CONTAINER OPACITY AND MEDIA COMPONENTS INFLUENCE ROOTING OF POTTED PHALAENOPSIS AND DORITAENOPSIS ORCHIDS

Abstract

Two experiments were performed to determine the effects of container opacity and different media components on rooting and vegetative growth of several clones of Phalaenopsis and one clone of Doritaenopsis hybrid orchids. Doritaenopsis White Moon and Phalaenopsis Sharon Bay were grown in 12 cm translucent or opaque pots containing a bark-based media. After 30 weeks at 29°C, plants in opaque pots had formed >7 roots outside of each container, whereas <2 roots per pot had developed outside the translucent containers. In a second experiment, Phalaenopsis Brother Showpiece, Brother Wild Thing, and Pink Twilight were grown in opaque 12 cm plastic pots containing different ratio of medium-grade Douglas fir bark, medium-grade coconut coir, long-fibered sphagnum moss, coarse perlite, chunky peat, and fine-grade charcoal. Plants were grown for 33 weeks at 29°C with a maximum light intensity of 300 μmol m s. In general, plants grown in a medium consisting of 1:1 (by volume) chunky peat:perlite or 2:1:1 perlite:chunky peat:coconut coir had fewer aerial roots than plants grown in a bark-based or 1:1 perlite:charcoal media. In addition, plants grown in the chunky peat:perlite mix or chunky peat:charcoal mix had the greatest increase in leaf span compared to the other medias. These studies provide scientific evidence for the advantages of translucent pots, and also indicate that chunky peat can be used as an alternative to a bark-based media. INTRODUCTION Phalaenopsis orchids have thick roots that arise from the base of the stem or lower nodes, functioning as one of three types: aerial, prostrate epiphytic, or substrate (Christenson, 2001). The primary role of substrate roots is for water and nutrient absorption, while the primary role of aerial and epiphytic roots is for adhesion to surfaces (Dycus and Knudsen, 1957). Although the apical meristem of aerial and epiphytic roots contains chlorophyll and can photosynthesize, the amount of carbon assimilation is less than the amount of carbon evolved through respiration (Dycus and Knudsen, 1957; Hew and Yong, 1997). Water and nutrient absorption through aerial and epiphytic roots is insignificant for plant growth unless these roots come in contact with the substrate (Dycus and Knudsen, 1957). Thus, aerial and epiphytic roots that do not penetrate the growing medium have limited function during commercial orchid production. An increasing majority of commercial orchid growers throughout the world use translucent or clear containers for Phalaenopsis production. Clear containers allow light to penetrate, which is believed to promote the entry of aerial and epiphytic roots into the substrate (Wang et al., 2007). There are two advantages of facilitating the entry of roots into the growing medium: 1) water and nutrient absorption is greater since more roots are in contact with the media (Wang and Greg, 1994), and 2) fewer roots are damaged during handling by the producer and consumer. However, to our knowledge, no published scientific studies have compared the use of clear and opaque containers for orchid production. Commercial growers of Phalaenopsis and Doritaenopsis orchids use many different organic and inorganic materials to formulate the growing media, including 115 Proc. IW on Ornam. Plants Eds.: N Chomchalow and V. Chantrasmi Acta Hort. 788, ISHS 2008 activated charcoal, aliflor, chopped coconut husks and fiber, cork, diatomite, Douglas fir bark, expanded clay, expanded polystyrene, osmunda fiber, peat, perlite, pumice, redwood bark, rock wool, sphagnum moss, tree fern fiber, vermiculite, volcanic rock, and washed gravel (Baker and Baker, 1991; Cullina, 2004; Slump, 2004; Wang, 2005). Among all of these materials, the primary orchid medium component used in the United States is Douglas fir bark, which is usually harvested by the logging industry on the Pacific coast of the United States (Wang, 2000). Unfortunately, Douglas fir bark has several shortcomings: 1) fresh bark has a low water-holding capacity and a high carbon to nitrogen ratio; 2) smallor medium-grade bark decomposes rapidly, resulting in high water retention; 3) it has low nutrient retention and poor uniformity; and 4) some people consider fir bark to be a non-renewable resource. In addition, the availability of Douglas fir bark could be limited in the future because it may host the fungus Phytophthora ramorum, which causes sudden oak death disease (Wang and Barnes, 2004). In 2004, the United States’ Animal and Plant Health Inspection Service (APHIS) issued a federal order restricting the interstate movement of nursery stock from all commercial nurseries in California, Oregon, and Washington (APHIS, 2004). Although both greenhouse-grown orchids and Douglas fir bark are currently exempt from this federal regulation, these exemptions may change in the future. Therefore, alternative orchid media need to be identified so that the reliance on Douglas fir bark is reduced. Aims of this experiment were to determine 1) how different media components influence root and foliar development of Phalaenopsis and Doritaenopsis orchids after transplant, and 2) the effects of container opacity and plant spacing on rooting of Phalaenopsis. If there are benefits to using clear containers, adequate plant spacing may be necessary to allow light to penetrate into the side walls of the container. In addition, alternative media sources for orchid production could alleviate the challenges associated with Douglas fir bark and provide growers with additional options when selecting an orchid media. MATERIALS AND METHODS Container Oopacity Bare-root Doritaenopsis White Moon (Doritaenopsis White Castle × Moon World) and Phalaenopsis Sharon Bay (Phalaenopsis Sharon Cohen × French Connection) plants with an average of 5.8 and 5.3 leaves, respectively, were received and transplanted into 12 cm translucent or opaque plastic containers (Poppelmann Plastics USA, Inc., Hickory, North Carolina, USA). The growing medium was comprised of 5:3:1:1 (by volume) medium-grade Douglas fir bark (Rexius Forest By-Products Inc., Eugene, Oregon, USA), medium-grade chopped coconut coir (Millenniumsoils Coir, St. Catharines, Ontario, Canada), coarse-grade perlite (OFE Intl. Inc., Miami, Florida, USA), and long-fibered Canadian sphagnum moss (Mosser Lee Co., Millston, Wisconsin, USA. Plants were spaced on perforated steel greenhouse benches at a density of 22 or 44 containers per m and were grown in a glass greenhouse at a constant 29°C. The photoperiod was 16 hours (06:00 to 22:00) consisting of natural photoperiods (latitude 43°N) with day-extension lighting provided by high-pressure sodium (HPS) lamps delivering a photosynthetic photon flux (PPF) of 20 to 25 μmol m s at plant height. Light transmission was reduced using woven shade curtains and whitewash applied to the greenhouse glazing so that the maximum PPF at plant height was 300 μmol m s. A vapor-pressure deficit of 1.0 kPa was maintained by the injection of water vapor. Plants were irrigated as necessary with reverse-osmosis water supplemented with a watersoluble fertilizer providing (mg L): 125 N, 12 P, 100 K, 65 Ca, 12 Mg, 1.0 Fe and Cu, 0.5 Mn and Zn, 0.3 B, and 0.1 Mo (MSU Special, GreenCare Fertilizers, Inc., Kankakee, Illinois, USA). After 30 weeks of growth, the number of roots on each plant that were growing over the edge of the container (aerial roots) and the number of leaves were counted, and leaf span (distance from the tip of the longest leaf to the tip of its opposite leaf) was