MISCELLANEOUS PUBLICATIONS 1251, U.S. DEPT. OF AGRICULTURE 



used, and commercially available fertilizers may 

 have to be substituted for the pure reagents we use. 

 This is entirely dependent on the purpose for 

 which the plants are grown. 



The growth conditions described here for each 

 species will produce vigorous plants and can serve 



as guidelines. Deviations from these conditions 

 can be used, but one must expect some variation 

 in plant development. Generally this variation will 

 not be of concern, particularly if the plants are to 

 be used for demonstration and not for critical 

 experiments. 



CONTROL OF ENVIRONMENT 



Since there are only four frost-free months in a 

 year at Fargo, facilities to control the environment 

 are required in order to grow plants throughout the 

 year. We use greenhouses and controlled environ- 

 ment chambers for this purpose to duplicate 

 growth conditions at different times. 



Greenhouses 



Many types of greenhouses are available. Com- 

 mercially they vary considerably in size, shape, 

 and construction materials (15). The covering may 

 be glass or translucent plastic. Glass is more trans- 

 parent, but it is susceptible to breakage and in- 

 creases heat retention. Regardless of the type of 

 greenhouse available, certain features are neces- 

 sary to insure its usefulness for growing plants 

 throughout the year. Provision must be made for 

 winter heating, and some type of supplemental 

 lighting is often needed during the winter. Neces- 

 sary cooling is usually provided in the summer by a 

 combination of some type of shading, cooling, and 

 appropriate ventilation. 



Temperature is controlled in the winter by 

 thermostatically actuated steam heat. In mild 

 climates proportional heat control may be desir- 

 able, but it is not necessary under severe winter 

 conditions where greenhouse heat losses are high. 

 The radiators are located around the perimeter of 

 the greenhouse where they are most effective. 



Shading of greenhouses in the summer is essen- 

 tial to prevent excessive heat buildup. It is gen- 

 erally accomplished either by painting the houses 

 with a lime-linseed oil shading compound or by 

 using a shade screen. One of the types available 

 may be obtained from the Chicopee Manufactur- 

 ing Co., Lumite Division, Cornelia, Ga. Shade 

 screen can be installed on rollers near the green- 

 house ridge and raised or lowered as needed. Al- 

 though shade screen may be expensive to install 

 initially, it will last for 6 to 10 years. In contrast, a 

 shading compound has to be applied each spring 



and removed by scrubbing each fall. These opera- 

 tions require considerable labor if the greenhouse 

 area is extensive. In addition, the shading com- 

 pound will weather during the summer, often pro- 

 viding inadequate shading during the last part of 

 the summer, and over the years it will etch the 

 glass and thus reduce the transparency of the 

 greenhouse during the winter. 



Temperature is further controlled by using ridge 

 or side vents, which may be hand or electrically 

 operated. These vents are often used in conjunc- 

 tion with forced-air evaporative coolers, which 

 bring air into the greenhouse. A high degree of 

 summer temperature control can be achieved if 

 both the evaporative coolers and motorized vents 

 are thermostatically activated. The covering of 

 vent openings with a fine screen such as Nitex 263, 

 available from Smico Sales Co., Minneapolis, 

 Minn., will exclude birds and most insects and 

 thereby reduce plant damage and the need to 

 fumigate more frequently for insect control. 



The temperature in the greenhouse during the 

 summer fluctuates between 21° and 35° C. During 

 the rest of the year one of the sections of the green- 

 house is kept at approximately 21° and the other 

 sections at 27° during the day. Temperature is de- 

 creased about 6° in all sections at night. This 

 provides a selection of temperature regimes for 

 growing different types of plants. 



Supplementary light controlled by timeclocks is 

 best supplied by fluorescent lamps without reflec- 

 tors. These provide minimum shading of the 

 plant benches when the sun is shining and can be 

 left in place permanently. We commonly use four 

 244-cm. fluorescent lamps (F96T12/CW 1,500 

 ma.) over each 122- by 244-cm. table. They supply 

 from 300 to 600 foot-candles (ft.-c.) of illumina- 

 tion, which controls day length and is sufficient to 

 maintain many species above the compensation 

 point during the winter. These lamps have an 

 average life of approximately 7,500 hours and 

 can be used in the greenhouse for at least two win- 



