September 2, 2012

Temp Humidity Ratio ~~ 3/1958 AVM

Col Bert Rosenbaum, U. S. Army
March 1958 African Violet Magazine, pgs. 46-47

During my eight years experience in growing African violets, I have worried over the many eccentricities of the African violet, such as why the whites, light blues, pinks, and holly leaf type generally need less light than the darker varieties — why do some violets use more water than others — why do the plants have a blooming cycle — why will some plants stand colder weather — why do some varieties stand higher temperatures — why do the leaves of some plants root faster, etc? Although these characteristics are sometimes irritating, they are exactly what make the Saintpaulia a wonderful experimental plant and promises us the possibility of a very superior plant for the future.
Since I feel that each of these conditions should be given at least a chapter in any book, I will endeavor to cover briefly and inadequately only one set of these conditions and how they are related. These conditions are temperature and humidity.

While many of you may have noted this relation of temperature versus humidity, maybe the opportunity to grow these plants in different parts of the world has given me an experimental advantage you have not had. I made my observations in Newfoundland; Dayton, Ohio; Mobile, Alabama, and Morocco, North Africa, my present Station. I shall describe the climatic conditions in each location. I am sure you will note that I have been given a rare opportunity to experiment.

Taking each location in order, I will describe my observations made while growing the plant at each of them.
Newfoundland: The climate had a mean temperature of approximately 240 F — the lowest temperature noted was minus 5' in winter months and highest temperature noted was a rare 840 F during the summer months. Nevertheless, the temperature inside was easy to control. The days were extremely long in the summer and short during the winter. However, growing conditions inside were excellent for the violets and they bloomed the year round with more abundance of blooms in the summer than in the winter. The humidity was high during the months of March, April, May and June, but the temperature stayed low enough to void any effects of the high humidity.

Dayton, Ohio: Dayton is just about our average weather for the United States with hot weather and high humidity during June, July, August and part of September. Here I had to devise means to combat the combination of heat and humidity. I found that a continuous flow of air over the plants kept them in good condition and generally in bloom the entire summer. Until I devised this means for a flow of air over the plants, I had only a few blooms on only a few varieties during the summer.

Mobile, Alabama: Mobile gives the violet grower a challenge unless he or she lives where there is a breeze or can force ventilation across the plants or Air Condition the room in which the violets grow. I touud that the mmonths of June, July, August and September were very hard on the plants because the average humidity was over 85 % day and night during tnese months and the temperature averaged about 90' F. Even at night the temperature rarely fell below 80' F, During these months it was very hard to start leaves and my entire culture of the African violet seemed out of balance. By lowering the temperature to an 85' F during the day and around 70' F at night, I noted a decided improvement. (Note the 15' F temperature change.)

Casablanca, North Africa: Of all places I have grown the plant, I think this is the most ideal. There is never a frost and the maximum temperature only gets into the nineties about eight or ten times a year. The average humidity at night is between 70% and 80% and during the day runs between 25% and 35%. The temperature outside varies about 25 to 30' F between day and night. Inside the temperature varies from 75' to 90' F from night to day or a spread of 15' F. Inside my violets have grown and bloomed proiusely all summer, while outside, (I have a place fixed with fluorescent lights in an open garage) the violets have grown fairly well with only a few blooms on a few plants. However, the growth was slower and the leaves very crisp.

My conclusions are:
1. That we worry about relative humidity when we should worry about moisture content of the air versus temperature.
a. Relative humidity is a ratio of saturated air at a certain temperature to what it actually does hold at the same temperature.
b. Vapor pressure is a true measure of the quantity of water in the air regardless of temperature, and represents the moisture that affect the plants. The ability of a violet to tolerate moisture is in a direct ratio to the temperature.
2. The following are my recommendations for combating humidity versus temperature:
When temperature varies from:
600 — 75 F
700 — 85 F
750 — 90 F
Humidity could vary from:
45 — 85% (Best results around 65%)
40 — 60% (Best results around 55%)
30 — 40% (Best results around 40%)
3. At any temperature above 90'F, air should be circulated through the plants and the humidity kept around 30% if possible; however, I believe a good growth and bloom can be expected with humidity up to 50% for a short period of time. If the humidity goes to 80 or 90%, as it does in Mobile, Alabama, and you cannot get a circulation of air through your plants, you can expect serious losses.
4. A fifteen degree variation of temperature between day and night, according to my experience, is best — provided the humidity can be controlled, and provided the humidity stays within a desirable range. I believe the 15 F temperature variation between day and night is more important than keeping the growing temperature at 65 F at night and 75 F during the day time. I say this because I have never had better success with my plants than I have here where the temperature inside during the day averages 88 F and at night it averages 74 F. Any greater variation will shock most of the varieties; although I noticed that Ohio Bountiful, White Madonna, Sweet Memory, Double One, Periwinkle, and my own variety, Purple Rajah, did very well even though the temperature varied outside about 25 F during the summer. Of course we all know that the African violet does have a tolerance to a variety of conditions and can adjust to a great degree in regards to numerous situations. As example, some people leave their plants sitting in water. (I personally believe this hurts the condition of the foliage even though the plant does bloom), also by carefully increasing a fertilization plan, the plants can utilize a greater amount of fertilizer, etc.
5. I do not recommend circulating a fine mist of water across plants where the temperature is above 90 F and the humidity is above 70%. This is likely to cause fungus growth and cause excessive rotting of the leaves. I would not follow a plan of regularly washing the leaves under these conditions. Instead, gently dust the leaves.

I really have no interesting information to relate regarding the growing of the African violet in Morocco. The "Vita" Nursery in Casablanca has been growing them for about three years, but they are very far behind the United States in its cultivation. I visited them and offered all the assistance I could as they are having extreme difficulty with nematodes and mites, and found them very appreciative. All of their plants are grown from seeds, and I saw no doubles. I naturally remedied that situation.
Let me extend my offer of cuttings from my seventy-five varieties to any member in Africa, provided they pay the postage charges. This is all from Morocco.             end

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