by : Robert J. Black from www.edis.ifas.ufl.edu
Fertilization

Fertilization is a controversial issue among orchid growers. Many debate the merits of inorganic and organic fertilizers and some even question the need for fertilizer. However, most growers use fertilizer and obtain excellent results. Research and experience have shown that if orchids are grown in tree fern, osmunda, peat, charcoal, or stone (which are slow to decompose or inert) a complete fertilizer with a 1-1-1 ratio should be used. However bark decomposes rapidly because it supports high populations of wood decaying microorganisms. These microbes do not injure plants but compete for available nitrogen; therefore, a 3-1-1 ratio fertilizer is recommended to compensate for this problem. Both 1-1-1 and 3-1-1 fertilizers are available in soluble, dry or slow release formulations. Fertilizers with the same basic ratio may be interchanged, but be sure to adjust for changes in concentration. For instance, a recommendation may be given in terms of 20-20-20, i.e. 1 or 1-1/2 tsp./gallon, but a 10-10-10 fertilizer may be all that is available. Use the 10-10-10, but keep in mind that this material is half as concentrated as 20-20-20 and compensate by using 2 to 3 tsp./gallon.

Soluble fertilizers are dissolved in water and applied in place of a normal watering. If 20-20-20 is used to fertilize orchids growing in osmunda, tree fern, charcoal, stone or peat, mix at the rate of 1 to 1-1/2 tsp./gallon of water. Plants grown in bark should be fertilized with a 30-10-10 or similar high nitrogen material at the same rate. Apply soluble fertilizers at monthly intervals.

Slow release type fertilizers, such as Osmocote, MagAmp, Pro-Gro. etc., release nutrients very slowly, the rate of release being dependent on temperature. Although their initial cost is higher, one application every two to three months is adequate and plants will be continuously fertilized during that time. Osmocote 14-14-14 should be applied at rates of 1 tsp. per 6″ pot every two to three months.

Dry materials, such as 20-20-20 garden fertilizer, can also be used for orchids at rates of 1 tsp. per 6″ pot applied monthly. Again additional nitrogen is needed for plants growing in bark.

Watering

How often should orchids be watered? This is a common question to which there is no one set answer. Watering frequency depends on such factors as pot size and type (plastic or clay), medium, location of the pot (hanging or bench), size of the plant in the pot, air circulation, shade levels, and general environment in the growing area. As a general rule, plants grown in small pots dry more rapidly than those grown in large pots, thus requiring water more frequently. Also plants grown in porous clay pots should receive water more often than those grown in plastic pots. When watering, saturate each pot so that moisture drains from the bottom of the pot and then do not water again until the surface of the medium becomes dry.

Water quality concerns many orchid growers, particularly those living in coastal areas. Salt water intrusion into freshwater wells is common along both coasts. Orchid growers should be aware that water with salt levels in excess of 875 ppm is detrimental to the growth of orchids and should not be used. Water is of good quality when the salinity level is less than 500 ppm and no special precautions are needed. However if salt levels range between 525 and 875 ppm, be sure to water thoroughly and leach heavily at each watering to wash residual salts from the medium. Use rainwater collected by placing large containers under downspouts from roofs.

Light

Most orchids require partial shade for optimum growth and flowering. High light intensities degrade chlorophyll causing the foliage to yellow, and frequently may burn the leaves. Recommended light levels for many varieties of Cattleya , Dendoribium , Oncidium , Vanda and Epidendrum range between 2000 and 3000 footcandles (21.6-32.4 K. Lux), or 70-80% shade. These shade levels may be obtained by several means including: (1) growing plants under saran cloth, (2) growing plants in the shade of trees. (3) shading the greenhouse with whitewash, and (4) growing under eaves of houses. Phalaenopsis , however, thrives when light levels are 1000-1800 footcandles (10.8-19.4 K. Lux), or 85-90% shade. Phalaenopsis leaves burn easily when subjected to high light intensities so shade should be provided at all times. Reed-stem epidendrums and terete leaved vandas grow in full sun and in south Florida are planted in open beds outdoors.

Temperature

Orchids need to be protected from cold temperatures. Generally 50°F (10°C) is considered the minimum temperature to which tropical orchids can be subjected without exhibiting symptoms of cold damage. Phalaenopsis and Vanda prefer night temperatures of 65-70°F. (18-21°C). Cattleya , Epidendrum , Oncidium and Dendrobium generally grow best when the night temperature is maintained between 60-65°F. (15-18°C), although these may recover when exposed to temperatures in the mid-30s (2-5°C) for short durations.

Few hobbyists are concerned with maximum temperatures. However, if orchids are grown in a greenhouse, be aware that temperatures of 110°F. (43°C) can occur under glass during the hot summer months and two hours exposure to 110°F. (43°C) and 10 minutes to 120°F. (40°C) will burn leaves and flowers.

Insects and Diseases

Orchids, like other plants, are susceptible to a number of insect and disease problems. However, in comparison with other ornamental pot plants, orchids are surprisingly less affected by them. Should pest problems arise, contact your local county extension office for a diagnosis and control.