Measuring and Controlling Temperatures during Spawn Growth

The methods used to monitor and control temperatures vary at each farm. The quantity of thermometers used for a room may vary. One may use glass, dial thermometers, or use remote sensing thermocouples with computer control. At least two thermometers are usually placed into the substrate in most mushroom houses, and these are left in the same location throughout the entire spawn-running period.

Several factors should be considered when determining how to and how often one should monitor substrate temperatures. The cooling and volume capacity of the air handling units, substrate nutrition and dry weight are several factors that will determine how easily the spawn temperatures can be controlled during its most active period. At the beginning of spawn growth, when the substrate temperature is uniform, thermometers are usually left in the same location. One should monitor substrate temperature twice a day and during the accelerated spawn-growing period, more often is advisable.

These stationary thermometers are used to represent the range of temperatures encountered in all areas of a room. Probing of the substrate temperatures should be done to insure that the stationary thermometers represent that area of the substrate or room and provide another tool to monitor and control the crop temperatures. Most growers invest time every few days, if not daily, to probe all areas within each house, both upstairs and down, to monitor differences and changes in temperatures. Hand-held rapid response thermometers are inserted into as many spots of compost as possible to monitor temperatures. This probing determines the range of temperature within the substrate and areas of the room. However, it is important to have personnel available to spend the time probing the room. Probing can be done by any trained and culturally clean individual, not necessarily a grower. More uniformity in the substrate's moisture, nutrition and dry weight will reduce the need for extensive comprehensive probing. When the grower has enough experience about how the crop will react and can anticipate when and how much of a heat surge may occur, less probing may be required. After spawning, before the spawn heat surge, during heat surge and before casing are the most critical times to probe.

Probing and or stationary thermometer temperatures are usually averaged into a mean temperature for the room. This average substrate temperature is used to make decisions for raising or lowering the air temperature. Decisions on whether to lower the air temperature or to raise it should be based on both the average substrate temperature and the frequency or distribution of higher temperatures. If the majority of spots are a bit warm then air temperature should be lowered a degree or two. Conversely, if the majority of areas are a bit cool 68 - 72 F, either the air temperature or the volume of air should be decreased since both of these factors can affect the substrate temperature.

The varying air volume during spawn growing to control substrate temperature may be a new consideration for some mushroom growers. The use of air temperature to control substrate temperature is normally how substrate temperatures are controlled. By lowering the air temperature, the substrate temperature is decreased. It may also be obvious that the more air moved through a house, the greater the amount of heat removed from the air in the house and, indirectly, the substrate. If the air is dry, substrate moisture will evaporate and evaporation is a form of cooling. The greater air volume moving through a house and the drier that air, the greater the cooling effect on the substrate.

Substrate temperature control is also dependent of the cooling capacity of the air handling system. Most single-zone bed farms design their air conditioning systems on the requirements during spawn run. Tons of cooling that is required to maintain temperatures range from 2 to 5 tons per 1,000 sq. ft. of growing space. Cooling capacity is also dependent on the location of the farm and weather conditions. Cooling for spawn growth also depends on the air volume, movement and distribution within the room. Substrate density also determines to amount of cooling required, the more substrate dry weight the more cooling that is needed. The condition of the substrate will affect the amount of heating that occurs the first few days of spawn growth. When substrate is not conditioned well, other microbes have food to use and will tend to grow and produce a heat surge early is the spawn growing period. Substrate moisture also affects the temperature control during spawn run. The drier substrate the harder it is for the heat to be removed and substrate will tend to heat more rapidly and for longer time. While wetter substrate has less air in the pore spaces, and heat is conducted more easily, making substrate temperatures easier to control. When hot spots are found the plastic covering the bed should be removed. In addition, additional fans may be directed on the hot spots or one may dig a hole in the substrate and allow the heat to escape.

The spawn-growing period is normally 10-18 days. Longer spawn run times can reduce yield and substrate moisture, thus influencing fresh quality. A short growing time will create more heat production during the time after casing. This additional heat will require more cooling and increase the drying of the casing. The cooler air temperatures will slow the spawn growth into the casing, delaying the flush and first harvest. Spawn growing period is considered complete when the spawn has completely colonized the substrate and the metabolic heat surge is subsiding.