Agriculture

Strawberry

Irrigation

Irrigation helps maximize strawberry yields in several ways. Regular watering produces large plants with large leaf surfaces, which in turn produce more flowers and later, more fruit. The practice also helps establish new plantings and runner rooting.

Irrigation can protect the crop from drought stress - especially during flower bud initiation in the fall - as well as frost damage and extreme heat. Water applications can also reduce damage to young plants and fruit caused by wind-borne soil particles. Applying water just after straw cover helps anchor the mulch to withstand fall winds. As discussed in the previous section, irrigation can also be used to incorporate fertilizers.

Strawberry - Water Interrelations

Strawberries have a low tolerance to drying and poor drainage because of their shallow root systems.

June bearers have 90% of their roots in the top 6 in. (15 cm) of soil, while day neutrals have 90% in the top 3 to 4 in. (7 to 10 cm). Both types of strawberries may have smaller root zones caused by winter injury or a narrowing of the rows during renovation.

This susceptibility to drought makes irrigation particularly effective for strawberry production. A 2.5 fold increase in yield has been recorded with some cultivars grown under irrigation versus non-irrigated strawberries.

It is most efficient to irrigate early in the day, when evaporation losses can be kept to a minimum. Straw mulch also greatly reduces evaporation from the soil.

Strawberry Irrigation Requirements

Day neutral strawberries may require higher rates of irrigation or more careful monitoring of soil moisture levels because of their shallow root systems. Trickle irrigation on light soils with a black poly mulch may require application rates as high as 0.1 gal/sq ft (5 L/m2) per day. Without mulch a rate of 0.75 gal/sq ft (37 L/m2) per day should be sufficient. (Dale and Pritts)

To determine if soil moisture is sufficient, use this hand test method:

  • Brush away the dry soil surface.
  • Take a handful of the newly exposed soils and squeeze it lightly.
  • If the soil ball breaks apart readily as you open your hand, the soil needs water.

A more exact procedure would be to use a soil moisture testing probe called a tensionmeter.

Avoid irrigating just prior to picking. Based on a two day picking interval, irrigate immediately after picking with 1/4 to 1/3 in. (6 to 8 mm) of water.

Risk of winter damage may increase when plants are under stress from lack of moisture. Plants should have access to a good level of moisture right up until mulching. The cold hardening process is mainly controlled by the shorter days and cooler temperatures and is not hastened by withholding moisture.

Normal autumn rainfall amounts should be taken into account to avoid over saturation of soils prior to covering.

Take care to avoid excessive irrigation, which can leach herbicides and nutrients out of the rooting zone. Too much moisture can diminish yields, reduce fruit quality and increase fruit rot development.

Key Irrigation Times for Strawberries

Key times for irrigation include:

  • establishment
  • initiation of berry set
  • maximum final enlargement of the fruit
  • before signs of wilting occur (otherwise berry size is greatly reduced)
  • after renovation of June bearers when fruit buds are forming
  • fall, just prior to covering, to reduce risk of winter injury

Other points to consider:

  • Excess irrigation leaches herbicides and nutrients from the rooting zone.
  • Excess irrigation can reduce yields, reduce fruit quality and increase fruit rot development.

Selecting an Irrigation System

Strawberry growers can choose from several irrigation systems and accessories. Factors to consider are site, investment cost, labour requirements and suitability of the system for the crop.

Trickle, pivot and large gun systems will apply water but are not suitable for frost control. Well designed systems have the proper size of sprinklers, pipelines, pumps and filters, are well laid out and are able to meet the needs of the crop. Consult an irrigation specialist or your local irrigation equipment dealer for details on systems and layouts.

A balance should be maintained between pipe costs and power costs. The larger the diameter of the pipe, the higher the investment cost. While smaller pipes are less expensive to purchase, more energy is required to pump the water due to increased water friction.

Planning an Irrigation System

A basic plan should contain the following:

  • a map showing the field boundaries and water source location
  • lift to deliver water to the field
  • location of natural gas and electrical power lines
  • highest and lowest points in proposed irrigated area
  • kind and depth of soil profile
  • rate water enters the soil
  • water-holding capacity of the soil
  • amount of water available for irrigation
  • power source
  • labour requirements - operation, maintenance and moving of system

A Solid Set Irrigation System for Strawberries

A solid set irrigation system is designed to apply water uniformly over the entire crop at one time. This type of system is expensive but can be operated without labour during the irrigation season. A solid set system can be used for frost control, and crop cooling, and can be adapted to irregularly shaped fields with topographical variation. Figure 8 shows a sample layout.

Components

A solid set system basically consists of:

  • a main line
  • lateral lines
  • sprinkler heads
  • risers
  • pump
  • power unit

Main Line - Aluminum pipe is used for an above-ground main line, while PVC pipe is used for a buried main line. The diameter of the line can range from 6 to 12 in. (15 to 30 cm).

Laterals - Aluminum pipe with diameter ranging from 2 to 4 in. (5 to 10 cm) is used. Coupling types used to connect the pipe vary between companies and generally are not interchangeable. Spring couplers are not ideal if the system is moved regularly. Rubber gaskets inside the coupler seal the line against leakage while the system is operating. Self draining couplers prevent rupture due to freezing but are not suitable for fertilizer application.

Laterals are normally spaced 60 ft. (18 m) apart, with sprinklers placed every 40 ft. (12 m) along the lateral. This is commonly referred to as a 40 x 60 ft. (12 x 18 m) pattern (see Photo 4). Depending on the sprinkler selection, a 12 x 18 m pattern should give a reasonably uniform water application when wind is less than 16 km/h (10 mph). At windy sites, a 30 x 40 ft. (9 x 12 m) spacing will provide more uniform water application.

Sprinkler Heads - The rotary sprinkler operates on the principle of an impact arm which is deflected by the jet of water from the nozzle. A spring under tension returns the arm to the original position causing an impact of the arm against the sprinkler body which results in rotation of the head.

If a system is operated below 100 C, ice can build up on the sprinkler heads and prevent the heads from rotating. Plastic heads can be more prone to ice buildup than metal heads.

An application of 1/10 to 1/8 in./h (2.5 to 3 mm/h) of water is required to control frost damage to plants. This requires a minimum of one revolution of the sprinkler per minute with the system described.

Ideally, the sprinkler should have one nozzle open and the other tip plugged. When both nozzles are open, excessive amounts of water may be applied and the smaller nozzle opening may plug with debris.

Sprinkler heads should not be placed opposite each other on the laterals that run side by side. Rather they should be offset, forming a triangular pattern. This pattern does not appear to be distorted as easily by wind.

Be sure sprinkler heads at the ends of laterals provide adequate coverage to the field perimeter.

A 2 in. (5 cm) diameter lateral can support up to 16 sprinklers. As a general rule, the difference in operating pressure between sprinklers should not exceed 20% of the operating pressure. For an application of 1/8 in./h (3 mm/h) using an 40 x 60 ft. (12 x 18 m) sprinkler pattern, the water flow per sprinkler should be approximately 3 gal./min. (11 L/min).

A 40 x 60 ft. (12 x 18 m) sprinkler pattern would require 46 sprinklers per ha (18 per acre).

Risers - The sprinklers are connected to a riser pipe that attaches to the lateral pipe couplers. Risers range in length from 6 to 24 in. (15 to 60 cm) Higher risers interfere with field sprayer booms and other field equipment.

Pump - A very reliable pumping system is essential, especially when water is needed to control frost.

It is questionable whether PTO pumps can provide the trouble-free reliability needed for frost control, although PTO pumps and several hand-move laterals may provide an inexpensive irrigation system for the establishment year.

For an application of 1/8 in./h (3 mm/h) the minimum pump capacity required is 44 gal/min/acre (500 L/min./ha). Select a pump that provides the required flow at a pressure sufficient to efficiently operate the sprinkler where the pressure drop in the system is greatest. This must take into consideration the line losses and head requirements.

Economic Irrigation Considerations

A solid set irrigation system costs about $2500/acre ($6200/ha). No return on investment can be expected in the establishment year unless the planting is a high density June bearing or day neutral type. In these cases, cash flows are generated in the planting year. The normally high capital costs of a solid set system can be delayed until the spring of the picking year.

Less costly alternative systems like the big gun, hand moved lateral and trickle systems can be used in year one.

Cost of operation is also a factor to consider when planning the system. For example, internal combustion engines require monitoring for refilling. Electricity as a power source is less costly to operate but may be expensive to install at sites that cannot readily be accessed by three phase power.

Special Applications of Solid Set Irrigation

Radiation Frost and Freezing Injury

Day neutrals can be grown as an annual with little regard for spring frost damage to the flowers. However, extending the harvest period beyond the natural first fall frosts by using irrigation for frost protection, can be economical.

Radiation frost occurs on clear, cool, calm and relatively dry nights in the spring when the air temperature near the ground drops below the freezing point. Plant tissues freeze as heat is lost from the leaves, flowers and fruit.

A second weather phenomena, differing from frost, is a freeze. This occurs when cool air masses move into the area, taking heat away from the plant by convection.

Frost damages the center of the flower. The center turns black while the petals and leaves appear uninjured. The blackening occurs within a few hours to one day after the frost. Frost can also damage the developing fruit, deforming the berries.

Frost injury rarely causes complete crop loss because the strawberry plant produces flowers over a two to three week period. The first flowers to open are the largest and face the greatest risk of frost injury. Closed buds are also sensitive to frost damage. Frost losses can range from 20 to 80% depending on the temperature and the duration of the frost, the cultivar, vigor, stage of development and the weather preceding the frost.

Ground level temperatures can be two to three degrees cooler than temperatures reported by Environment Canada, which are taken at shoulder level. Therefore, thermometers or electronic temperature alarms should be located in low spots where frost would usually occur first.

Mild frost damage can be controlled by management practices other than solid set irrigation. These include not tilling the soil during the frost risk period in early spring. Untilled soil acts as a heat sink during the day and this heat is released during the night to protect the plants. Irrigation prior to a frost will provide an increased heat release from the soil by conductivity. The use of polyethelene covers will reduce damage to flowers caused by light frosts and cold winds.

The Frost Control Principle and Frost Injury Levels

As water freezes, heat is released, keeping the plant parts above the freezing point. This frost control method is effective down to about - 6.60 C. The key to utilizing this principle successfully is to leave the irrigation system operating until after sunrise to ensure that the ice will melt.

A temperature of -10 C at the plant level may cause slight injury to open flowers. A medium injury to open flowers can be expected at -20 C. Temperatures below -30 C at the plant level will cause severe injury to buds and developing berries.

Key tools for frost control include an automatic thermal alarm, several probe sensors or thermistors and several accurate thermometers. The thermistors are placed in the flower buds of two random plants, preferably with one plant located in a low area of the field and the other beyond the frost protection system. The thermometers should be placed at the thermistor locations as a double check. A comparison of the readings at the two sites will help determine when to stop the irrigation system. These are the key steps for frost control:

  • Start the sprinklers just before the air temperature at ground level reaches 00 C.
  • Apply 1/10 to 1/8 in/h (2.5 to 3 mm/h). At this rate, 60 to 75 gal/acre (655 to 822 L/ha) of water is applied.
  • Do not stop when ice begins to form on the plants.
  • Operating time needed to control frost varies from three to fifteen hours, but usually takes eight to ten hours. A slight breeze or cloud cover during the night may temporarily raise the temperature above the freezing point, but the temperature can drop below freezing just as quickly if conditions change.
  • Ice may form anywhere from 1/16 to ½ in. (1.6 to 12 mm thick) Depending on the intensity and duration of the frost. This does not seem to injure low growing strawberry plants.
  • Leave sprinklers on until the temperature increases enough to start melting the ice that has formed.

For specific steps in frost control see section on the care of established planting.

Frost Control Using Less Water

Growers with limited water supplies or poorly drained soils can achieve frost control by applying water at intervals starting at air temperatures slightly below 00 C. This delay in starting up the irrigation can reduce the water used for frost control providing careful temperature readings are taken. This method is based on the theory that cell salts allow lower temperatures to occur within the cell without causing freezing damage. Since bud temperatures are 1 to 2 degrees warmer than freezing air temperatures, irrigation can be initiated later. This can reduce the time the irrigation system is in operation for frost control.

Water requirements for frost control can be reduced up to 50% by:

  • monitoring the internal bud temperatures using a thermistor placed in the bud
  • delaying the starting time of irrigation until the internal bud temperature is close to critical temperatures (-2.20C) rather than using the air temperature (00 C)
  • applying irrigation at varying intervals, based on the length of time it takes for water film to freeze on the buds
  • stopping irrigation at sunrise.

Environmental conditions like high winds during a frost can hinder the effectiveness of frost control practices. The key is to maintain a film of water on the bud at all times and no injury will result (Dana).

Wind machines can only be used successfully under temperature inversion conditions, when a warm air layer lies over a cold air layer on the ground. A radiation frost will more often be the cause of damage to strawberry flowers on the Prairies.

Crop Cooling

Light irrigation will cool crops and may lead to better yields. The final enlargement of the fruit can occur only when there is a minimum of water stress on the plant because moisture is needed to fill the cells of the fruit. Under hot, dry conditions, water may move out of the fruit into the leaves, resulting in smaller size and lower quality fruit. This condition is called Sunscald. The frequency of irrigation during the harvest season can affect berry size, appearance and sweetness. In experiments where small quantities of water were applied in eleven irrigations during the harvest season rather than in two widely spaced irrigations, berry yield increased from 4.4 to 7.2 tons/acre (9.8 to 16.1 t/ha) and the berries were 86% larger in size. They were more attractive and held longer in storage but were not as sweet as those produced using the two irrigations.

Irrigation for air conditioning or crop cooling should be applied around noon on hot, dry days when temperatures reach 300C and relative humidities are below 30%.

This procedure lowers the leaf temperature, raises the relatively humidity of the air and reduces the transpiration rate and water stress on the plant. Plants require small amounts of water spread over several applications. Generally, rates of 1/10 in./acre (2 mm/ha) of water for two or three hours are sufficient. Where nozzle size does not permit a low application rate per hour, higher application rates per hour can be used if time of application is kept short and repeated periodically throughout the hot portion of the day.

Pesticide Injection

There are not registrations for pesticide injections through irrigation systems in Canada. This practice is not recommended because back flow contamination to the water source is a serious danger. As well, the efficiency of this pesticide application method is questionable.

Trickle Irrigation

Many strawberry growers use solid set irrigation because of the frost control and air conditioning potential of this system. However, trickle irrigation can be used successfully when growing late flowering cultivars, when water supplies are limited, or when flow rates are low.

Four advantages of tricker irrigation are high efficiency of water use, water patterns that are unaffected by winds ease of fertilizer injection and low initial capital cost ($800/acre or $1976/ha). More and more growers are switching to this type of system.

Disadvantages of trickle irrigation include risk of damage caused by field equipment and rodents, as well as a higher depreciation rate compared to a solid set system.

Components of the trickle irrigation system include: pump, filter, main line, subline, laterals and emitters. Plastic is usually the main material with a linear tape system. The tapes have hole spacings 12 in. (30 cm) apart. The most critical part of an efficient trickle irrigation system is the filter because organic material and precipitates can plug the emitters.

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