Saturday, February 21, 2009

Build a terrarium this winter

By David Trinklein
Associate Professor Plant Sciences

The cold days of winter force gardening enthusiasts to peruse their hobby indoors and terrariums represent an ideal way to accomplish that goal. A
terrarium is a tightly closed clear glass or plastic container filled with
small plants. It also has come to mean an open, transparent container for
growing and displaying plants. Terrariums are most useful for small plants
that do not adapt well to normal home atmospheres. When properly planted
and located, they provide a way to grow many plants with minimal care.
Transparent containers for growing plants date back 2,500 years ago to
ancient Greece. However, the terrarium as we know it is credited to Dr. N.
B. Ward of London. Dr. Ward was interested in growing many types of ferns
in his backyard but had not been successful. While studying a sphinx moth
emerging from the chrysalis he had buried in moist earth in a closed
bottle, he was amazed to see a seedling fern and some grass growing
inside. He watched them grow for four years, during which time not one
drop of water was added, nor was the cover removed. This led to
development of "Wardian Cases," which were large, enclosed containers for growing delicate plants in the home or transporting precious plants over
long distances.
The terrarium most often used today are small ornamental versions of the
Wardian case. We recognize two different types of terrariums: closed and
open. Closed terrariums are more traditional and retain more humidity than
do open ones. The latter, however, do provide higher humidity for plants
than do dish gardens. Open terrariums and dish gardens require more
frequent watering, but danger of disease buildup is reduced.
Critical to the establishment of a successful terrarium is the selection
of an appropriate container. A terrarium container must be made from a
transparent material such clear glass or plastic. As long as it is clear,
almost any type of container may be used: empty fish bowls, fish tanks,
brandy snifters, old glass jars, jugs, bottles, etc. Additionally, there
are containers specially designed for use as terrariums. All closed
containers should have transparent covers. Containers with small openings
also are satisfactory. Containers with large openings without covers can
be used but will require more frequent watering to maintain the high
humidity needed by some plants.
The growing medium for terrariums also is important. It must be
biologically inert, well drained and high in organic matter. Prepackaged
peat-like mixes sold at garden centers and nurseries or where plant
supplies are sold are excellent choices. Adding fertilizer is not
necessary, since most packaged mixes contain a starter charge of
fertilizer; plus, plants in terrariums should not grow rapidly to keep
from "outgrowing" their container. Light fertilization with a houseplant
fertilizer may be done after plants are established.
Plants are the "jewel in the crown" of terrariums and many different
plants are suitable for use. Plants that have a low and dense growth habit
usually are best. Don’t mix plants requiring widely different conditions.
Terrarium plants differ relative to optimum light and temperature
conditions. Those plants requiring medium light should be placed in good
light near a window, or receive supplemental artificial light. Terrariums
with this type of plants should be placed within several feet of a bright
window, but not in direct sun. Few plants tolerate low light for extended
periods. Those terrarium plants that tolerate low light will tolerate a
location no more than about 10 feet from a bright window. Plants requiring
high light are not frequently used but should be placed close to a window,
often in direct sun. Cacti and succulents are examples of the latter. Do
not put closed containers in full sun.
Since most terrarium plants are tropical in nature they require warm
temperatures. A night temperature of 65 F degrees is ideal for this type
of plant; day temperatures normally should be about 10 degrees higher.
"Cool temperature" is a designation that primarily fits woodland plants in
woodland terrariums. These plants should have night temperatures about 50
to 55 F degrees. In the home these temperatures may be difficult to find,
but placed on a window close to the glass with a drape pulled behind them
at night, a pocket of cool air will develop during the winter. Day
temperatures also should be cool but are not as critical. Table 1 lists
some of the more popular terrarium plants along with their light and
temperature preferences.
When designing the terrarium, combine plants for variation in size, color
and texture. Since terrariums usually are viewed from one side, the soil
should be sloped for viewing from that side. Plants also should be
arranged so that taller plants are toward the back. A low, coarse-textured
plant is often desirable for a dominant focal point near the front. Don’t
build a collection of variegated or unusual plants. They compete with each
other and don’t give a unified pattern. Use rocks, sand, wood and other
natural materials to create cliffs, rock ledges, dry stream beds or lush
tropical forests. Hills and valleys will make the scene more interesting
than a flat surface.
Once all necessary materials have been obtained, it is time to construct
the terrarium. In general, about one quarter of the terrarium’s volume
should be used for the growing medium and drainage material. Charcoal and
pebbles should be placed in the bottom of the container for drainage.
These may be mixed together, but the charcoal usually will be most
effective if placed in a half-inch layer above the layer of gravel,
crushed pots, marble chips or other drainage material. Sphagnum moss,
placed over the layer of gravel and charcoal, prevents the growing medium
from sifting into the drainage area.
Next, add the growing medium. It should be slightly moist so as not to be
dusty, but not too moist that it is sticks to the sides of the container.
For most containers, a growing medium minimum thickness of one and
one-half inches is necessary to provide suffi cient volume.
To assemble the terrarium, take the plants from their pots and remove
extra growing medium to expose the roots. Trim off any leaves that are
yellowed, damaged or show any indication of disease or insect damage. Trim
off some roots from plants that were extremely pot bound. Promptly place
the plant in the container, so that the exposed roots do not dry. In the
closed container, try to keep foliage from touching the sides of the
container. Leaves touching the glass will collect water and be more
subject to rot.
After planting, mist over the plants to wash off any growing medium that
sticks to leaves or sides of the container. If the medium was properly
moist at planting, heavy watering will be unnecessary. The water misted
over the leaves is adequate to settle the medium. Don’t cover the
terrarium, and repeat the misting after one day. Allow the container to
remain open until the foliage has thoroughly dried. Then, if the terrarium
is the closed type, apply the cover. Watch the newly-planted terrarium
closely for several weeks for signs of diseases or other problems.
As stated previously, plants in terrariums should not grow rapidly.
Therefore terrariums seldom need fertilizer. Don’t plan any fertilization
for at least a year after planting. If after the first year the plants are
yellowish and seem to lack vigor without any other apparent problems, a
light fertilization may be necessary. Use a water-soluble houseplant
fertilizer at about one-fourth the rate recommended for normal
houseplants. Do not allow any of this fertilizer solution to be left on
the foliage.
Although a terrarium is designed for growing plants indoors with minimum
care, it is not an inanimate object. Some plants will thrive, others may
die. Occasionally it will become necessary to remove certain plants or add
others. When adding plants, take all precautions described for planting
the new terrarium. It is always possible to add new problems when adding
new plants.
Table 1. Plants suitable for use in terrariums
Common nameScientific nameLightTemp
African violet Saintpaulia spp medium warm
Aluminum plant Pilea cadierii medium warm
Arrow-head plant Syngonium podophyllum medium warm
Artillery plant Pilea microphylla medium warm
Asparagus fern Asparagus plumosus med.- brt.warm
Baby tears Helxine soleirolii medium warm
Begonia Begonia spp medium warm
Bird’s nest SansevieriaSansevieria trifasciata hahniiany warm
Bloodleaf Iresine herbstii bright warm
Chinese evergreen Aglaonema spp low - med.warm
Coral berry Ardisia crispa medium warm
Creeping fig Ficus pumila medium warm
Croton Codiaeum variegatum bright warm
Devil’s ivy (Pothos) Epipremnum aureum medium warm
Dwarf gloxinia Sinningia pusilla medium warm
Dwarf pomegranate Punica granatum nana bright warm
Emerald Ripple Peperomia caperata medium warm
English ivy Hedera helix medium cool
Flame violet Episcia cupreata medium warm
Foam flower Tiarella cordifolia med.- brt.cool
Gold dust dracaena Dracaena godseffiana medium warm
Goldfish vine Columnea microphylla medium warm
Heart-leaved philodendronP. scandens oxycardium medium warm
Maidenhair fern Adiantum cuneatum medium warm
Miniature peperomia Pilea depressa medium warm
Neanthe bella palm Chamaedorea elegans medium warm
Nerve plant Fittonia spp medium warm
Oxalis Oxalis spp bright warm
Piggy-back plant Tolmiea menziesii medium warm
Podocarpus Podocarpus macrophylla medium warm
Pink polka dot Hypoestes panguinolenta bright warm
Prayer plant Maranta spp medium warm
Rosary vine Ceropegia woodii med.- brt.warm
Sander’s dracaena Dracaena sanderiana medium warm
Satin pellionia Pellionia pulchra med.- brt.warm
Spider Aralia Dizygotheca elegantissimamed.- brt.warm
Spider plant Chlorophytum comosum lowmed. warm
Strawberry begonia Saxifraga sarmentosa medium warm
Swedish ivy Plectranthus australis lowmed. warm
Table brake fern Pteris spp medium cool
Tahitian bridal veil Gibasis geniculata medium warm
Var. ovalleaf peperomia P. obtusifolia variegata medium warm
Venus fly trap Dionaea muscipula bright warm
Watermelon peperomia Peperomia sandersii med.- brt.warm

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