This
article deals with basic greenhouse culture of palms, problems that can be
encountered, special requirements of greenhouse growing, and other topics
of interest on the subject.
Greenhouse culture is a means of providing a plant an improved growing environment within an artificial structure to improve its growth and survival. It is paramount to describing what a grower in a colder or dryer environment does to mimic the conditions that a grower in a warmer, more moist environment has naturally in his backyard. There are heated greenhouses that maintain a minimum temperature. There are cold frames that only use the passive solar heat with no supplemental heat. And, there are greenhouse that even cool the air. Most greenhouses maintain a higher indoor humidity. All greenhouse are an attempt to give the plant preferable growth conditions and control and stabilize the environment.
So, in describing greenhouse culture, one must discuss what is ones goal with the greenhouse and in what locality does one live. Greenhouses are used for seed germination, vegetative propagation, maximizing growth, creating tropical environments, commercial production of plants, and just for fun. For propagation, one wants to maintain good humidity, adequate warmth, and less than full sun. For pushing growth on Bismarckia, one would want a dry greenhouse with good light, very high temperatures and very little humidity. For high elevation Geonomas, one would want a cooler greenhouse with a narrow temperature spread and high humidity. Thus, one can understand that no simple formula or set of rules will apply to all greenhouse culture. Despite these limitations, I will discuss important topics regarding greenhouse container culture of palms, especially as reflected by my experience in Southern California. Remember to use the greenhouse when it is needed. If a species grows just as fast and just as well outdoors, why use the greenhouse?
Why have a Greenhouse? There is a debate over what is the best size to introduce a palm to your outdoor environment. Does a species do better if exposed to cold as a two leaf seedling or should one wait for a larger juvenile plant? Although this may vary from species to species, most growers have come to realize that a species success with cold may be improved if one starts with a larger plant, preferably at least one half meter in size, typically in a 10 to 20 liter container. This may necessitate someone using a greenhouse to get the plant up to this critical mass. This would not apply to those readers living in a tropical or warm subtropical zone. For those living in more temperate regions, however, this is a critical problem and the greenhouse may make the difference between success and failure on getting the plant to obtain enough size to be grown outdoors. For decades I saw certain species remain as a two leaf seedling year after year with outdoor culture. The same seedling in the greenhouse in the same time would have been well on its way to a good size for outdoor planting.
Temperature
A well constructed and operated greenhouse controls and stabilizes the growing environment. This is accomplished utilizing thermostats, humidistats, fans, coolers, and other greenhouse apparatus. A must is mounting several hi-low thermometers to know what the greenhouse is really doing. The most important goal is to maintain a minimum temperature during cold weather. A minimum temperature of 18.5 degrees C would be acceptable for almost all palms, but this may be financially prohibitive. Ive found that a minimum of 10 degrees C allows me to grow about 98% of the palm species Ive come across. Regarding maximum temperatures, there are very few species that want temperatures in excess of 32 to 33 degrees C. Certain genera, such as Breahea, Bismarckia, Nannorrhops, and Washingtonia like temperatures even higher, but these are the exception. Your Chamaedorea, Howea, Lepidorrhachis, Hedyscepe, Geonoma, Rhopalostylis, Asterogyne, and many mountain species will succumb when faced with such extreme high temperatures. To avoid exceeding such maximum temperatures, one can use cooling apparatus, intake and exhaust fans, interior air circulation with water mist, and overhead sun protection in the form of shade cloth or white wash paints. Most growers implement several of the previous techniques.
Light
Sunlight can be quite intense in the greenhouse. Without protection, the leaves of some of your palms will become bleached out and yellow. One can utilize shade cloth or water based white wash pain on plastic greenhouses. Different densities of shade cloth and different concentrations of the paint can fine tune the light in any particular area of the greenhouse. Observation of plant appearance is more valuable than a light meter. Too much light will produce faded, yellow and sometimes burnt leaves on the palms. Too little light produces green but stretched out and unstable plants that lack vigor. Adjusting your shade in either direction will solve each of these two problems and consistency can be obtained by utilizing a light meter.
Humidity
Humidity should be maintained at about 50 to 60% for most species. This can be accomplished by spraying the greenhouse walkways with water on a regular basis or by utilizing electronically controlled misting devices. The optimal humidity levels obviously depends on what is being grown, but I have found this level to be adequate for most species. Too little humidity leads to leaf desiccation, frequent watering requirements, red spider and other pest infestations, and overall poor performance. Too much humidity makes the greenhouse slippery with algae and slimy and sooty molds. One will get algae buildup on the soil and leaves. This can lead to fungal problems and rot. A purchased humidistat can monitor and control humidity levels. Adequate air circulation can help overcome some of the algae/mold problems and is crucial in other ways.
Air Circulation
Adequate air circulation is the most forgotten factor in the greenhouse environment. Greenhouses have two types of fans. First, thermostatically controlled exhaust fans which mainly serve to cool the greenhouse by blowing out hot air and passively pulling in cooler outside air. They likewise provide some interior air circulation. Second, circulating fans that are set on timers, independent of temperature, and circulate air within the greenhouse. For larger greenhouses, exhaust fans are quite powerful blades over a meter in size. One can utilize intake fans for cooling but this is limited by the physical agitation such fans would cause on the plant material inside the house. On small greenhouses, temperature sensitive hydraulic devices can open vent windows at the top of the house. One could alternatively just open the doors in the morning and close them at night. Interior circulating fans, however, are important regardless of the size of your greenhouse. They gently circulate air among the plants and are critical for good healthy growth and preventing various greenhouse maladies. Typically they are set to operate 30 minutes on and 60 minutes off in any given locality of the greenhouse.
Benches Getting the palms off the ground aids in their growth. This is limited by the size of your plants. Benches avoid the colder ground temperature and help plants get more air circulation. It also makes plant inspection easier and lessens weed problems. Remember to space your plants with air space between them.
Potting Soil for the Greenhouse
Comments on potting soil are similar to those discussed under Container Plants, but in the greenhouse one has to make sure soil drainage is good because of the higher humidity within the greenhouse. Drainage is created by adding air space into the potting soil. Topsoil and very fine sand slow drainage by filling air spaces. Drainage is increased by adding perlite, pumice, peat moss or chunky bark material. In time, however, the bark and peat moss that you added to increase drainage will decay and slow the drainage. Such aged soil is typically wet and sticky and may have a thin layer of green algae or mold on the surface. Such soils can lead to root rot. A well formulated soil should last two years in the greenhouse. When soils breakdown, repotting into fresh soil is needed. The most basic of well draining greenhouse potting mix would be 50% each of peat moss and perlite. Wetting agents, gypsum, and dolomite can be added to this. Peat moss (and ground sphagnum moss) do have intrinsic antifungal characteristics. However, such a light mix needs a very precise watering program.
Watering
All the rules regarding watering described in the Container Culture still apply in the greenhouse. However, because of higher ambient temperatures, pots may dry out more quickly in the greenhouse, especially during a hot dry summer. During the winter, especially if the greenhouse is closed up, pots stay wet much longer. Watering will also be affected by inside humidity levels, air circulation and the soil formulation. One still checks for dryness of the exposed potting soil to determine need for watering and sets a water schedule accordingly. It is best to water early in the morning or late in the day. Occasional leaching of the soil will help prevent salt buildup. One cannot recommend an optimal watering schedule as the variables above may be totally different in any given greenhouse. Occasionally spraying the foliage might refresh the plant and prevent insect problems. However, continual overhead watering as from fixed misters can lead to algae and bud rot problems. It is best to apply the water directly on the soil. Seedlings and small plants require more careful observation for drying. Newly repotted plants need repetitive initial watering to totally saturate the soil. A triple application of water until it is seen running out the drainage holes usually suffices. For the first several weeks, check newly replanted palms every few days to guarantee adequate soil moisture.
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