[Selecting an Interior
Plant: selecting | environmental factors |
SELECTING AN INTERIOR PLANT[continued]
Ch. 9, pp. 4 - 9
Light, water, temperature, humidity, ventilation, fertilization,
and soil are chief factors affecting plant growth, and any one of
these factors in incorrect quantity will prevent proper plant
Light is probably the most essential factor for indoor plant
growth. The growth of plants and the length of time they remain
active depend on the amount of light they receive. Light is
necessary for all plants because they use this energy source to
photosynthesize. When examining light levels for tropicals,
consider three aspects of light: intensity, duration, and quality.
Light intensity influences the manufacture of
plant food, stem length, leaf color, and flowering. A geranium
grown in low light tends to be spindly and the leaves light green
in color. A similar plant grown in very bright light would tend to
be shorter, better branched, and have larger, dark green leaves.
Indoor plants can be classified according to their light needs by
high, medium, and low light requirements. The intensity of light
a plant receives indoors depends upon the nearness of the light
source to the plant (light intensity decreases rapidly as you move
away from the source of light). The direction the windows in your
home face will affect the intensity of natural sunlight that
plants receive. Southern exposures have the most intense light,
eastern and western exposures receive about 60% of the intensity
of southern exposures, and northern exposures receive 20% of a
southern exposure. A southern exposure is the warmest, eastern and
western are less warm, and a northern exposure is the coolest.
Other factors which can influence the intensity of light
penetrating a window are the presence of curtains, trees outside
the window, weather, seasons of the year, shade from other
buildings, and the cleanliness of the window. Reflective
(light-colored) surfaces inside the home/office will increase the
intensity of light available to plants. Dark surfaces will
decrease light intensity.
Day length or duration of light received by
plants is also of some importance, but generally only to those
plants which are photosensitive. Poinsettia, kalanchoe, and
Christmas cactus bud and flower only when day-length is short (11
hours of daylight or less). Most flowering indoor plants are
indifferent to day-length.
Low light intensity can be compensated by increasing
the time (duration) the plant is exposed to light, as long as the
plant is not sensitive to day-length in its flowering response.
Increased hours of lighting allow the plant to make sufficient
food to survive and/or grow. How-ever, plants require some period
of darkness to develop properly, and thus should be illuminated
for no more than 16 hours. Excessive light is as harmful as too
little light. When a plant gets too much direct light, the leaves
become pale, sometimes burn, turn brown, and die. Therefore,
during the summer months, protect plants from too much direct
Additional lighting may be supplied by either
incandescent or fluorescent lights. Incandescent lights produce a
great deal of heat and are not very efficient users of
electricity. If artificial lights are to be used as the only
source of light for growing plants, the quality of light
(wavelength) must be considered. For photosynthesis, plants
require mostly blues and reds, but for flowering, infrared light
is also needed. Incandescent lights produce mostly red, and some
infrared light, but are very low in blues. Fluorescent lights vary
according to the phosphorus used by the manufacturer. Cool-white
lights produce mostly blue light, and are low in red light.
Foliage plants grow well under cool-white fluorescent lights,
which are also cool enough to position quite close to plants.
Blooming plants require extra infrared which can be supplied by
incandescent lights, or special horticultural-type fluorescent
Overwatering and underwatering account for a large percentage of
tropical plant losses. The most common question gardeners ask is,
How often should I water my plants? There is not a
good answer to this question. Some plants like drier conditions
than others. Differences in potting medium and environment
influence water needs. Watering as soon as the soil crust dries
can result in overwatering.
Plant roots are usually in the bottom 2/3 of the pot,
so do not water until the bottom 2/3 starts to dry out slightly.
You can't tell this by looking. You have to feel the soil. For a
6-inch pot, stick your index finger about 2 inches into the soil
(approximately to the second joint of your finger). If the soil
feels damp, don't water. Keep repeating the test until the soil is
barely moist at the 2-inch depth. For smaller pots, 1 inch into
the soil is the proper depth to measure.
Water the pot until water runs out of the bottom. This
serves two purposes. First, it washes out all the excess salts
(fertilizer residue). Second, it guarantees that the bottom 2/3 of
the pot, which contains most of the roots, receives sufficient
water. However, don't let the pot sit in the water that runs out.
After a thorough watering, wait until the soil dries at the 2-inch
depth before watering again.
When you test for watering, pay attention to the soil.
If your finger can't penetrate 2 inches deep, you either need a
more porous soil mix or the plant is becoming root-bound.
Most house plants tolerate normal temperature fluctuations. In
general, indoor foliage plants grow best between 70° and 80°
F during the day and from 60° to 68° F at night. Most
flowering indoor plants prefer the same daytime range but grow
best at nighttime temperatures from 55° to 60° F. The
lower night temperature induces physiological recovery from
moisture loss, intensifies flower color, and prolongs flowerlife.
Excessively low or high temperatures may cause plant
failures, stop growth, or cause spindly appearance and foliage
damage or drop. A cooler temperature at night is actually more
desirable for plant growth than higher temperatures. A good rule
of thumb is to keep the night temperature 10 to 15 degrees lower
then the day temperature.
|A layer of gravel or pebbles increases the
|Another way of controlling moisture is to
water sphagnum peat moss around the smaller pot.
Atmospheric humidity is expressed as a percentage of the moisture
saturation of air. To provide increased humidity, attach a
humidifier to the heating or ventilating system in the home, or
place gravel trays (in which an even water level is maintained)
under the plant containers. This will increase the relative
humidity in the vicinity of the containers. As the moisture around
the pebbles evaporates, the relative humidity is raised.
Another way to raise humidity is to group plants close
together. You can also spray a fine mist on the foliage, although
this is of doubtful effectiveness for total humidity modification.
Do this early in the day so that the plants will be dry by night.
This lessens the chance of disease, since cool dampness at night
provides an ideal environment for disease infection.
Indoor plants, especially flowering varieties, are very sensitive
to drafts or heat from registers. Forced air dries the plants
rapidly, overtaxes their limited root systems, and may cause
damage or plant loss. Plants are sensitive to natural or blended
gas. Some plants refuse to flower, while others drop flower buds
and foliage when exposed to gases. Blended gases are more toxic to
plants than natural gases. Tomato plants are extremely sensitive
to gas. They will turn yellow before the escaping gas is detected
by household members, and are sometimes used in greenhouses as
indicator plants for excessive ethylene gas (resulting from
incomplete combustion in gas furnaces).
Indoor plants, like most other plants, need fertilizers containing
three major plant food elements: nitrogen (N), phosphorus acid
(P), and potassium (K). They are available in many different
combinations and under a multitude of brand names. Each brand
should be analyzed on the label, indicating specifically how much
water- soluble elemental nitrogen, phosphate, or potash is
available in every pound of the product.
The majority of these fertilizers are about 20-20-20.
The first figure indicates available nitrogen; the second,
available phosphate; and the third, water-soluble potassium.
Commercial fertilizers used for indoor plants are sold
in granular, crystalline, liquid, or tablet forms. Each should be
used according to instructions on the package label.
Frequency of fertilizer application varies somewhat
with the vigor of growth and age of each plant. Some need it every
2 weeks, while others will flower well for several months without
needing any supplement. As a general rule, fertilize every 2 weeks
from March to September. During the winter months, no fertilizer
is needed because reduced light and temperature result in reduced
growth. Fertilizing at this time could be detrimental to some
Reduced growth, brown leaf-tips, dropping of lower leaves, small
new growth, dead root-tips, and wilting are all signs of high
soluble salts. These salts will accumulate on top of the soil
forming a yellow to white crust. A ring of salt deposits may be
formed around the pot at the soil line or around the drainage
hole. Salts will also build up on the outside of clay pots.
Soluble salts are minerals dissolved in water. Fertilizer
dissolved in water becomes a soluble salt. When water evaporates
from the soil, the minerals or salts stay behind. As the salts in
the soil become more and more concentrated, plants find it harder
and harder to take up water. If salts build to an extremely high
level, water can be taken out of the root-tips, causing them to
High soluble salts damage the roots directly, and
be-cause the plant is weakened, it is more susceptible to attack
from insects and diseases. One of the most common problems
associated with high salt levels is root rot.
The best way to prevent soluble salt injury is to stop
the salts from building up. Water correctly. When you water, allow
some water to drain through, and then empty the drip plate. Water
equal to 1/10 the volume of the pot should drain through each time
you water. DO NOT ALLOW THE POT TO SIT IN WATER. If you
allow the drained water to be absorbed by the soil, the salts that
were washed out are taken back into the soil. Salts can be
reabsorbed through the drainage hole or directly through a clay
Plants should be leached every 4 to 6 months. You
should leach a plant before you fertilize, so that you don't wash
away the fertilizer you just added. Leaching is accomplished by
pouring a lot of water on the soil and letting it drain
completely. The amount of water used for leaching should equal
twice the volume of the pot. A 6-inch pot will hold 10 cups of
water, so 20 cups of water are used to leach a plant in a 6-inch
pot. Keep the water running through the soil to wash the salts
out. If a layer of salts has formed a crust on top of the soil,
you should remove the salt crust before you begin to leach. Do not
remove more than 1/4 inch of soil. It is best not to add more soil
to the top of the pot. If the soluble salt level is extremely high
or the pot has no drainage, repot the plant.
The level of salts that will cause injury varies with
the type of plant and how it is being grown. A plant grown in the
home may be injured by salts at concentrations of 200 ppm. The
same plant growing in a greenhouse, where the light and drainage
are good, will grow with salts at 10 times that level, or 2,000
ppm. Some nurseries and plant shops leach plants to remove excess
salts before the plant is sold. If you are not sure that has been
done, leach a newly purchased plant the first time you water it.