Greenhouse crops are irrigated by means of applying water to the media surface through drip tubes or tapes, by hand using a hose, overhead sprinklers and booms or by apply water through the bottom of the container through subirrigation, or by using a combination of delivery systems. Information on the different watering practices are available titled "Irrigation Systems" in our "Massachusetts Greenhouse Industry Best Management Practices Guide".
Hand watering is often a difficult task to master in greenhouse production. The goal is to adequately saturate the root media to maintain the plant until the next irrigation.
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. Inexperienced waterers are advised 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 to measure the weight of the pot.
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.
Water plants 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.
Here are helpful "how to" articles on watering greenhouse crops:
Krug B. 2014. Master the Art of Watering. Greenhouse Grower, October issue, pp 56-60.
Krug B. 2012. Water Management-More an Art Than a Science. e-Gro Alert 1(15)
Getter K. 2014. Have you thought about your greenhouse watering strategy lately?. Michigan State University Extension.
Fisher P., Huang J., Freyre R. & R. Dickson. 2015. Too Wet or Two Dry. Grower Talks Magazine
2012 Burnett S., Meyer G., Van Iersel M. and R. Hansen. Save Water with Automation and Sensors. Greenhoue Grower.
Back Pocket Grower. (click training): 5 Point Moisture Scale for Irrigation of Seedling Plugs and Cuttings
A 1994 SARE project conducted by a grower in Massachusett titled "Trough Irrigation versus Traditional Overhead Water in a Commercial Greenhouse Operation" compared costs of hand watering vs trough irrigation. Although the costs are out of date, other information may be helpful.
University of Massachusetts - Amherst
Greenhouse Crops and Floriculture Program