“Watering is one of the main factors that affects how your crop is going to grow. Always irrigating at the right time, at the right place, and using the right amount of water” is key to improving water use and plant health in the greenhouse or container nursery setting, Dr. Rosa Raudales, greenhouse Extension specialist at UConn, said in a recent webinar. “Watering is complex because it requires making a decision based on multiple parameters.”

Teaching others how to water isn’t easy. What plants get water? When? How much? How often? Species, container size, plant size, environmental conditions and where the plant is located in the greenhouse all influence how much irrigation is needed and how it needs to be applied.

“Teaching and learning how to water containers is one of the most difficult subjects when training your staff,” she emphasized.

Why Water Well?

Watering your plants well is a must. If plants are over-watered, algae will rapidly grow. The algae layer that develops on the container medium prevents water from penetrating into the soil. Any algae that develops on the floor can make things slippery and hazardous. Algae also provides a route for pathogens to become established. Shore flies are attracted to algae and weaken the plants they attack, allowing disease-causing pathogens easy access. “Algae is always present in the greenhouse,” Raudales said.

Root rot diseases, such as those caused by Pythium species, are another concern with over-watering. Too much water in plant soils is detrimental, and moisture often provides protection to plant pathogens. When soils or growing media dry out, pathogens are less likely to survive. But dry growing media cannot deliver nutrients.

“Pathogens are always present. If we promote the right conditions, pathogens can multiply and cause disease,” Raudales said.

Providing the right amount of water to promote healthy plant growth isn’t a one-size-fits-all job. Frequently, plants in containers on the edges of rows are often under-irrigated, while interior containers may be watered appropriately. Additional spot watering of these edge row containers keeps plant growth uniform, which is the goal in greenhouse production.

Getting the correct amount of water to the plants’ root zone is key to watering success. The root zone includes the microbes (beneficial, neutral and pathogenic) which continually interact with the roots. Proper environmental conditions in the root zone keep a healthy balance of microbes and nutrients. Water is needed to move nutrients into the plant, promoting plant growth and providing plant cells with turgidity, keeping the plant strong and upright.

Oxygen is also needed in the root zone, and it helps roots respire and keep pathogens from proliferating. Different growing media have varying physical properties, including air spaces. Water and gases occupy these spaces. Nutrients move in the water and attach to particles of the media.

“Roots use oxygen. We need to make sure that we provide enough oxygen in the root zone for the plants to be able to function properly,” Raudales said. “Good balance provides enough water, but also enough oxygen.”

Photosynthesis is also dependent upon water. Plants increase photosynthesis in the presence of adequate amounts of water. If water is scarce, the rate of photosynthesis is not optimized. As water becomes available, photosynthesis increases. If water is too abundant, photosynthesis levels off; too much water becomes detrimental as photosynthesis drops.

Variables

Watering well depends upon more than adding a set amount of water to keep the moisture level from becoming a detriment to plant health and growth. That’s because so many factors play a role in determining what that healthy range of soil moisture is and how to achieve it.

“I believe that if you understand how to read the substrate, and how to make adjustments based on the container, you will be able to make good decisions” about watering, Raudales said.

The physical properties of media include roughly 85% pore space. Air and water compete for these spaces.

Container capacity is the amount of water that remains after the container is filled with water and allowed to naturally drain down. The saturation zone contains fewer pore spaces and contains more water than the rest of the container. This zone is located at the bottom of the container. Gravitational water is the water that drains out freely.

Knowing what container capacity looks like in any given medium is important for assessing proper irrigation amounts. Different substrates have varying particle sizes. Sand, rice hulls and vermiculite have large particle sizes and contain more air space, and water drains more freely. Peat and coir hold more water due to their smaller particle size.

Smaller containers, in proportion, have less oxygen than larger containers, and will saturate and dry quickly, complicating irrigation needs. Larger containers have more space for oxygen and are less prone to rapid saturation and drying times.

The zone of saturation changes with container shape, not just the size. Flats contain very small saturation zones and dry quickly. Containers that are tall and narrow have deeper saturation zones and don’t dry as rapidly. Container with the same volumes, but different shapes, will respond differently to irrigation.

Compaction also affects air space. If a medium is packed tightly, its air space decreases and its watering needs differ from those of the same medium packed lightly in the same container.

“We’re talking about irrigation. But a lot of the success of irrigation comes down to how we manage the growing media,” Raudales explained.

Plants which have outgrown their containers don’t have space for water. The plants won’t have enough root hairs to absorb the water that is available either. Keeping plants potted up before this happens is important.

The material from which the container is made also impacts irrigation needs. Plastic hanging baskets will require less watering than baskets which use coir, as coir dries out quickly.

“If you don’t understand how those dry up differently, it’s going to be difficult to water properly,” Raudales said of various media, container shapes and sizes. “Understand those key aspects of growing media.”

Teaching Irrigation

It’s hard to explain how to water. The crop, the season, the stage of growth, the growing medium, environmental factors, water quality, container size and shape and other factors all influence irrigation needs.

Having a strategy to quantify the amount of water to provide can help make watering more consistent in your operation, no matter who does it. While it’s better to err on the side of too dry than too wet, getting things just right is the goal.

Sorting plants by watering needs can help staff do a better job. Spacing plants out, to prevent dry outer rows, can give more uniform results. Measuring moisture levels using a visually and tactile observation scale is a method of standardizing irrigation. The appearance of the soil surface as well as the texture of the medium can be observed. Instructions on what level of dryness indicates a need to water, and what level is a complete watering, can be useful to standardize irrigation practices. Weighing containers at carrying capacity, and weighing after irrigation, to standardize desired irrigation parameters is another approach.

“What you do with that hose will determine how that crop will grow,” Raudales concluded.