Irrigating tender fruit

With the hot dry summer that is currently being predicted, irrigation will be essential in maintaining tree health and enhancing fruit size.

Irrigation plays an important role in establishing good root systems in new plantings in orchards and vineyards. Well irrigated trees are less stressed and more hardy heading into winter. Irrigation enhances growth and shoot length, and ensures flower bud initiation and sufficient blossoms for next year. For growers with little or no crop this year, they may look at managing irrigation to avoid increased vegetative growth but balancing the benefits of return bloom. Growers with a crop this year will need to use irrigation to increase fruit size, marketability and overall crop yields. Irrigation should be adjusted based on orchard floor cover (permanent vs cover crop), crop load, and insect or disease pressure.

One of the greatest challenges to growers is determining when irrigation is needed. The two important periods for peach irrigation are during cell division (from bloom to 30 days after bloom), and during cell expansion (approximately 2-4 weeks before harvest). Irrigation is best if started early in the season (mid-June in "normal seasons") and is continued on a regular schedule.

Some of the most common methods for determining when to irrigate include feel testing, water budget modeling using climate parameters (rainfall, evapotranspiration), and soil sensors.

"Feel" testing is one simple method to determine whether the soil needs irrigation. At 50% of saturation (or field capacity), clay and clay loam soils will be somewhat pliable and will form balls under pressure when squeezed in the hand. Sandy loam soils will appear to be dry and will not form a ball under pressure. While this "feel" method is cheap and fast, its accuracy leaves a lot to be desired.

Water budget modeling includes the use of climatic conditions (rainfall and evapotranspiration rates) to determine when water should be applied to the crop. The advantages for using water budgeting for scheduling irrigation is that there is minimal equipment required, it is accurate, simple to use, and it can be adapted for other crops. The water budget approach for scheduling irrigation of fruit crops can be broken into several basic steps, which are slightly different for high-volume sprinkler-irrigated orchards than for those where low-volume drip or micro-sprinklers are used. The steps include estimating: the amount of available water in the root zone, the allowable soil water depletion in the root zone and the water use rate of fruit crops. This information is then used to decide when to irrigate. For more information on the use of water budgeting for irrigation in fruit crops refer to OMAFRA factsheet Irrigation Scheduling for Fruit Crops. Weather INnovations Incorporated (WIN) provides information on the amount of precipitation and evapotranspiration rates on their website. Environment Canada also provides daily precipitation amounts on their website.

Although there are several different soil moisture probes available. The one most commonly used by OMAFRA in tender fruit orchards in recent years is the C-Probe. This probe records volumetric soil moisture data by using capacitance sensors. The sensors are mounted on a column that is inserted into a PVC access tube in the ground. A C-Probe may have up to six sensors on the column. The sensors can be placed at any depth required by the grower. Typically, most sensors are placed near the portion of the plant's root zone that draws the most water from the soil. WIN often uses a C-Probe with moisture sensors at 10 cm (4 in), 30 cm (~1 ft) and 50 cm (~2 ft). The data from each sensor is processed by an electronic circuit on the C-Probe and is transmitted to a telemetry communication unit. These probes provide site specific information on the soil moisture levels at different soil depths. C-Probes can be purchased through WIN at a cost of ~$1000.

How much do you irrigate? A general rule of thumb for tender fruit trees under "normal conditions" would be 25 to 38 mm (1 to 1.5 inches) per week early in the season and 38 to 50 mm (1.5 to 2 inches) during final swell. Also, each mature tree requires 36 litres (8 Imperial gallons) of water per day during July and August. Avoid extended dry periods followed with excessive volumes of water to reduce the incidence of split pits.

Regardless of the method of determining if irrigation is necessary or not, it is likely irrigation will be important in maintaining tree health and crop viability in 2012. If we continue to have below normal rainfall this summer growers should carefully monitor their water supplies. Lack of precipitation can also mean a reduced irrigation water supply. If your supply should run low you may be able to get an emergency Permit To Take Water from the Ministry of the Environment for another nearby source in the area that is not being used. You would need consent of the landowner to access and use the supply.

If lower than normal precipitation persists the Ontario Low Water Response program will ask growers to cut back water use. You can access water supply data through the Low Water Response program at your local Conservation Authority.