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by John Strang, Extension Specialist in Fruit and Vegetable Crops Plant Establishment Irrigation is critical during plant establishment, which is usually from the first to the third year after planting, depending on the fruit crop. During this period plants have not established an extensive root system and are prone to drought stress. Early in the life of a fruit crop growers work at trying to develop an extensive plant root system and an above ground plant structure to produce fruit on. Lack of water can drastically reduce the time to bearing and increase production costs substantially. Weed control is particularly important in a dry season, because weeds compete with fruit crops for available water. Weeds should be eliminated beneath the fruit crop canopy and the row middles should be kept closely mowed to reduce water loss. Winter Hardiness Development Since fruit crops are perennial plants they must develop adequate levels of winter hardiness to survive the winter. Many biochemical processes occur in the fall to prepare plants for winter. Under drought stress these biochemical processes are inhibited and the development of winter hardiness is slowed. Tree fruit crops begin developing winter hardiness at the shoot tips and the process moves down the plant. The last portions of tree fruit crops to harden off are the lower trunks and branch crotches. Heavy crops of fruit that take longer to mature, due to the size of the crop coupled with drought stress, significantly delay the development of winter hardiness. On the other hand light to moderate drought stress in early fall slows the growth of fruit crops and begins the hardening off process. Fruit Cracking and Split Pits Drought stress followed by a rain or irrigation can lead to fruit cracking. Under conditions of water stress the fruit skin ceases growth. If this is followed by the rapid introduction of water, fruit volume increase particularly rapidly and skin expansion does not keep pace, resulting in cracking. Certain varieties and fruit types are more prone to this type of injury. Stayman Winesap apples, Reliance grapes are well known for this problem. Another physiological problem associated with water stress followed by water being readily available is the occurrence of split pits in peaches and plums. Split pits are more prevalent in early maturing peaches and water stress aggravates the problem. Fruit Quality A slight or moderate drought period combined with a cloudless sky actually improves fruit quality. This is due to increased sunlight exposure for photosynthesis and a concentration of the fruit sugars and flavor components. Permanent Wilting Point The permanent wilting point is the point at which plants wilt and remain wilted for 24 hours a day. Under this condition there is not enough soil moisture left in the soil to rehydrate the plant overnight and water needs to be applied to avoid plant die back and /or death, which may occur the following season. If the plants are wilted early in the morning the permanent wilting point has been reached and irrigation is necessary to save the plants. Blueberries Highbush blueberries have a shallow root system and most roots are concentrated in the upper 18 inches of soil. Thus, they can not use water that is stored deep in the soil. They are particularly sensitive to a lack of water and annual irrigation will double blueberry yields. In fact a serious lack of water in a drought that lasts 20 to 30 days can result in catastrophic yield losses the following year and extreme drought can lead to plant dieback. Trickle irrigation is the irrigation method of choice for most blueberry plantings and this is enhanced by the addition of a four foot wide band of sawdust mulch down the row, which is four to six inches deep. Strawberries Strawberries also have a shallow root system. Drought stress early in the season and during harvest substantially reduces fruit size and yield. Drought stress immediately following harvest on matted row plantings delays the production of runner plants and significantly reduces yields the following season. Runner plants that root down after mid-August are not very productive the following year and are essentially weeds. If a hot dry summer is predicted and irrigation is not readily available, growers may wish to narrow their rows at renovation to a width of 10 to 12 inches instead of eight inches. By throwing ½ inch of soil over the remaining plants the old plants will reroot higher up on the crown and be vigorous productive plants the following season. Blackberries and June Bearing Raspberries A lack of water for several weeks prior to harvest on these crops causes the berries to dry up and drastically reduces yields. Irrigation is particularly important on these bramble crops following harvest when the primocanes that will produce next years crop are growing. Fall Bearing Raspberries As with June bearing raspberries, fall bearing or everbearing raspberries benefit from irrigation during their fruit development periods, primarily in July, August and September. Grapes Irrigation is critical for the first two to three years to establish grape plantings. However, following this the plants are fairly resistant to drought due to their extensive root systems. In fact moderate drought concentrates the sugars and flavor components in wine grapes and leads to the production of some of the best wines. Drought will improve the flavor of grapes grown for fresh market, but adversely affect berry size. Fresh market grape growers will often thin the fruit clusters on each fruiting shoot to the largest cluster just prior to bloom or just after bloom to increase cluster and berry size. Apples and Pears Water is critical for achieving adequate fruit size on apples and pears. The fruit continue to size all season long on a incremental basis. If water is lacking, fruit quits increasing in size until water again becomes available. In drought years increased fruit thinning will help remaining fruit increase in size due to reduced competition for water and nutrients. Peaches, Nectarines and Plums The critical time for peach and nectarine water availability and for plums to a lesser extent is the period three weeks prior to harvest. As a stone fruit begins to grow, it increases rapidly in size to the point of pit hardening. During the pit hardening phase the fruit quits increasing in size and the time spent in pit hardening determines whether the peach, nectarine or plum matures early or late. Following pit hardening, the final swell occurs for roughly the last three weeks before harvest. Water availability is critical during the final swell period if adequate fruit size is to be achieved. Fruit thinning early in the season is important in increasing fruit size. |