Guidelines For growing The High Elevation New Guinea Dendrobuims:
The species for which this guide was created are primarily the mountain cloud forest inhabitants of sections Oxyglossum as well as the cooler Pedilonums & Calyptrochilus members. There are many other species in both of these sections and not all require the cool conditions as described. These instructions are primarily aimed at species such as: Dendrobium cuthbertsonii, Den. vexillarius, D. violaceum, D. hellwigianum, D. masarangense & D. seranicum, etc. of section Oxyglossum (OXY) and D. subclausum of the Calyptrochilus (CALY), D. caliculimentum, D. alaticaulinum, etc. of section Pedilonum (PED) just to name a few.
These and similar species are best accommodated in a cool greenhouse with a fairly high humidity. While a greenhouse does simplify care, they could also be grown under lights if attention is given to cultural requirements. NEW GUINEA does experience definite seasonal wet/dry periods but the highland region receives them-in much moderated form. Their "summer" or growing season may experience day temps between 55'-85'F. (depending on elev.) with humidity upwards of 90% day and night. The "winter" or "dry" season often receives 50'-65'F. days with dips as low as 32'- 40'F. on clear nights. This often coincides with clearer, brighter, and lower humidity days. While the precipitation and humidity may be lower during this season, their root zones remain moist and seldom, if ever, dry out. This is due to the condensation, mist from passing clouds, and the fact that the "dry" season is not totally dry at this high an elevation. Fortunately for orchid growers, many of these species seem willing to adapt to a less extreme of an environment than what they often experience in nature!
The following is a general guideline for the culture of a wide assortment of these species:
Lighting: They require good light (1800-2200 foot-candles is a good average). Not enough light will result in weak, easily damaged and disease prone foliage, poor blooming, and a general lack of vigor.
Moisture / Humidity: Most of the species really prefer fairly high humidity, up around 70%. Too little and these often miniature plants will dry out too quickly or even desiccate (dry up) and too much and they can be subject to disease attacks and/or Root loss. Strive for a lightly damp media / mount that never becomes "bone dry" nor remains wet for extended periods. Do allow the roots to dry briefly between watering. Water quality is a concern because many do not appreciate a high mineral content in their water. I use collected rain water for most of the year. Many people who are unlucky with their usual water supply will use De-ionized, Distilled, Reverse Osmosis or other sources of good quality water in order to optimize growth.
Temperatures: Most of the species will respond best to "cool to intermediate" range (50-55F nights) temperatires. In keeping with standard Orchid cultivation practice, daytime temperatures are best allowed a 10-20F degree rise from the nighttime low. It is most everyone's goal to strive for a nighttime temp as close as possible to the "Ideal 50-55" nights and then to simply locate them in as cool a place as you can possibly provide during the summer months. Many people will either place them outside in the summer (in a shady, "cool" spot), or often better yet, grow them in a cool & humid greenhouse. Another particularly good method is to grow them under fluorescent lights in one's basement as this location is oftentimes both a cool and naturally humid space.
Regarding Summer Temps: Maintain them as cool as possible, 55-60F nights are a good goal but they will tolerate higher temps. They seem to fare better than Masdevallias during the summer as long as their roots are healthy.
A note of caution: I have been suspect of 'Superthrive' and other similar agents. Normally these products are harmless and may, in some cases, stimulate growth & rooting but I have also noticed a potential problem with their use on sensitive plants. I do not recommend them as an overall foliar dip. It seems that more than once I've noticed outbreaks of bacterial infections following their use. I assume that whatever ingredients contained in these products may be encouraging growth of more than just the desired plant. If following application, the plants' foliage & crowns do not dry sufficiently or if the humidity surrounding the plant remains excessively high, there may be localized outbreaks of disease causing leaf drop & crown rots. If you are using these, you need not discontinue their use but do use them with caution.
Feeding: These upper elevation Dendrobiums are typically not heavy feeders. Approximate bimonthly feedings of a half strength balanced fertilizer (unless you grow in a bark based mix) to healthy plants spring through fall, less in winter is usually sufficient.
Substrate / Media: Plants are safest when grown mounted on bark, cork or tree fern slabs with a pad of moss or other moisture retaining material beneath them. Potted plants can be grown as well but very careful attention must be paid to prevent over watering and media decomposition and the inevitable root loss that goes along with it. A healthy, well-developed root system is paramount to good culture.
Seasonal Changes: A strict adherence to "NEW GUINEA SEASONS" does not seem necessary for proper growth but may hold a key to any shy bloomers that one may encounter.