants such as echeveria, panda plant, or haworthia.  For a bog, use mosses, ferns, and even carnivorous plants such as the Venus fly trap. 
  
For a rock garden, consider perennials such as saxifrage, sedum, sempervivum (hens and chicks), erodium (alpine geranium), or low dianthus (pinks). Keep in mind, though, if using perennials that they may need some winter rest in a cool area. This may be an unheated, yet non-freezing garage, with indirect light.
   
Materials you'll need, beside plants, include proper soil (humus soil for woodlands, sandy for deserts, for instance), sand or fine gravel for drainage, and charcoal bits to keep the soil sweet.  For desert themes, you may want to layer colored sands in the bottom.  White or colored aquarium gravel, and miniature figures or fairy garden accessories (available at many garden and craft stores), can be used for accents.  Sphagnum sheet moss is the usual liner for traditional and woodland terrariums.
   
Tools you'll need are scissors, a dowel rod, wire hooks or fork, water sprayer, funnel or tube, and brush or paper towels.  Long-handled bamboo or similar tongs are useful for placing plants, pebbles, or accents.  A small paint brush, as used in crafts, helps in cleaning leaves.
   
First, if using a moss liner, insert it green side out  into the bottom quarter of the bowl.  Then add a layer of gravel or sand.  Sprinkle the charcoal bits on top.  Finish with the soil, moistened.  The container should now be about a quarter to third full. Add a little extra soil, as it will likely settle a half inch or so over the coming months.
   
Check your plants for pests before placing in the container.  If foliage plants, it may be a good idea to dunk them in a mild, soapy bath and rinse before planting.  This may help get rid of any lurking pests.  Then check your terrarium frequently for pests, especially the undersides of leaves.  If palm seedlings, check for mites, although mites prefer drier environments.  Mealybugs may be the more commonly found, and if so, swab them with a cotton swab and rubbing alcohol.
  
Insert the plants, roots first, into holes made with a fork.  Start with the largest plants first, filling in with those smaller.  Use the dowel rod to place plants, and firm soil around them.  Spray the plants with water (unless of course a dry garden), clean the inner sides of the container, then add your decorative accents.
   
Keep the terrarium in bright but indirect light.  In direct light it can get too hot, cooking the plants.  Since the moisture and humidity remains in the container much longer than with houseplants, take care not to overwater.  Closed containers will remain moist much longer than those with open tops.  If your container is closed with a lid, open and check it every few days to give it some fresh air, and to make sure mold isn’t growing inside.
   
Most containers used for terrariums don’t have drainage, so if you overwater you’ll end up with a bog garden only suitable for specific plants.  You can tell when to water if the plants wilt, of course, or if the soil looks dry and lighter in color. Not watering too much is probably the most important key to success.  If in doubt, best is not to water.  For a new terrarium, or one with young and small plants, you may need to water only a couple times a month.  For those with well-grown plants, and little soil or sandy soil, you may need to water a couple times a week. 
   
You may need to water cacti and succulents very seldom-- once or twice a month-- and during sunny days.  If watering when cloudy, the soil may stay wet too long and these arid-loving plants can rot.  When watering any terrarium plants in winter, make sure the water is room temperature and not icy cold. Avoid fertilizing, or do so very sparingly at very low rates, to keep plants smaller longer.
    
If a large container, such as large rectangular fish tank, you may want to put a layer of gravel on the bottom and very small pots of miniature plants on top, as in a greenhouse.  Plant choices here might be miniature African violets, small seedlings of cacti and succulents, or rooted cuttings of small-leaved houseplants such as some gesneriads, peperomia and begonias.
   
There are many attractive containers you can purchase for terrariums, such as miniature greenhouses and Victorian-style large glass cases with many sides.  Round glass “rose bowls” are available at many florist shops.  Those with sides lower than the plants are used to make “dish gardens.”  Look around the kitchen for clear containers such as glass tea cups or mugs, large spice jars, gallon cider jars (the thin necks will be harder to work through), or the tops of plastic soda bottles cut off and placed over pots.
   
If you find a clear glass or plastic container, the size and shape of a Christmas ornament, this can be used for a one-plant hanging terrarium.  I have one with a tillandsia air-plant which needs very little water, just an occasional misting. You also may find suitable and unique containers at craft stores, flea markets, yard sales, antique shops, and on the internet. The next time visiting any of these, think “terrarium containers.”
 

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