NED NASH
This is the second installment of a five-part series that first appeared in the American Orchid Society BULLETIN Vol. 52, 1983. This five-part series, while over 25 years old, still remains an extremely valuable resource for orchid growers. This article has been edited to conform to modern taxonomic nomenclature and the availability of pesticides/insecticides and growing media.
THE SECOND most common problem that people experience with Cattleya culture, after insufficient light, is improper watering. An understanding of the most common habits of Cattleya and related species greatly helps in understanding their watering needs. As discussed earlier, most horticulturally important Cattleya-types live as epiphytes in the middle elevations (2000-5000-feet) of the sub-tropics.
WATERING AND HUMIDITY
There are important lessons to be learned here. The roots of a cattleya, because they are by nature largely exposed and dry quite rapidly, simply will not tolerate extended periods of wetness in culture. Unless the roots are allowed at least to practically dry between wettings, they will rot. The absorptive layer of the roots, the velamen, absorbs water like a sponge. When it is fully charged with moisture, no gas exchange with the atmosphere occurs, and this leads to rot. Although cattleyas are generally grown in pots for convenience and ease of handling, their need for cyclic drying and wetting still remains. We will be discussing potting media and potting later in this series but suffice it to say that a relatively coarse and free-draining medium is necessary for cattleyas.
A coarse potting medium and resulting free drainage help to duplicate the cyclic, wet-dry nature of the plants' native habitat. They also provide the liberal air circulation about the roots that the plants enjoy. Yet the coarsest mix and the best draining pot simply will not help if one is too liberal with watering. Many growers tend to "fuss" over the plants too much. Because of their exotic reputation many people are basically scared by cattleyas. Unfortunately, people seem to be more afraid of under-, than over-watering their cattleyas. Many more plants have succumbed to over-watering than vice-versa, although under-watering does have its attendant problems. One has to remember that cattleyas have evolved to be drought-tolerant and will not tolerate too much water at the roots. Don't go overboard on this advice, either! Practice moderation. Allowing plants to dry between watering does not mean creating a desert for days on end. If a plant is dry, by all means water it!
"Well", you may be asking yourself, "how does one know for sure when a cattleya needs water?" Good question. There are many diagnostic tests one can perform to determine the water content of a pot. First of all, the differential drying rate of plastic and clay pots must be considered. Because clay is porous, it "breathes," hastening the drying of the potting medium it contains. This sweating of moisture from clay pots can give a hint as to whether the plant needs water. Lift the pot. If there is a ring of moisture on the bench under the pot, there is plenty of water in that pot. A trick that works well with both clay and plastic is to insert a freshly sharpened lead pencil about halfway into the medium. If the wood is dark with moisture when the pencil is withdrawn, the plant does not need watering. Today's lightweight potting media in combination with the lightness of plastic pots can give valuable information as to the amount of moisture in the pot. Lift a plant in a plastic pot immediately after you have watered it. It will be relatively heavy. Lift that same pot immediately before you plan to water. If the plant really needs to be watered, it will be quite light. With a little practice, the weight and balance of a plant in a plastic pot can tell you just how much water is left in the pot and will enable you to estimate when the plant will need water again.
Obviously, very few of us have either the time or the inclination to go to each plant in our collection and test it individually. However, if the plants are set up in "size-place" (pots of the same size together), and one remembers the intervals between watering for a given pot size, the initial learning period can be greatly reduced. It is a good idea to double-check your presumptions on watering intervals every so often so as not to become complacent.
Seasonal influences are the primary factors affecting watering intervals. Areas like Florida and Hawaii experience seasonal variations in day-length and the amount of cloud cover or rainfall, however slight. As one moves away from the equator, these factors become increasingly important. Plants generally require less water during the winter months, owing to shorter and cooler days which slow the plants' growth rate. (Although less pronounced today because of the complex nature of most hybrids grown, a very dry rest during winter was quite necessary to the health of Cattleya species grown years ago.) The extent to which watering frequency is reduced will depend on the severity of the winter in your area. Plants whose growth is slowed by a reduction of total insolation (short days and considerable cloud cover) require less water and will remain wet longer between watering. Not only do the plants actually use much less water, but the normal evaporation from the pot is also slowed by the lower, average temperatures. This is one of the primary reasons that cold-climate greenhouses require better air circulation than warm-climate greenhouses. The increased air movement aids the evaporative process, thereby helping the pots to dry more quickly.
Conversely, warmer and longer days, with the increased sun they bring, will necessitate more frequent watering. Not only are the plants in active growth with many new roots forming, but the increased admission of fresh air evaporates moisture from the pots more quickly as well. It is especially important to take advantage of this quickening of growth in northern or cooler areas. The growing season can be quite short in these areas, and it is important to utilize it efficiently. This means monitoring the plants' water needs closely so as to avoid slowing their growth by insufficient water. This is not the contradiction it may seem. As I stated earlier, "If a plant needs water, water it!" Only when the plants' water needs are met will they perform to their true potential.
Adequate humidity goes hand-in-hand with proper watering practices. In nature, cattleyas are of course subject to fluctuating humidity. This is moderated by the creation of a micro-climate in forested areas by the collective transpiration of the resident plants. At night, the forest's atmosphere may be saturated with moisture as the temperature drops. The species will experience relative humidity in the range of ca. 20% to 100%. More typically, the range would be 40-80%. This is the range they will prefer in your greenhouse.
A brief explanation of relative humidity will be relevant here to help understand its relationship to other environmental factors. A given volume of air will hold a fixed amount of water vapor at any given temperature. For example, a cubic liter of air will hold 10 ml of water vapor at 70F with a 100% relative humidity (RH). (These figures are for the sake of discussion only.) If there are only 5ml of water vapor in that liter of air at 70F, we say the RH is 50%, as the volume of air only contains one-half the moisture it potentially can. Cool air will hold less moisture than warm. If we take that same liter of air and warm it 10F to 80F, it can hold more water vapor than it could have at 70F. We will assume that it can hold twice as much, or 20 ml. Our test liter of air at 70F with 5 ml of water vapor and at 50% RH, if warmed to 80F, will be at 25% RH as it holds only one-fourth of the water vapor that it potentially could.
In a closed greenhouse, we are dealing with nearly fixed volumes of air and water vapor. It is easy to see the effect that temperature change will have on relative humidity. The effect can be neatly explained as an inverse relationship. That is, RH drops as temperature rises, and vice-versa. Conversely, a plant's humidity needs are in direct relation to temperature. With rising temperatures, more humidity is necessary to prevent the plant's transpiration rate from outstripping its water supply, causing subsequent stress. At night, or on cool, sunless days, the air may reach 100% RH as the cool air has less potential water capacity. This is a rather dangerous situation if left too long, as a saturated atmosphere is highly conducive to fungal and bacterial problems.
In real life, however, a truly "closed" greenhouse is seldom achieved. Whether or not it is even advantageous is open to debate. Vents, doors, uncaulked seams - all do their part in altering the RH in a greenhouse by allowing interchange with the outside atmosphere. We like to at least crack the vents on warm days, as this allows better air circulation. However, this also serves to lower the RH as moist air escapes through the open vents. Damping-down, the wetting of floors and benches, is a good solution. This adds to the potential amount of water vapor in the house. A good test for dryness is to observe whether the greenhouse floor is dark with moisture or light-colored and dry.
Young seedlings require more water and humidity as they do not have the well-developed, water-storage organs of larger plants. More frequent watering is called for. If you are fortunate enough to be home to tend your plants during the day, the seedlings may be lightly misted as necessary. For those of us who must have a job to support our orchid (and other) habits, here is a good trick. After the seedlings are deflasked and planted into community- or corn-pots, bend a wire loop approximately ten inches across and insert it into the pot(s). Place a clear plastic veggie bag over the loop and presto! - you have a "mini-greenhouse." The bag need only be removed for watering. I find about 10 weeks "in the bag" is enough to establish young seedlings and to partially adapt them to outside life.
FERTILIZING
The substrates upon which cattleyas grow in nature, trees and rocks, provide few if any nutrients. Cattleyas do, however, enjoy a steady source of fertilizer from many external environmental sources. Birds and other animals leave their droppings behind, leaf detritus collects around the base of the plants, and nutrient solutions are washed over the plants from above by the frequent rains. We can draw important conclusions from this information that will apply not only to cattleyas but to other epiphytic orchids as well. Since the nutrients are supplied sparingly by outside factors, we can infer that cattleyas are moderate, not heavy feeders. Because the nutrients are being supplied relatively constantly, generally with precipitation, cattleyas will prefer more frequent, but lighter doses of fertilizer. Last, although the nutrients are being provided in a more unorthodox manner than with terrestrial plants, the nutrients are supplied in a more or less "balanced" feed solution.
How do these conclusions relate to successful Cattleya culture under artificial conditions? We have already discussed the preference of cattleyas for more frequent feedings of moderate strength because this more closely duplicates the conditions they are used to. We orchid growers eat two to three times a day in moderate amounts rather than gorging once a week (at least I think so). Here at our nursery we fertilize at every watering with one-half the recommended dosage of our custom-blend fertilizers. It is acceptable to feed full strength every two weeks — if one is lazy or pressed for time — but more frequent feedings definitely enhance both growth and flowering.
Organic media utilizing fir bark were experimented with in England many years ago, but they were considered unsatisfactory. Because growers were so used to osmunda and never fertilized, it was felt that orchids did not require additional feeding. Plants grown in fir bark and not fertilized do poorly indeed, as the bark requires nitrogen as well as the plant. Interest in fir bark media was renewed in the early 1950's in California because of a ready and cheap supply. A fertilizer suitable for use with these types of media was developed by O. A. Matkin. It was found that these types of media require a relatively high ratio of nitrogen, as the organisms present in the medium that break it down require nitrogen and will take it from the plant if sufficient is not provided. For this reason, organic mixes containing fir bark or tree fern are watered with a fertilizer balanced to a 3-1-1 or 3-1-2 ratio; the two "extra" parts of nitrogen are for the mix, leaving a truly "balanced" 1—1—1 feed solution. We use one-half strength 30-10-10 every watering, altering with a low nitrogen 10-30-30 fertilizer every fourth watering. The low nitrogen fertilizer helps to encourage root and flower production (phosphorus and potassium do this) and to harden the plant somewhat. A point should be made here in regard to commercially prepared, balanced fertilizers. A bit of study will show that most, if not all of these growth formulations prepared for fir bark growing have the basic 3-1-1 or 3-1-2 ratio (i.e., 30-10-10, 27-9-18, etc.). Since the nutrients are derived from the same salts no matter which company blends them, there is really very little difference between commercial preparations. Distinctive dyes are usually added to differentiate products, but the dye is really most important as an indicator that the fertilizer solution is coming through the hose [Editor's note:  Since this article first appear a number of studies related to fertilizer needs have been conducted.  We now know that optimum fertilizer is directly related to water quality and that excess nitrogen is not as important as first thought since the excess nitrogen contributes to the decomposition of the potting medium.  We also know now that urea nitrogen is not efficiently used by orchids and that urea-free fertilizers are a better choice.]
A situation similar to that of the 1940's and 1950's now exists. The supply of good quality fir bark (remember those halcyon days of kiln-dried Weyerhauser bark?) and tree fern is declining rapidly, much as osmunda fiber did thirty years ago. For this reason, many growers are now experimenting with inorganic media. Because inorganic media generally provide no nutrients, culture in them is essentially hydroponic. That is, the medium provides only support and water retention. A truly balanced fertilizer is required [urea-free fertilizers are especially important here]. There are many good commercially prepared products available. It is important that the fertilizer be designed specifically for this type of use, as one can run the risk of under- or over-fertilizing with resulting harm to your cattleyas.
Part 3 of this series on Cattleya culture will deal more specifically with potting and potting media. —Armacost & Royston, 3376Foothill Road, Box 385, Carpinteria, California 93013.
