Department of Agricultural Engineering

G reenhouseHumidity Control

by J . N . Walker and G. A . Duncan

University of Kentucky . College of Agriculture . Cooperative Extension Service

Agriculture . Home Economics . 4-H . Development

The natural plant environment consists of manyfactors, such as temperature, l ight, carbon dioxide, andhumidity, which must be in balance for satisfactory growth.Of these factors, humidity has received the least study andattention. Nevertheless, it has been shown that humidity isa very important factor in terms of growth, incidence ofdisease. and product quality. Both high and low humiditieshave adverse effects. However, the occurrence of longperiods of high humidity are normally considered to bemost serious,

Effects of High Humidity

Studies have shown that humidities over 90 percentfoster rapid development of leaf mold (Cladosporum

fulvum) and botrytis fruit and stem rot (Botrytis cinerea).Infection of leaf mold on tomatoes actually occurs athumidities above 80 percent; whereas, below 70 perc€nt,problems with infection are slight.

High humidity conditions, however, do favor overallplant growth. This is normally related to a reductionin plant water stress. A number of reports indi-cate that taller plant growth occurs inhigh humidity environments. Yieldof tomato fruit both in termsof number and weight arereported to be greater underhigh humidity environmentsas long as diseases are con-t ro l led. Unfor tunate ly , thehigher y ie lds of tomatoesunder high humidity condi-tions occur with a relatedreduction in quality. Blotchyr i pen i ng , f o r i ns tance , i sreported as being greater inhigh humidity conditions.

Effects of Low Humidity

Low humidities are associated with reduced yield andgrowth as well as other problems. Fruit maladies, such asradial cracking of tomatoes, are generally increased whenlow humidity conditions occur. Overall, however, the mainproblem in greenhouse production is to prevent highhumidities rather than to avoid low humidities.

Effects of Time of Occtrrence

The time of occurrence of high humidity is alsoimportant. Though the diseases associated with high humid-it ies can occur under both day or night high humidity con-ditions, the problems are much more severe if the highhumidity conditions occur at night. In fact, the occurrenceof high humidity during the day wil l not normally lead tosevere problems if the leaves are dry upon entering thenight period and the night humidities are kept to moderatelevels. lf the inverse occurs, i.e., high night humidities and

low day humidities, severe disease problems are muc*rmore common.

Gontrol of High Humidityin Cold Weather

The best way to reducehigh humidities during coldweather is to uti l ize the con-densation potential of thecold interior greenhouse glaz-ing surfaces. lf anyone hasbeen in a greenhouse duringcold weather periods, theycannot help but have ob-served the heavy condensa-tion on all glass or plastic

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UNIVERSITY OF KENTUCKY. KENTUCKY STATE UNIVERSITY. U,S, DEPARTI \ ,4ENT OF AGRICULTURE, AND KENTUCKY COUNTIES, COOPERATING

coverings. Condensation can become so heavy that drippingfrom the roof becomes a serious problem. In fact, to en'courage run-off without dripping, roof slopes are kept to26,60 (6;12 slope) or greater.

The condensation is due to a cooling of the air im-mediately adjacent to the covering to a temperature belowthe dewpoint temperature of the air. When this occurs, theair cannot "hold" the moisture at that condition and it"condenses out." lf the air in the greenhouse is very sti l l ,condensation wil l occur from the air in the immediateproximity of the covering unti l the absolute humidity inthis air is reduced and becomes equal to the saturationhumidity corresponding to the temperature of the glazingsurface. Once this occurs, water vapor wil l move slowly bydiffusion from the remaining greenhouse air to the coldsurface with additional condensation occurring. However,since the diffusion movement is a relatively slow process,the humidity in the interfoliage region can approach satura-tion levels.

lf, however, the air is moved along the boundary sur-faces instead of being sti l l , condensation wil l occur as theair is moved along the boundary, with a subsequent reduc-tion in humidity. When this air is then warrned to thetemperature maintained in the house and moved back intothe plant region, the relative humidity of the air wil l belower than it was before it was moved along the cold glaz-ing surface. (The colder the glazing surface, of course, thegreater the humidity reduction.)

Assuming some moisture would be rervaporatedfrom wet soil or plant t issue, it is possible to estimate theresultant interior humidity for various outside temperatureconditions. For a greenhouse being heated to 68oF, thepredicted humidities are shown in Figure 1. As can be seen,the humidities in a single-covered house would be lowerthan those in a double-covered house. In single glazedhouses, humidities would theoretically be reduced to 90percent or less when the outside temperature drops 9oFbelow the interior temperature. For doublecovered houses,the exterior temperature must drop 170F below the interiortemperature to l imit the humidity to 90 percent. lt isimportant to remember that these levels of humidity reduc-tion occur only when positive air movement is provided tomove the moist air from the plant foliage area to the coolboundaries. Air movement levels of approximately 40 feetper minute (FPM) throughout the house are considerednecessary.

The effectiveness of humidity reduction which can beachieved by cold surface condensation can be seen in Figure2. The temperature-humidity record shows that on the twocold nights when the outside low temperatures were 40oFand 49oF, the relative humidity for these two nights wasrelatively constant, being about 78 percent and 84 perc€nt,respectively, and was below the levels associated with leafdiseases. On the two evenings when the outside temperature

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o r o 2 0 3 0 4 0 5 0 6 0 7 0

OUTSIDE AIR TEI 'PERA?URE, OF

Figure 1. Predicted interior humidities in a 68oF greenhouse withpositive air movement during cold weather periods.

was relatively warm, i.e., 600F and 58oF, the humidity inthe greenhouse did not fall and was near saturation for thefull night period. The low humidities during the day areassociated with ventilation periods.

The effectiveness of using cold boundaries and posi-tive air movement in the greenhouse to control severe prob-lems with leaf diseases has been demonstrated. ln onestudy, leaf mold infection occurred with tomato plants dur-ing the early fall prior to the onset of substantial coldweather. Subsequently, the severity of the disease was vari-able corresponding to the outside weather conditions. Thespread of disease was reduced during periods of low outsidetemperatures and increased during a warm trend after thecold period.

During the cold weather periods, a slight reduction ofhumidity can also normally be observed with each on-offcycle of the heating system. A 2oF rise in temperature dueto an on cycle of the furnace will be accompanied by anapproximately 4 percent drop in humidity. As the air tem-perature then cools, the humidity wil l again increase to thelevel at the beginning of the heating cycle. lf a heatingsystem failure occurs, the humidity wil l,rapidly rise towardssaturation levels as the internal temperature approachesoutside air temperatures.

Contro! of High Humidity During Warm Weather

The control of high humidity during periods whenthe temperature is near or above that within the greenhouse

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can be accomplished prirnarily by ventilation, or heating, ora combination of the two. During the spring and fall, venti-lation alone is normally sufficient. A study of v\reatherrecords showed that for these periods, the naturally occur-ring outside humidities are not generally near saturation. AtLexington, Kentucky, which is a humid region of theUnited States, the humidity is between 90 and 100 percentfor only an average of 1.6 hours per day for the spring andfall months. The mean hours per day when the RH isbetween 90 and 100 percent for Lexington, Kentucky,foreach month is given in Table 1. Weather data for otherlocations in the United States have also been analyzed;however, the conditions for Lexington, Kentucky, wereamong the worst locations considered. The hours of occur-rence of temperatures above 600F are also given in Table 1.

Since natural humidities do not normally exceed 90percent, positive ventilation during spring-fall weatherperiods wil l l imit the duration of high humidity. This has

_ : _ : | : - : - l-o:

Figure 2. A continuous inside temperature and relati\re humidity recording for four days in October, 1965. Minimum outside night tem-peratures were 40,49, 60, and 58 for the four days.

Table 1. Mean hours per day when the temperature is above 60oFand the relative humidity (RHl is betriveen 90 and 100 percent for

Lexington. Kentucky.

Lexinqton, KentuckV*Above Between

Month

JanuaryFebruaryMarchAprilMayJuneJulyAugustSeptemberOctoberNovemberDecember

0.491.689.24

16.81

23.2519.541 0 . 1 12.73o.42

6eF 9&t0ei R.H.

0.30 0.10

22.00 4.9423.80 6.03

0 . 1 30.060.862.81

6.O23.941 . 7 00.350.07

*Mean of 5 years data (1951-19561.

been experimentally demonstrated in a greenhouse where ahigh thermostat setting of 80pF during a warm weatherperiod resulted in very little ventilation and high interiornight humidities wfiereas a lower thermostat setting of60oF during a similar weather period resulted in greateramounts of venti lation and lower interior humidities. Dur-ing the period when the thermostat was set at ahigh level,the humidity inside the house was near saturation levels foran average of 9.5 hours but dropped to only 4.2 hoursforthe low temperature setting.

When the natural outside humidities are high, such asduring the sLrmmer, or for short periods during the spring orfall, venti lation alone wil l not l imit the interior humidity.During these periods, the most practical solution is theaddition of small amounts of heat. Heating the air loF wil lresult in a reduction in relative humidity of approximately2,5 percent. In one study, a timer set to operate a green-house heating system for one minute of every 10 minutesduring a humid warm weather period resulted in an increasein temperature of 3 to 4oF and a 5 to I percent drop inhumidity. The heating was not sufficient to result in anopening of the greenhouse dampers. This drop in humidityresulted in a decrease in leaf mold in infections during thetest period, ln many greenhouse production areas, humiditycontrol during warm weather periods is obtained by con-tinuous low level night heating in combination with a slightopening of venti lation dampers.

Gheck Your Humidity

lf you think high humidity conditions are causingproblems in your house, obtain a good reliable relativehumidity indicator and hang it in your greenhouse. Recordreadings 4 to 6 times daily, especially late at night and earlyin the morning before sunrise, for a week or two to see ifhigh humidity is occurring for several hours. lf so, thiscould well be your plant growth problem.

Summary

The problem of humidity control in greenhouses isnormally that of l imiting the occurrence of long periods ofhigh humidity, particularly at night. High humidity isassociated with a high incidence of plant diseases. lf dis-eases can be controlled, plant growth will generally begreater at high humidities than at lower humidities; how-ever, some problem is also often encountered with a reduc-tion in produce quality under high humidity environments.Similar reductions in quality and a reduction in growth areoften encountered with low humidities, but this is seldom aproblem in greenhouses,

During cold weather periods, the condensation poten-tial of cold surfaces can be uti l ized to l imit the humiditywithin a greenhouse. Positive air movement must be

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provided to transport moist air from the plant region to thecold greenhouse surfaces if satisfactory results are to beacfi ieved. When external air temperatures are 9 to 12oFbelow the interior greenhouse temperatures, humidities canbe maintained within the house below g0 percent. Fordouble-covered houses, the outside temperature needs to be170F below the inside temperature for similar results.

During warm weather periods, particularly during thespring and fall, the natural outside humidity is generallylow and merely ventilating wil l l imit the occurrence of highhumidities. This may require lowering the ventilationthermostat setting at night. During high humidity periodswhen the weather is warm, effective humidity control wil lrequire the addition of some heat. The cycling of theheating system on a one minute out of 10 minute intervalgave effective control in one study. In some cases, a smallamount of venti lation should be provided in combinationwith heating to bring about effective humidity control.

Bibliography

Cotter, D. J. and J. N. Ltthlker. 1967. ; 'Occurrence andBiological Effects of Humidity in Greenhouses." Proceed-ings of XVI I lnternational Horticultural Congress, Vol. l l l ,pp. 353-368.

Cotter, D. J. and J. N. Walker. 1966. "Climate-humidityf nterrelationships in Plastic Greenhouses." Proc. Amer. Soc.Hort Sci. (ln Press)

Freeland, R. O. 1936. "Effect of Transpiration upon theAbsorption and Distribution of Mineral Salts." Am. J.Botmy,23;355-362.

Guba, E. F. 1938. "Tomato Leaf Mold as Influenced byEnvironment." Mass. Agric. Exp. Sta. Bul.850.

Small, T. 1930. "The Relationship of Atmospheric Tem-perature and Humidity of Tomato Leaf Mold" (Clado-sporium fulvum). Ann. Appl. Biol. 17:71-8O.

Walker, J. C. 1952. Disease of Vegetable Crops. McGraw-Hill Book Company, Inc., 529 p. New York, Toronto andLondon.

Walker, J. N. and D. J. Cotter. 1968. "Condensation andResultant Humidity in Greenhouses During Cold Weather."Transactions of the ASAE. 11(21:263-266.

Walker, J. N. and D. J. Cotter. 1968. "Control of HighHumidity in Greenhouses During Warm Weather Periods."Transactions of the ASAE, 11121:267-269.

Walker, J. N. and D. J. Cotter. 1970. "Control of HighHumidity in Greenhouses." Proceedings of Vll Internation-af Congress of Agricultural Engineering, pp.127-132.

Went, F. W. 1957, "The Experimental Control of PlantGrourth." Chronica Botanica Co., Waltham, Mass.

lssued in turtharance ot Cooperaiive Extension work, Acts of May I and June 30, 1914, in cooperation with the U.S. Department of Agricultur€. Charles E- Earnhart, Director olCoopsrativg Extension Sgrvice, University of Kentucky College ol Agricultur€, Lexington, and Kentucky State University, Frankfort.

fssued 7-73, 12M to 1l-85, 2]t-2-87