Watering Greenhouse Crops

J. Raymond Kessler

      Incorrect watering of greenhouse crops is probably the most frequent source of cultural problems of any operation in the greenhouse. Done by hand, it is a seemingly simple process. In fact, it can be quite boring for some, or a time of quiet solitude for others. Yet hand watering requires experience and concentration to apply water evenly and in the right amount to all containers on a bench. Because of its simplicity, watering is often relegated to less experienced personnel. If an employee applies water at the wrong time or in incorrect amounts, crop quality may be reduced.

      An essential management decision is to assign watering responsibility to those with the most experience. This does not mean that hand watering must be performed by the experienced individual, but this person must carefully supervise both when water is applied and how much. Part of the supervisor's responsibility should be to walk through each crop at least daily and plan what crops will be watered and when. Because there are benefits to completing the watering tasks early in the day, this scouting should be done either late in the evening or early in the morning.

      Accomplishing the watering tasks on time should receive the highest priority in a greenhouse operation. During certain seasons of the year, the work force in a greenhouse may be strained to the limit to get other production jobs accomplished. However, provision must be made for correct watering practices. It can be tempting to put the task off or water plants before they need it to fit a tight situation, but this often results in reduced quality.

      Hand watering a crop uniformly requires a good deal of skill and practice. Experienced growers should work carefully with new personnel to explain and demonstrate how the task is done. Then check on the employee to see that instructions are followed. Show the employee the result of errors and how to correct them.

      As many crops as possible should be on some form of automated watering system. Several cost effective systems are available. Automated watering systems can apply water far more uniformly than hand watering. In a small to medium sized greenhouse range, automated systems can be set up so that the manager can perform all the watering alone. During the process of determining what crops need water and turning on the watering systems, time is available to the manager to scout the crops for disease, insect, or nutritional problems. Larger ranges often divide the responsibility among several experienced growers. However, each crop should have one person and only one in charge.

The Watering Decision

      The critical decisions involved in watering plants comes down to how often to water and how much water to apply.

Underwatering

      Plants that are not watered frequently enough may wilt which retards photosynthesis and slows growth. Wilting reduces cellular elongation in developing regions of the plant and results in shorter internodes, smaller leaves, hardened, brittle stems, and in some cases leaf abscission. In extreme cases, leaves may burn around the margins and root hairs may curl and die. Prolonged under watering may cause nutrient deficiency symptoms and stunting.

      Under watering may also be manifest in terms of not applying enough water to wet the entire medium in the container. Some growers mistakenly believe that a small amount of water applied to the medium surface will eventually spread throughout the pot. However, water moves in a wave front downward through the medium. If insufficient water is applied, the downward movement of water will stop part-way down the pot and the medium below will remain dry. For example, if a container requires 6 ounces of water to completely saturate the medium in a container and only 3 ounces is applied, the medium will be wetted only half-way down. If this kind of under watering is repeated, roots in the dry zone will die or new roots may not penetrate into the dry zone.

Overwatering

      Over watering may be manifest in terms of applying too much water at one application. However, in a well drained medium with adequate porosity, it is difficult to apply too much water at one application. In fact, applying a lot of water at one time so water drains from the drainage holes of the pot is called leaching. Leaching is one way the grower can reduce the amount of fertilizer salts in the medium when too much is present. However, frequent, heavy leaching can remove too much fertility and cause imbalances among nutrients.

      Over watering may also be manifest in terms of watering too frequently. Mild over watering in terms of applying water too frequently can result in lush, soft growth and plants that will not tolerate periods of moderate dryness well, e.g. during shipping or in the retail store. They may also be more susceptible to diseases. Mild over water results in the development of "water roots". These roots are long and thin with very few root hairs and a key symptom of mild over watering. Gross over watering, keeping the medium wet all the time, will restrict the oxygen supply to roots and cause root death. Ironically, the first sign of gross over watering is wilting, nutrient deficiencies, and stunting because dead roots are no longer able to absorb water and nutrients from the medium. Plants over watered in this way may wilt during bright sunny days and recover during the evening.

Rules for Watering

1.   Use a well drained medium.

Use a growing medium with the proper balance of texture and structure and good drainage and aeration. This means achieving a balance. Water should move through the medium and drains from the pots quickly yet retain and supply enough water to the plant until the next irrigation. A poorly drained medium is difficult to compensated for culturally. If the medium does not drain well enough, you will either underwater in an effort to maintain aeration or over water at the expense of aeration.

2.   Water thoroughly each time.

The amount of water to apply at each watering should completely saturate all the medium in the container and depends on the container size and the physical properties of the medium. “Container Capacity” refers to the total amount of water that the medium in a container can hold once the excess has drained away. In greenhouse production, water should be applied to reach container capacity plus 10 to 15 percent leachate. For example, if 10 ounces of water is needed to reach container capacity, then 11 to 11.5 ounces should be applied. This will result in 1 to 1.5 ounces of water draining from the pot. The 10 to 15 percent leachate is necessary because greenhouse crops are often fertilized using luxury amounts of fertilizer. Therefore, it is easy to build up excess or dangerous levels of fertilizer. The 10 to 15 percent leachate at each watering removes any excess fertilizer not used by the plant. This rule applies to the application of fertilizer solution as well as water.

3.   Water just before moisture stress occurs.

Water should be applied when the earliest symptoms of water stress occurs. These symptoms occur before the foliage begins to wilt. Learning the signs can take a lot of experience and will vary from one crop species to another. Look for the following:

      1.   Media based largely on peat moss will change color from dark brown to tan as it dries. Observe these color changes and relate them to plant growth.

      2.   Peat base media also changes weight from noticeably heavy when just watered to very light when dry. Make it a habit to pick up representative pots on a bench to learn the feel (weight) of a dry medium.

      3.   For many plants, the first tissues in the leaf to exhibit moisture stress are the cells of the epidermis. Light reflected from these cells on the leaf surface is sharp and clear when the cells are fully turgid so the leaves appear a clear green. However, as these cells loose turgidity and relax, light is not reflected as well and the leaves take on a gray-green cast. For some plants, this color difference may be distinct, while in others, it may be more difficult to detect.

      4.   Make it a habit of dropping plants out of the pots to examine the medium and the root system. The roots of plants that have received water too frequently will be long and thin with few root hairs. Those that have not been watered frequently enough will have short, stubby roots and the root hairs may be curled or brown. A healthy root system should have an abundance of straight, white root hairs just behind the growing tips. The appearance of a healthy root system varies from one species to another.

Watering Systems

Hand Watering

      Through much of the history of greenhouse production, plants in containers have been watered by hand from overhead. Today this involves a source of water through a hose often applied using a watering wand and water breaker. Watering wands may be 2 to 6 feet long and are used to extend the users reach. Water breakers disperse the water stream into a spray to reduce the impact of the water on the medium surface.

      Effective, uniform hand watering takes some practice and skill. Pot sizes of four inches and smaller (including flats) are usually watered with the wand held more upright, applying the spray in a general way. Larger pots (six inches and larger) can be watered more uniformly by moving the breaker pot-to-pot in a steady rhythm. Uniform application of enough water to each pot to achieve container capacity plus 10 to 15 percent is difficult using hand watering. The applicator should stop periodically, drop some plants out of the pots to see if water has reached the bottom of the pot.

      One problem with hand watering a large crop over a large greenhouse range is the shear logistics of completing the task so that all the plants receive water when needed. If a crew of three people starts at one end of the crop and it requires six hours to complete the task, plants at the other end of the crop may be wilted before they receive water. Not only is this detrimental to crop quality, but given the cost of labor, it's not economical when compared to the cost of installation and potential savings of an automatic watering system.

Microtube Watering

      Microtube watering is probably the standard form of automatic watering system for large containers (>5 inches). Water is delivered to each pot through a thin-diameter polyethylene microtube held on the medium surface by a lead or plastic weight to keep the tube in the pot. The weight further serves to break the force of the water. Each microtube on a bench should be the same length for uniform watering. Multiple microtubes are supplied with water from a larger (usually ¾-inch) header running down the center of the bench. Each header connects to a water main at each bench. Each bench may be turned on or off by an electric water solenoid installed where the header branches from the water main. Many benches or whole greenhouses may be divided into watering zones, the size of which depends on the capacity of the water supply. Watering may be controlled by devices as simple as on/off switches or as complex as an environmental control computer, though irrigation timers are common.

      If properly designed, microtube watering can water pot crops very accurately. Irrigation specialists should be consulted about a new microtube watering installation. The most common installation error is to run the header tube too long. If the header tube is too long, plants close to the water main will start to receive water before plants at the far end. Watering of all the plants stops when the water is turned off. Therefore, plants close to the water main will receive more water.

      Determining the amount of water to deliver to a crop using a properly designed microtube watering system involves determining the amount of time to open the water solenoid. As follows:

      1.   Place 5 or 6 measuring cups on the bench in random locations with one microtube in each cup (measures total water applied). Elevate the cups using empty pots the same size as those used to pot the crop.

      2.   Place 5 or 6 pot saucers under plants with microtubes so the leachate can be captured and measured with measuring cups.

      3.   Run the microtube watering system until several ounces of water drains from the bottom of all pots. Clock the time the water ran.

      4.   Subtract the average amount of water captured as leachate from the average total amount of water applied. This determines container capacity.

      5.   Repeat with new plants to determine the amount of time to achieve saturation plus 10 to 15 % leachate or calculate the time required using ratios.

      Tube irrigation may also be applied to hanging baskets overhead. The header is fixed to a metal galvanized pipe or other support from which the baskets are hung. The microtubes hang down from the header into each hanging basket.

 

Subirrigation

      Subirrigation is a completely different approach to watering plants that involves supplying water through the drainage holes in the bottom of the pot to be absorbed by capillary action up through the medium. It has the following advantages:

      1.   Because water is not applied overhead, the foliage remains dry and reduces the chance of disease.

      2.   Because the medium only absorbs the water it can hold, it's difficult to over water a crop.

      3.   Subirrigation requires about half the concentration of fertilizer needed in overhead irrigation.

      4.   System design makes it easy to re-capture the water and fertilizer solution runoff for reuse. Thus, it is more environmentally friendly.

      Several application methods have been used over the years and recent developments have caused a revival in the acceptance of this watering method. The following are some common applications:

Mat (capillary) watering uses a sheet of 3/16" to ½" thick synthetic mat that is laid on top of a layer of polyethylene plastic. The pots sit directly on the mat. Water may be delivered to the mat using watering tubes or an ooze hose. The mat is kept constantly wet to maintain capillary connection between the mat and medium in the pots. One problem with mat watering is that the constant moisture encourages algae growth on the mats.

Gutter benches use greenhouse benches that are fitted with shallow metal or plastic gutters that the pots are placed in. Water is delivered to each gutter by a header at one end of the bench. The bench is slightly angled so when the water is turned on, the gutters flood and water runs off the other end to be captured and recycled.

Flood benches usually involve a plastic tray or insert installed on each bench constructed with groves in the bottom. Pots are spaced within the trays. Water is delivered to flood the tray to about one-half inch deep. Once the pots have absorbed enough water, a drain is opened to remove the water.

Flood floors work just like flood benches except on a much larger scale. Whole greenhouse floors are constructed out of concrete for flooding. This is usually only practical at the time the greenhouse is being built but existing greenhouse have been retrofitted.