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| Figure 17. Approximate growth curve of red raspberry fruit. |
A plentiful supply of water is especially important from early spring (summer for fall primocane systems) until after harvest. It will help to promote growth of robust shoots early in the season and to maintain good fruit size during the harvest period.
Irrigation should be started before plants show signs of wilting. The amount of water required by brambles varies with weather conditions, plant stand, and plant vigor. However, the maximum or peak moisture-use rate is considered about 0.20 to 0.33 inch per day. This peak moisture-use rate can be of considerable value as an irrigation guide when moisture-storage capacity of the soil is known.
Brambles obtain practically all their moisture from the top two feet of soil. Soils vary in the amount of water that they can hold in the top two feet. For example, well-drained sandy soils may hold only two inches of water, and loams may hold up to four inches. It is generally considered that irrigation should be started when 50 percent of the available moisture in the soil has been used. It can be seen that with peak-use rates, this situation would occur in five to 10 days, depending on the soil type and rainfall.
The "feel" of the soil when squeezed into a ball in the hand can also be a useful irrigation guide. The soil sample should be taken from the depth of maximum rooting (about six to 10 inches). When the soil will not hold together in a firm ball when squeezed, the moisture content has dropped to a stage where irrigation should be started. A ball that is sticky when squeezed contains enough moisture. Instruments for measuring soil moisture are also available, and an investment in a good instrument is well justified on larger plantings.
In considering how much water to apply, it is generally advisable to use enough to bring the available moisture in the top two feet of soil back to 100 percent. Remember that irrigation equipment is only about 75 percent efficient, so about 1-1/3 inches of water would need to be discharged from the sprinklers to supply one inch of available soil moisture. Irrigation should not be applied at a rate faster than it can be absorbed by the soil.
Excessive irrigation is wasteful, can be harmful, and should be avoided. It can result in soft fruit, which does not handle and ship well. It leaches nutrients, especially nitrogen, below the root zone. Excessive soil moisture also hinders aeration, normal root growth and development, and can lead to serious root disease problems.
As a rough guide, raspberries require 1 to 1-1/2 inches of water per week, either from rain or irrigation, from bloom time until the end of harvest (Figure 17). Irrigation may be required in August if dry weather occurs (particularly the fall-bearing system that is producing fruit), but generally it is not desirable to irrigate in late summer and fall so that canes can harden off for the winter. Lack of adequate water at final berry swell will reduce berry size.
The use of fairly tall risers to get the sprinkler heads above the bramble plants is advisable. This permits more uniform coverage than is obtained from sprinklers on low risers. Also, with high risers, there is little risk of injury to berries and foliage from the water being discharged from the nozzles.
Cultural practices will usually need revision if irrigation is practiced. Diseases such as anthracnose, spur blight, and mildew may be more prevalent if irrigation is not accompanied by adequate disease-control measures. Good drainage is essential if irrigation is to be used. Good soil-management practices should not be neglected even though irrigation is available.
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| Figure 17. Approximate growth curve of red raspberry fruit. |
Microirrigation irrigation (trickle) can used in bramble production. The principal advantages are low labor requirements, low water and energy usage, a more uniform moisture level, and little effect on fruit rot. Disadvantages include the need for a clean water source; the need for periodic replacement; possible damage to the irrigation system from cultivation (tillage); susceptibility to rodent damage and vandalism; and the fact that this form of irrigation does not provide the option of protection from heat or frost.
Two general types of drip irrigation are available.
Emitter Type
This system consists of half-inch lateral lines along the plant row into which some kind of fixed or adjustable flow outlet (emitter), calibrated to deliver one-half to one gallon per hour, is inserted. These emitters range from small diameter plastic tubing inserted into the lateral line (the rate of flow being regulated by inside diameter and length of tubing) to adjustable flow self-cleaning, plastic emitters.
Soak Type
The half-inch lateral is replaced by a soaker that is either laid or buried along the row. These soakers range from half-inch plastic pipe, in which holes are punched at intervals, to double-walled laterals with openings at fixed distances, or laterals that ooze water their entire length. Various types of these systems have been laid on the soil surface, buried in the soil, or suspended from the upper trellis wire. If the material is buried in the soil, the trickle line is buried four to eight inches deep to prevent damage from tillage equipment.
All systems require a good water filter, regardless of the water source, and a pressure regulator. Pressures used vary from three to 20 psi. Preliminary experience indicates the need to apply water at one-half to one gallon per hour between every other plant (six-foot intervals). The length of time needed to apply the water varies with plant development and weather conditions. Continuous applications may not be desirable for brambles.
Detailed specifications for both portable sprinkle and trickle-type irrigation systems, including water requirements, are available from agricultural engineering specialists, through county agricultural Extension agents, or equipment dealers.