Over-watered Plants: Stunted plants may be a sign of over-watering. Plants can be easily over-watered during overcast, cloudy weather, when plants with different water needs are grouped together or by an inexperienced waterer. Over-watering deprives roots of oxygen and increases susceptibility to root diseases such as Pythium and also leads to algae growth and infestations of fungus gnats and shore flies. Check roots regularly for signs of over-watering by gently removing plants from their containers. Plants that have been watered too frequently will have roots that are long and thin with few root hairs. If plants continue to be over-watered, roots will eventually turn brown and rot.
Determining When to Water: There are many things to consider before watering including: The pot size; the stage of the crop; the current weather and tomorrow’s weather; the time of day and moisture level of the media. Use several criteria to decide when to water and refrain from following a set schedule.
The weight of containers is commonly used to determine the moisture level of the media and when to water. Pick up individual containers and if it feels heavy, even though the surface is dry, then do not water. If containers are light in weight, even though the surface looks wet, investigate further to make sure the water is thoroughly wetting the media. Train new waterers to remove the pot and inspect the moisture level throughout the entire profile to become familiar with the weight of containers in relation to the moisture level. A scale is also a useful tool when training new employees.
Some growers successfully use a water gauge that measures how much water is used. This can be a container placed among the pots, or if a drip system is used, a container with an emitter placed it. Experience will dictate how much water is needed to thoroughly saturate a container.
Some growers use their finger to feel the media in shallow containers (the surface can be dry, while wet deep in the pot, especially large containers).
Moisture sensors may be an option (primarily used to automate irrigation in drip irrigation systems). They vary in cost and reliability. Some moisture sensors are sensitive to electrical conductivity and temperature.
Hand-watering Tips: For most applications, use a simple breaker with a valve behind the breaker and adjust the pressure to deliver a gentle flow of water out of the breaker in a uniform manner. Avoid using too high water pressure or large droplets that can wash out the media, compact the media (which then holds too much water) and damage plants. This is especially important for newly transplanted crops.
Train employees to water by lowering the breaker down near the soil surface of the pot and move from pot to pot, watering each plant individually (rather than to hold the wand high overhead like a shower). This allows the waterer to control the amount of water that goes into each pot by holding the breaker at each pot for a consistent 1-2-3 count (depending on the size of the pot). This minimizes the amount of foliage that gets wet and water and fertilizer is directly delivered into the pot efficiently.
When watering, bring the substrate of the entire crop to container capacity (the point where the substrate cannot hold any more water against the pull of gravity). This will encourage deep root growth and help to minimize spot watering.
Let the soil partially dry between watering, but avoid drying down to wilt.
Master the Art of Watering, Greenhouse Grower
Water Management More of An Art than a Science, E-Gro bulletin
Have You Thought About Your Greenhouse Watering Strategies Lately? Michigan State University Extension
Too Wet or Too Dry, Grower Talks
Back Pocket Grower (click training): 5 Point Moisture Scale for Irrigation of Seedling Plugs and Cuttings
Save Water with Automation and Sensors, Greenhouse Grower
Photo: water gauge in greenhouse
Tina Smith, UMass Extension
with input from Leanne Pundt, UConn Extension