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Karen Delahaut, Outreach Specialist, UW-Madison
Watering may seem intuitive to someone who makes their living growing and selling plants but watering can be both an art and a science when it comes down to how much is enough yet not too much. Irrigation is not just how much water to apply but also the frequency, duration, and method.
Inadequate irrigation will result in plants that wilt, have off-color foliage, smaller leaves, shortened internodes, and marginal or interveinal necrosis. Underwatering can result when an insufficient amount of water is applied to plants or the interval between irrigation sessions is too great and the plant reaches the point of water stress. Water-stressed plants are also more prone to attack by spider mites which will compound the problem.
Overwatering, on the other hand, is usually from too frequent irrigation and not too much water at each irrigation session. Foliage becomes yellow or wilts. Leaves are soft and internodes are unusually long. Excess water will decrease gas exchange in the media and kill the roots. If you look at the root system, it will be smaller than normal and the roots themselves will be soft and dark. Inexperienced growers may mistake the wilting associated with a dying root system for a plant under water stress and the plants are given more water—just what they don’t need! Another common symptom of overwatering is edema which is characterized by raised, blister-like brown lesions on the leaves of susceptible plants. Although the lesions are blister-like, they are not comprised of necrotic tissue. Edema usually occurs when the media is cool and wet while the air is warm causing the excess water to accumulate in the leaves.
So how do you determine when to water to provide the correct amount? First, make sure that your potting media is well-drained so that excess water can leach out. Also, familiarize yourself with the water requirements of the plants you commonly grow. Some species will require more water than others. Stage of plant growth will also affect its water requirements with seedlings and cuttings needing more frequent irrigation until they develop an extensive root system. And environmental conditions will also play a key role in determining the amount and frequency of irrigation.
There are several methods that can be used to determine whether the plants need water. They all vary in their accuracy and sophistication. The touch method is probably the least reliable of all. The moisture you feel may not be available to the plant as most available moisture will be held in the lower portion of the pot. Timers are equally as inefficient because they don’t take into account the changes in environmental conditions such as heat, light, humidity, and ventilation. Weighing plants can determine whether the weight of the container has dropped below that at which much of the available water has been used. Although the overall weight of a plant will increase as the plant matures, the difference in weight between a saturated soil and one that requires water will remain constant. This will take a little experimentation on your part to determine at what weight loss the plants require water. Start by watering the plants to the point of saturation. Then, at frequent, regular intervals, sample several random pots and weigh them. More sophisticated and accurate methods use a tensiometer or computerized environmental monitoring equipment to determine a plant’s water needs. Tensiometers measure water tension in the soil. The greater the tension, the more difficult it will be for plant roots to extract water from the media and may indicate a need to water.
How you water can also impact plant health. Most potted plants are irrigated by ebb and flow, trough irrigation, capillary mats, drip irrigation, or spaghetti tubes. Bedding plants are water by hand watering, boom sprayers, stationary overhead sprinklers, and ebb and flow systems.
Overhead watering, such as with hand watering and boom sprayers, will increase your risk for disease outbreaks while drip, ebb and flow, trough, and capillary mats will not. Hand watering is the most economical method when it comes to equipment costs but more of the water will be “wasted” and not reach the plant. Hand watering can also be very imprecise depending on who is doing the watering—often an untrained employee.
Drip irrigation uses emitters to place the water right were you want it in the pot without wetting the foliage. The emitters supply a slow, steady drip into the pot. Spaghetti tubes are similar to drip systems but water is supplied as a continuous stream. Fertilizers and fungicides or insecticides can also be applied through drip irrigation systems. The system is more expensive to install but will avoid wasting water on non-crop areas. Drip irrigation systems will also require regular inspection to assure it’s in proper working order and that emitters aren’t plugged. An advantage to drip and spaghetti tube irrigation is that the aerial plant parts remain dry thereby reducing disease problems.
Ebb and flow, trough and capillary mat systems water the containers from the bottom so the foliage never gets wet. Plants that like higher humidity levels will do particularly well in an ebb and flow system since the presence of water on the bench or floor will raise the humidity. Ebb and flow systems must be designed so the amount and frequency of irrigation can be adjusted as needed. With this system, benches or floors are flooded with water that is taken up by the plant roots via capillary action. One concern over ebb and flow irrigation is the potential for oxygen stress to the roots. This is less of a problem with a trough or channel subirrigation system because the water is aerated as it flows. Aeration will also destroy any pathogens in the recycled water as well. Capillary mats are commonly used with small pots that are set directly onto the mat. The mat is wetted and the water is pulled into the pot through capillary action. Sometimes it’s necessary to include wetting agents in the potting media to assure that they will pull the water from the mat. The biggest downside of capillary mats is that they often become overgrown with algae which then results in shore fly outbreaks.
By fine-tuning your irrigation practice, you can produce quality crops more economically with less waste in terms of water and fertilizer. You can also avoid costly disease and insect outbreaks if you choose the appropriate method for your situation.
