Orchard Management


Balancing water demand with limited supply has never beenmore important than now.


I


have covered water from a number of different angles over the years, but this one is special. In a semi- arid environment, climate and its impact on plant need for water is of paramount importance.


Our crop suitability maps are largely based on soils, but the overriding factor is climate. Water is key. Temperature, frost-free days, humidity, evaporation from plant and soil surfaces and proximity to bodies of water are all factors in the climatic mix affecting plant growth and product quality.


Most of the growing areas in the Okanagan are serviced by gravity-fed water from dams in the hills. This is not news, but it is significant as the depth of snow pack and its water equivalent, the earliness of the freshette,


temperatures and rainfall during freshette determine when dams are full and when they stop filling. As soon as they stop filling, sometime from mid-


June to mid-July, they start to get drawn down by use for agriculture and domestic (inside use and gardens). The earlier the draw-down starts and the extent of heat, cloudiness and rainfall which in part determines plant demand (cropping levels, type of crop and crop age and tree row volume are all additional factors) will determine how the water lasts through the growing season into September. The average net moisture deficit as estimated by the Ministry of Agriculture and science based information is 800 mm per acre per year (or 31 inches). This number is lower in the northern part of the Okanagan Valley and higher in the


By Peter Waterman A critical ingredient for quality


southern half. The net moisture


deficit is the amount of water that is needed in addition to the local rainfall to grow most crops. Whether the net moisture deficit


reaches that number is dependent on annual conditions of rainfall,


cloudiness etc. Over the next 50 to 100 years-plus, that number will increase due to climate change. At the very least there will be a 10 percent increase in plant demand which equates to approximately three inches. Our crops are not desert plants; they do not have the types of leaf surfaces that are waxy, spine-like, with fewer stomates or holes for the exchange of oxygen/carbon dioxide and moisture, like true desert plants. They cannot survive hot dry conditions for very long without shutting down


production factories in the leaves, let alone


drought conditions that have an impact on crop quality and plant survival.


There are ways to offset or


compensate for hot, dry conditions


and even drought conditions. Pruning can affect leaf surface and evaporative demand. Heading cuts instead of thinning cuts encourages shoot growth, if done too hard in an off crop year coupled with excessive There are ways to offset or


compensate for hot dry conditions and even drought conditions. Pruning can affect leaf surface and evaporative demand. Heading cuts instead of thinning cuts encourages shoot growth. Too much shoot growth coupled with high nitrogen fertilizer creates an imbalance between fruit and leaves, resulting in calcium losses from the fruit to the leaves which translates into poor quality fruit for storage.


The same thing goes for excessive fruit numbers for the balance of leaves. The fruit will suffer. Demands in high heat will have the leaves shut down, which puts fruit sizing at risk. The above practices are largely done


prior to petal fall. Additional measures can have an impact on efficient water use as the season proceeds and when hot weather or drought occurs. Grass and weeds in the tree row pull moisture from the soil and the tree root zone. Drip irrigation is approximately 90 to 95 percent efficient, whereas sprinklers are 50 to 60 percent efficient in their ability to deliver water to the tree. In light sandy to gravelly soils a drip line on either side of the tree may be best. Efficient irrigation can be coupled with mulching to further reduce moisture loss through evaporation from the soil surface around the trees. I hasten to add that fruit trees respond very quickly to low humidities and dry air conditions. Leaves show decrease in thickness due to water loss and loss of turgor or pressure in the leaf within minutes.


As one proceeds down the tree system, thickness of shoots, branches, limbs and trunks due to moisture stress is easily measured by scientists in the field. In Israel during the early 1990s I saw irrigation systems tied to leaf and shoot moisture stress. This is extremely advanced but we do have electronic systems that ensure plant demands are being met without excessive use of water. Electronic evaporimeters are available, as are moisture blocks that can be read regularly.


If growers want to be able to produce good volumes of high quality fresh fruit (soft fruit to grapes) moisture monitoring, types of irrigation systems and close observation of plant and soil conditions and responses is plain good management.


As we move forward, growers must help ensure that adequate water is available. The Okanagan is one of the few areas where such an extensive range of crops can be grown. Climate change over the next 50 to 100 years will make this increasingly necessary. — Retired orchardist and horticulturist Peter Waterman can be reached at peter@omedia.ca


British Columbia FRUIT GROWER • Summer 2012 19


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