Greenhouse - Growing Cooler Affects Media Temperature and Can Lead to Problems

Growing cooler can save on fuel costs in some greenhouse crops. However, it can also have some negative effects from having cooler media temperatures. The following is a good article on the subject from Michigan State University.

Don’t forget that cooler air temperatures can mean cool media temperatures. Media temperatures control nutrient and water uptake and can have a powerful effect on plant growth. This effect is most noticeable with plugs and small plants.

Optimum media temperature varies by type of plant and especially by plant age. Young plants require higher media temperatures than older, almost fully developed plants. Optimum temperatures are usually in the 60° to 65° range for most crops. Focus your efforts at providing optimum temperatures in the propagation/germination areas and early in the crop. As the crop matures, cool media temperatures will have less of an effect on plant growth.

Media temperature may be up to 10°F lower than air temperature with overhead heating systems when the plants are on benches. The difference between air and media temperature can be even greater when the plants are on the ground or have just been watered with cold water. The best way to determine media temperature is by using a media/soil thermometer. These thermometers are rugged and inexpensive but require calibration just as with thermostats and other temperature sensing equipment. Another way to check media temperature is by using an infrared thermometer. These units are more expensive but can measure temperatures from a distance, a useful feature when checking the middle of prop beds or hanging baskets. You should check them against a calibrated media/soil thermometer because the color of the media and/or pot can influence their readings.

Temperature gradients between air and media are caused by a number of reasons. Water evaporating from the media surface or the surface of the pot or plug tray cools the media especially in propagation or germination beds. With overhead heating systems warm air may be blocked before it reaches the media by the top of the plants or because the containers are placed on the ground or a solid bench surface. The ground is a huge heat sink and is very slow to warm up, especially in cloudy weather. Raising pots or flats just an inch or two off the ground or solid bench increases temperature significantly by preventing movement of heat to the cold ground/bench and increasing air movement around the roots.

Probably the greatest reason for cool media temperatures is watering with cold water. When the plant is young, the greatest component (by weight) of the media is water. Water absorbs a great deal of heat without changing temperature and the movement of energy (heat) from air to water isn’t very efficient. Since air doesn’t hold a great deal of energy (heat) per cubic foot you must circulate a lot of air around the pot/plug to increase media temperature. The most efficient way to increase media temperature is to water with tempered water. Seventy degree F water coming out of the end of the hose usually means the hot water heater is set at 100°F or slightly higher. Also consider reducing evaporation cooling by using bed covers or tenting instead of overhead misting in propagation beds.

Most of us are familiar with Phosphorus (P) deficiency symptoms (stunting, purpling of stems, leaf petioles and undersides of leaves) caused by cold media but low media temperatures also influence the uptake of water and all other nutrients. When the roots are not active because of low temperatures, water isn’t taken up and the plants can wilt even when the media has adequate water available. This is common when the sun suddenly comes out after a period of low light. Make sure your employees are trained to check media moisture levels rather than just react to the sight of wilted plants by watering. Calcium (Ca) moves with the water being transpired. If the root isn’t active, water isn’t moving into the plant and neither is Ca. Low temperatures also cause higher relative humidity around the plant which slows the transpiration of water, and therefore, the movement of Ca into growing points. Below 60°F media temperature Ammonium Nitrogen (NH4) is not converted into Nitrate Nitrogen (NO3-) by bacteria in the media. NH4 can build up to toxic levels if you don’t increase the percentage of NO3- in your fertilizer.

Lastly, the fastest way I know to kill roots is by drowning them. Cool media doesn’t dry out quickly and roots can be starved for oxygen for long periods of time. Make sure your media is highly aerated, with lots of large pores to allow water to drain and oxygen to penetrate. However, highly porous media doesn’t hold a lot of water so adjust your watering practices during warmer weather.

Reprinted from "Save fuel but don’t cause problems due to low media temperatures" by Dean M. Krauskopf, Southeast Michigan Greenhouse Educator, in the October 28, 2005 edition of the Greenhouse Alert newsletter from Michigan State University.