70 August 2005 • Greenhouse Grower

by YIN-TUNG WANG, MATTHEW BLANCHARD, ROBERTO LOPEZ

and ERIK RUNKLE

MANAGING the rootzone of potted orchidscan be one of the mostcritical aspects to grow-

ing a healthy crop. In this second arti-cle of our four-part series, we presentresearch-based information on howmedia components, watering and fer-tility influence growth of potted pha-laenopsis (Phalaenopsis spp.) orchids.

Media ComponentsIn their natural habitats, most pha-

laenopsis orchid species are epiphyticplants, meaning that they grow on treetrunks and limbs. Their roots are ex-posed to air movement and absorbmoisture from the humid air, as wellas from rains and dews. Because ofthis, when we grow phalaenopsis incontainers filled with an artificialmedium, we must consider aeration,capillary action, water and nutrientholding capacity, stability and weightof the medium components, as well ascost and consistency. Coarse materialsare often used to allow plenty of airmovement through the media.Ground bark from the Douglas fir and

costal redwood trees has been used forover half a century for growing or-chids. Although Douglas fir and costalredwood are host plants forPhytophthora ramorum, the fungus thatcauses the Sudden Oak Death disease,orchids grown in bark have been ex-empt from the quarantine list.

Nearly all commercial orchid grow-ers make their own media. Althoughmost modern orchid media still containfir bark, one or more of the water ab-sorptive materials such as sphagnumpeat, perlite, sphagnum moss, coconuthusk chips and diatomite is mixed withbark for improved plant growth (Figure1). For example, research at Texas A&MUniversity showed that plant growthwas vastly improved in a medium con-sisting of 20 percent coarse sphagnumpeat and 80 percent fir bark, comparedto fir bark alone.

BarkFresh-ground bark does not hold

much water, which can be problematicwhen bare root phalaenopsis arenewly planted. When bark starts tohold more water and nutrients, it de-composes quickly and ties up some ofthe nutrients it holds. In fact, theremay be no detectable amount of ni-trate-nitrogen in a bark medium.

Fir bark particles do not allow cap-illary action and do not readily trans-fer moisture from the bottom of thepot towards the surface. A mediumof only fir bark can hold too muchwater once it begins to degrade,which can lead to severe root rot andpoor plant performance.

The imported aged bark obtainedfrom the Monterey pine, grown inNew Zealand for lumber, has beenused by an increasing number of or-chid growers in the United States. Ourresearch found that aged Montereypine bark holds more moisture andnutrients and maintains a higher pHthan fir bark (Figure 2). In addition,this aged pine bark does not decom-pose as quickly.

P R O D U C T I O NO R C H I D S

In the second article of this four-part series, find out how to cash in on orchids,the second hottest potted flowering plant in the United States.

The OrchidGrower

Figure 1. Some of the components used in orchid media.From left: An orchid mix composed of fir bark, coarse perlite,coarse peat, and shredded sphagnum moss; fine grade pumice; coarse sphagnumpeat; medium grade Douglas fir bark; fine grade aged Monterey pine bark; andcoarse perlite (sponge rock).

Figure 2. Phalaenopsis planted in amix of coarse perlite, coarse sphag-num peat and fir bark (left) or agedMonterey pine bark (right). Plantswere provided with a constant fertilityof 100 ppm nitrogen from Peters 15-5-15 Cal Mag. Photo courtesy of TexasA&M University.

71 August 2005 • Greenhouse Grower

Sphagnum MossPure sphagnum moss is probably

the single best material for growingyoung phalaenopsis orchids in warm(tropical and subtropical) climates.Many progressive growers plantplantlets from flasks into plug trayswith sphagnum as the sole growingmedium. Taiwan is the leading ex-porter of phalaenopsis orchids, and

nearly all are pro-duced in sphagnummoss (Figures 3 and 4).Although moss from NewZealand is of higher quality, manygrowers use Chilean moss be-cause it is less expensive.

Moss has a low pH and absorbslarge quantities of water and mineralnutrients. Thus, growers must payclose attention to watering and fertili-ty, especially those located in moretemperate climates where plants losewater less rapidly. In addition, inexpe-rienced consumers often overwaterplants sold in sphagnum, leading toroot rot and plant decline. For thesereasons, many growers in northern cli-mates avoid using a sphagnum moss-based medium.

TransplantingPhalaenopsis imported into the

United States arrive as bare rootplants, and thus immediate trans-planting is necessary (Figure 5).Regardless of the medium compo-nents, transplanting requires attention

to detail and is quite laborious. Rootsshould be spread out in pots beforemedium is added. It is very importantto position medium between the rootsso there are no large air pockets in thepot. A stick is sometimes used to pushdown and compact the medium tohold the plant in place. However, ifthe medium is compacted too much, itcould hold too much water and haveinadequate aeration. It is also impor-tant to ensure that plants are plantedat the correct level. Plants planted toodeep may have more disease problems

P R O D U C T I O NO R C H I D S

Figure 3. An excellent crop of pha-laenopsis being grown in soft, clearplastic pots filled with sphagnummoss at Taiwan Sugar Corp.

For Details Circle No. 19 on Postcard or at www.greenhousegrower.com

Figure 4. A phalaenopsisplug grown insphagnummoss showingwhite, healthyroots.

72 August 2005 • Greenhouse Grower

and could rot, and those planted tooshallow may not root into the mediumproperly and may not have adequatesupport for flowers.

Phalaenopsis roots that are exposed tolight have chloroplasts and performphotosynthesis. A majority of pha-laenopsis growers in Taiwan and theNetherlands, and many in the UnitedStates, use clear pots that allow light topenetrate into the pots. Root photosyn-

thesis likely contributes very little tototal plant photosynthesis. However, be-cause roots avoid darkness, roots ofplants grown in clear pots generally stayinside the pot better than the roots ofplants grown in opaque pots (Figure 6).

WateringGrowing a good crop of orchids re-

quires skillful and attentive watering. Thesucculent phalaenopsis roots should dryslightly before being wetted again.Orchids with pseudobulbs (the enlargedstem) store water in them and can with-stand periods of drought. However, pha-laenopsis do not have pseudobulbs andtherefore are intolerant of extended dryconditions.

Phalaenopsis have succulent leavesthat do not show signs of stress until sev-eral days of a water deficiency. Researchhas shown that the uptake of carbondioxide (used in photosynthesis) steadilydeclines when water is withheld. Whensubjected to two or three weeks of dry-ness, the excess loss of moisture willcause the succulent leaves to “flop” andcan cause one or more of the lowerleaves to turn yellow and abscise.

In general, medium must be al-lowed to dry before being wateredagain, but it should never be allowed

to dry out completely. Even inhot summer climates, pha-laenopsis in a water absorptivemedium require watering onlyevery five to seven days in 6-inch (15-cm) pots and at three tofive day intervals in 4-inch (10-cm) pots. Less frequent wateringis appropriate in cooler climates.

Maintaining a high relativehumidity reduces watering fre-quency and is conducive to leafgrowth. We recommend the rela-tive humidity to be around 70percent to 80 percent. Long (half-inch) brownish or greenish succulentroot tips are signs of active plantgrowth. Low humidity can quicklycause the edges of flowers to becomedry and papery.

FertilityThere have been – and still are – mis-

conceptions about proper fertility of or-chids. In the past, phalaenopsis weremistakenly thought to need very littlefertilizer because plants were oftengrown in improper growing media andthus plant growth was poor. Researchin the past decade has revealed thatphalaenopsis is a moderate feederwhen grown in more water-absorptivemedia. When complete fertilizers areused, there is little or no effect of fertil-izer type on leaf growth and floweringwhen grown in a medium containing 20percent peat and 80 percent bark. Plantssupplied with 200 ppm nitrogen atevery watering were superior to plantsfertilized with 100 ppm of nitrogen (orless) at every watering. Therefore, werecommend incorporation of a com-plete fertilizer delivering 150 to 200ppm nitrogen at every watering.

Another misconception is that ahigh concentration of phosphorus en-sures excellent flowering. Researchhas indicated that many growers havebeen using excessive concentrations ofthis nutrient. In one study, phosphorusvarying from 22 to 242 ppm (with 100or 200 ppm of nitrogen) did not influ-ence growth or flowering of pha-laenopsis. Preliminary research indi-cates that 25 to 50 ppm phosphorus isadequate to grow an excellent crop.

For growers in cold climates, consid-er using a lower fertilizer rate duringthe flowering cycle due to reduced veg-etative growth. Fertilization can bewithheld completely once an inflores-

cence has reached 10 inches (25 cm) inlength, or after the first flower hasopened, without affecting flowercount or size.

SalinityRoots of phalaenopsis are very sen-

sitive to salinity and can be injuredwhen salts accumulate in the medium.When salinity of the irrigation water(before adding any fertilizer) increasedfrom 0 to 1.4 dS/m, root fresh weightdecreased in one study. Many orchidgrowers have installed reverse osmo-sis (RO) equipment to provide waterthat is low in dissolved salts. Ifstraight RO water is used for irriga-tion, supplemental calcium and mag-nesium must be used to avoid poten-tial deficiency problems.

A desirable electroconductivity (EC)of the water before adding fertilizer is0.5 dS/m or less. We recommend thatgrowers maintain a medium leachateof 1.5 dS/m or lower when using thepour-through technique. When medi-um EC becomes too high (2.0 dS/m),timely leaching with clear water isrecommended.

Look for next month’s GreenhouseGrower for the third article on produc-tion of phalaenopsis. It will focus onlight and temperature requirementsfor vegetative growth and flowering,height control and managing diseasesand insects. GG

About the authors: Yin-Tung Wang is pro-fessor of floriculture at Texas A&MUniversity; yt-wang@tamu.edu. MatthewBlanchard and Roberto “RoLo” Lopez aregraduate assistants and Erik Runkle is as-sistant professor and floriculture extensionspecialist at Michigan State University;runkleer@msu.edu. The authors thank theFred C. Gloeckner Foundation, theAmerican Orchid Society, Taiwan SugarCorporation, Project GREEEN, variouslocal orchid societies, and private green-house companies and individuals that sup-port orchid research.

P R O D U C T I O NO R C H I D S

Figure 6. Phalaenopsis grown in opaque pots (left)and clear plastic pots (right). The roots of pha-laenopsis are better contained in a pot that istranslucent to light.

Figure 5. Bare root phalaenopsisplants packed in shredded newspaperfor exporting from Taida Orchids inTaiwan.