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Growing Requirements
by David Horak
Fertilizing Orchids
It used to be assumed that as epiphytes, or air plants, orchids did not need fertilizing because in nature the plants are commonly attached to tree limbs with little or no humus or other obvious organic source of nutrition available. While we now know this is not the case, the nutritional needs of many orchids are still a mystery to orchid growers.
Much of what plants need in order to thrive is readily available in the growing medium or the environment, but there are 13 mineral nutrients that become depleted and must be replenished on a regular basis. The three primary macronutrients, nitrogen (N), phosphorus (P), and potassium (K), often shortened to NPK, are usually indicated in a fertilizer as a numerical ratio such as 30-10-10, 20-20-20, or the like. The three numbers indicate the percentage of each nutrient in the formula. In addition, plants need smaller quantities of other minerals, such as the secondary macronutrients calcium (Ca), magnesium (Mg), and sulfur (S), and seven micronutrients, or what are frequently termed trace elements, boron (B), copper (Cu), iron (Fe), chloride (Cl), manganese (Mn), molybdenum (Mo), and zinc (Zn).
All orchids, including Catasetum Orchidglade, need to be lightly fertilized. Flush plants occasionally with plain water to avoid the buildup of mineral salts in the pots, which may damage the root tips.
Specific fertilizer compositions and regimens vary tremendously from grower to grower. Inorganic fertilizers that use combinations of mineral salts, soluble in water, are the easiest to use and most commonly available. Manufacturers offer a variety of different formulations specific to the needs of different plants in either liquid or powder form.
In the early days of orchid cultivation, osmunda fiber was the medium of choice; it required almost no additional nutrients to succeed with orchids. But for the past 40 years or so, as fir bark has become the most common growing medium, regular supplemental fertilizing has been necessary since the bark provides little in the way of nutrition for the plants as it breaks down. Some growers fertilize at full strength every fourth watering when the plants are actively growing. I support the time-worn adage of feeding "weakly weekly." Orchids are generally considered to be light feeders. So, when orchids are actively growing, commercially available fertilizers should be used at about a quarter of their recommended strength for three successive waterings. The fourth should be a pure water rinse to flush out any soluble salts that have accumulated. This is referred to as "constant feeding." For many years, good general advice has been to begin fertilizing in the spring when new growth appears using a fertilizer such as 15-30-15 about once a week. This fertilizer helps get the plants started. By mid-May to June, as the plants begin to really grow, move to a high-nitrogen formula such as 30-10-10, which provides an abundance of nitrogen for rapidly growing foliage and compensates for any nitrogen that may be taken up by the bacteria that break down the bark. In late summer or early fall, as the growths are maturing switch to a low-nitrogen, high-phosphorus, and potassium formula such as Peter's 10-30-20 Blossom Booster, used to encourage flowering. By October reduce fertilizer applications to once a month or less until spring. This approach suits many growers well. Others have adopted a simpler solution. They use a balanced formula such as 20-20-20 or 17-17-17 at a rate schedule similar to that outlined above, since excess nutrients are not utilized by the plants. Both of these commonly used approaches have served growers well for many years.
It has long been thought that high phosphorus content in fertilizer is responsible for stimulating and increasing blooming in orchids. However, recent research at Michigan State University, published in Orchids magazine in June 2003, suggests that the phosphorus needs of orchids are relatively low and that increasing phosphorus levels can increase blooming only if there is a preexisting deficiency. In general, it is the decrease in excess nitrogen that is responsible for more abundant blooming. Since water quality is so important to the way plants utilize nutrients, the researchers at MSU have developed two formulas—one for mineral-rich, or hard, water such as most well water (19-4-23), and a second for relatively pure water such as RO (13-3-15). The MSU researchers' approach does not so much contradict tradition as it clarifies what actually happens in the plants. The results achieved in growth and flowering with the MSU orchid collection have so far been remarkable and compelling. The bottom line is that orchids need supplemental nutrition and that there are many ways to address their needs.
Judicious application of fertilizer has played its part in bringing Miltonia Les Chines 'Linda Hara' AM/AOS to flower. For beginners it's best to use a fertilizer specifically developed for orchids and a widely accepted application schedule, as fertilizing with too strong a dosage or too often can cause leaf-tip burn or dieback.
In general it is best to use fertilizers formulated with ammoniac or nitrate sources of nitrogen rather than urea, which does not break down as readily under normal orchid-growing conditions and can be washed out of a pot before it becomes available to the plant. This information is available on the labels of most fertilizers in the form of percentages and can be useful in deciding whether to purchase a particular brand. Giving too much nitrogen throughout the year generally results in a number of problems: Lush green growths that may be too soft and can't support themselves well are more susceptible to disease and either inhibit or prevent flowering.
Regardless of the specific fertilizer or formulation, it is important to consider the other nutrients besides nitrogen, phosphorus, and potassium. Many fertilizers now include all the necessary macronutrients and micronutrients. For those that do not, fertilizers such as Peter's 15-5-15 Cal Mag or micronutrient supplements such as Peter's S.T.E.M. can be used to provide these elements. If you grow just a few plants, try distributing crushed oyster or eggshells on top of the medium or working it into the medium to provide calcium. You can use Epsom salts at the rate of one teaspoon per gallon of water once a month to provide magnesium. Or sprinkle a very small amount of crushed dolomitic limestone on top of the mix to provide both nutrients.
Some growers prefer natural fertilizers such as diluted fish emulsion or composted manure, soaked in water and strained. Both provide the necessary macronutrients as well as trace elements. Disadvantages are a fixed NPK ratio and foul odor.
Fertilizer applied in liquid form while watering tends to be the most controllable and convenient method of feeding. Solid fertilizers such as the time-release types Nutricote and Osmocote provide constant amounts of fertilizer at every watering over a specific period of time, determined by the rate at which the fertilizer breaks down. These are not recommended for general use on all orchids as they can deliver too much fertilizer and burn leaves and roots. They are often a useful supplement to a regular fertilizer program for heavy feeders that grow quickly such as cymbidiums, lycastes, and catasetums. There are numerous other supplements such as vitamins and silicon that are passionately advocated by some growers. There are constantly new fads in fertilizers and regimens, but beginning orchid growers are probably best served with a fertilizer specifically developed for orchids and a widely accepted application schedule.
David Horak has been growing orchids for more than 25 years. He is the curator of Orchids and the Robert W. Wilson Aquatic House at Brooklyn Botanic Garden. He is the current president of teh Greater New York Orchid Society and the chairman of the New York International Orchid Show.
Photographs: Charles Marden Fitch