House Plants

1. Q. Surely certain, obscure, environmental factors could have an influence on houseplant health?

A. Researchers at the University of Florida have discovered that mercury found in some indoor paints is highly toxic to two of the most popular foliage plants, Ficus benjamina and Dieffenbachia. When plants are exposed to mercury-containing paints used as a mildew retardant, severe leaf drop problems occur.

2. Q. Will houseplants purify air or do they just add oxygen?

A. According to scientists at the National Space Technologies Lab in Bay St. Louis, Miss., several houseplants appear to have air-purifying qualities. These plants, including spider plant, golden pothos, peace lily and Chinese evergreen, can cleanse the air of carbon monoxide, nitrogen dioxide and formaldehyde.

In a laboratory test, a spider plant placed in a sealed chamber filled with formaldehyde gas reduced the concentration by 85 percent within 24 hours. In an average-sized house, as few as 15 plants might significantly cleanse the air.

3. Q. What is "Filtered Sunlight" ?

A. Generally when someone recommends that you put a plant in filtered sunlight it means that the plant needs a south or east exposure. For house plants, this means it ought to be reasonably close to the window. For outdoor plants it means some light shading overhead would be preferable. Your hand can measure light intensity reasonably well in your home. Pass it over the plant and in front of the light source. If you see a sharp, clear shadow the light is high in intensity. A softer, more diffused shadow means medium light, while a faint shadow shows low light.

4. Q. How should houseplants which have been outdoors during the summer be prepared to move into the house?

A. Houseplants often grow rapidly during our hot, humid summers and may have a gangling, unkempt appearance by fall. To re-establish their compact form, prune any spindly or errant branches. Herbaceous plants can simply be pinched back, but use pruning shears on woody selections. Be sure to remove any stems or leaves that are dead or diseased.

Repot those plants whose roots are growing out of drainage holes or coming to the surface of the soil. For others, repotting is best postponed until spring. Use a container that is only one size larger than the old one' otherwise, top growth may be retarded while the roots try to fill up the extra space. If repotting isn't necessary, be sure to clean the outer surfaces of your containers. After a summer outside, they may have to be scrubbed with a brush.

Next, clean the leaves. Accumulations of dust and dirt can interfere with a plant's ability to turn light energy into food. A gentle spray from the garden hose and a light wipe with a sponge are usually adequate for this job.

Even if you haven't seen any bugs on the plants, an application of insecticidal soap is a wise precaution. Spider mites and other pests thrive in a dry, heated house, and they multiply rapidly. What was once a minor infestation could sweep through all your plants in no time. If your plants have an insect problem, be prepared to reapply the insecticidal soap as indicated on the label.

Once plants are inside, locate them where they will receive as much natural light as possible. When you water, do so thoroughly--but less often during winter. And fertilize sparingly. For most foliage houseplants, soluble fertilizer given once a month should be plenty until spring.

5. Q. When should a plant be repotted? Is there a certain time interval or does size of plant determine when repotting should occur?

A. Generally, plants are sold in containers or pots of adequate size to support the plant. However, vigorously growing plants will require repotting every year or two. With regular fertilization and thorough watering practices, plant roots require little growing media to support top growth. Some signs to look for which could indicate that your plant needs repotting are:

  • Decreasing leaf size on new growth
  • Plant wilts rapidly after watering
  • Roots appear at soil surface
  • Lower leaves turn yellow.
It is essential that all plants are potted in a well-drained growing medium mix. If there are no drainage holes in the pot, provide a drainage layer of about one inch of horticultural charcoal, when transplanting. In the meantime, you may find that pots without drainage holes need less frequent watering. The new pot should be approximately two inches wider and deeper than the original. Growing media should be of equal parts of some or all of the following: peat moss, vermiculite, perlite, bark or sand. This combination allows for a light, loose, airy mixture that breathes. At the same time, the peat moss helps retain uniform moisture so the growing medium does not dry out too quickly. A factor often neglected is that roots need oxygen. Avoid heavy, packed, clay-like soils. When positioning the plant in the new pot be sure to leave at least a one to one and one- half inch reservoir at the top. This facilitates easy watering and reduces over-flow. Also take care in potting the plant to the original depth. Plants potted too deep often develop diseases at the ""soil line.'' Lastly, water the newly repotted plant thoroughly. This helps stabilize the growing medium and also provides moisture for the stressed root system.

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