Like the blueprint for a building, the layout plan shows the location of ranch roads, soil types, home site, bee spaces, work areas, irrigation system, possible water, electric, gas, septic tank, or sewer lines, wind machines in cold spots, and the placement of specific tree varieties best suited to the soil type and microclimate. It may also show plans for a future fruit removal system. It is here that the rough farm plan and costs start to take shape.
Orchard Layout
Groves are commonly laid out in irrigation blocks, and within the blocks trees and irrigation lines are set out as nearly horizontal as possible to reduce water pressure problems. Tree rows are established along these horizontal (lateral) lines with the distance between rows varying according to the tree variety. Where there is a choice on northern or eastern slopes, the 20’ dimension (the distance between roes) should run north and south to permit maximum sunlight in the aisle and to let the cold air escape more easily.
Parts of the land may be cold enough to warrant the use of the Bacon variety, one of the most cold resistant varieties. Other parts of the land may be warm enough for Hass or Reed varieties.
The choice of variety will determine the number of trees needed per acre and will have an effect on the ultimate costs.
You should discuss with your nurseryman the relative merits of tube-grown trees versus field-grown burlap-balled trees, keeping in mind that from seed to 30” nursery trees, it is a 14- to 24-month process. To insure a supply of trees at the proper time, arrangement should be made early with your nurseryman. Nurserymen usually ask for a deposit to grow and hold your trees for you.
Tree Shape
In avocados, tree shape must be considered when planning for spacing in a grove layout. For example: Fuertes tend to be spreading while Hass are less spreading, and Bacon, Zutano, Covacado, and Reed are upright in their natural growth (Figures 8, 9, 10).
After the variety has been selected and tree spacing has been determined, the tree locations can be marked with a short wooden stake to develop the grove layout plan. The top portion of the stake can be dipped in a can of brightly colored paint to improve visibility.
Irrigation systems are usually laid out in blocks, a block being the amount of land that can be irrigated at one time. This is known as a “set” and is the largest area that can be irrigated using the available water pressure and volume. Using a sprinkler system in a mature grove, with trees irrigated by Star or impact type sprinklers, a set is approximately one acre. Under drip irrigation, with 8 emitters per mature tree and with a 2” meter, a set may be as much as ten acres due to the slower rate of water application.
Choosing an irrigation system in the development of new land should pose no major problem for owners who are new to avocados. The choice of either a drip or sprinkler system can be defended. The accompanying comparison chart reveals some of the advantages and disadvantages of the two most widely used systems (Table 2). There are many component items on the market today and your development designer will be able to recommend the combination best suited to your land.
Sprinkler
The sprinkler or standard type irrigation system is an arrangement of main lines, sub-main lines and lateral lines with their associated valving. Each tree has its own riser to which the sprinkler is attached. The piping is made of polyvinyl chloride pipe and is cut, welded with solvent, and buried in trenches at the grove site with usually only the risers showing above ground. For lateral lines, assembled ½” PVC is literally pulled into the soil by a device attached to a track laying tractor, leaving a riser in the proper location for every tree. This homemade implement opens the ground, inserts the pipe with attached fittings and backfills the trench in one pass. The equipment is capable of installing almost 5 miles of laterals per day.
Sprinklers come in three basic categories: spitters, revolving, and impact. These come in different orifice sizes to apply given amounts of water at a given pressure over a definite-sized water pattern. After long use, the sprinkler orifices may become enlarged by the wear of foreign material in the water and may require tip replacement.
Young trees are normally started using ¼ circle spitters. As the trees become larger these may be exchanged for ½ circle, and later full circle, spitters and end with revolving sprinklers that provide full coverage of land with a diameter of 25’. Thus, the gradual increase in the irrigated area keeps pace with the growth of the root system over the years.
It has long been thought that mature trees of the spreading types like Fuerte and Hass transpire 40-60 gallons of water per day during the warmer periods of the year, and that growers should budget for 3½ acre feet per acre per year when applied by sprinklers.
We used to think “slow delivery” sprinkler tips with a .4 gallon per minute rate of delivery on each sprinkler arm or a .5 tip on one arm and a blank on the other were slow. By contrast, a drip emitter may deliver as little as ½ gallon per hour. As you can see, the opportunities to waste water by run off on sprinkler systems are rather sizeable. This fact, coupled with known and anticipated increases in water costs, makes planning for the most efficient and beneficial use of water crucial.
Drip
If you choose the drip approach to irrigation, a typical installation might resemble the following sketch.
The first California drip irrigation systems in avocados were installed less than ten years ago. The industry began by laying a drip irrigation hose on the soil (Fig. 12) surface next to the trunk of the young trees, and installing an emitter at that point. When the tree was in the ground about two months, an additional emitter was added about 3 feet away from the first emitter, with the hose repositioned in such a way that the tree trunk equally divided the two emitters.
This plan envisioned that as the tree grew, additional emitters would be added to the surface hose and that perhaps there would be eight or more emitters by the time the trees reached adulthood. To accommodate larger trees, an additional surface hose line could be added on the other side of the tree with the necessary emitters, or a “pigtail” could be attached by a “T” to the original surface hose line. The pigtail would be placed near the tree drip line, which would be equipped with the additional emitters.
Poor quality
Figure 11. Young grove on spitters with no erosion control. Basins around trees will serve for the first few months but should be allowed to “melt” down before winter. Otherwise, basins may trap the cold around the bud union and damage or destroy the tree.
Poor quality
Figure 12. Close-up of a drip system. The hose coming off the riser is called a “pigtail.” For newly planted trees, two emitters placed 3’ apart on the pigtail are used. As the tree grows, the pigtail can be unwound with more emitters added at 3’ intervals.
Later, buried PVC pipe laterals seemed a good idea, as in a sprinkler system. At each tree location, a riser came off the lateral to which an assembly of hose and emitters were attached. A drip hose assembly can be attached directly to the buried lateral. The reason for connecting to the buried pipe is that there could be less damage from future grove practices like picking or weed control.
Because of the nature of drip irrigation, use relatively low water pressures, typically 15 psi, on the tree rows. Correct pressure is sometimes achieved by installing a small pressure regulator on each lateral.
Poor quality
Figure 14. Obstacles are no obstacle when PVC hose is used for irrigation lines. The large, unmoved rock in the foreground is “overcome” by the flexible hose (right center of rock) allowing both the row of trees and spitters to continue in a straight line. This grower is using a spitter irrigation system.
Poor quality
Figure 15. Irrigation systems that are properly designed and maintained can offer a substantial cost savings in water and fertilizer. The system shown above is collectively known as a “water head” and consists of a water filter (the horizontal cylinder) with a removable screen for cleaning, an injector tank in which the fertilizing materials are placed and carefully monitored into the distribution line, and special valving to prevent back-feeding of the fertilizer in the water source line.
For the most uniform application of water it is desirable that differences in elevation along each lateral be less than 10’. However, in some circumstances, this problem can be relieved by the use of pressure compensating emitters to equalize the water delivery between the high and low areas in a given line.
There are many variations of the drip system from which to choose (Fig. 15). Generally, the initial cost is apt to be cheaper than a sprinkler system, and it is expected that there would be important added savings in the water, irrigation labor, and fertilizing costs. With a drip system, there is less waste and more efficient use of moisture and nutrients by putting them just where the tree wants them. With a drip system it is mandatory to fertilize through the system.
Filters
Two sources of water are generally available for irrigation in most avocado growing areas -- water from wells or from the local irrigation district. Other sources might include spring or stream water, but these sources are rarely used. Regardless of the source of the water, it is usually necessary to filter it before it is introduced into the irrigation system. A filter is a device that separates foreign matter from the irrigation water before entering the system. In some situations where water may enter the system under fluctuating pressures, it is necessary to use a pressure regulator to equalize the pressure before the water enters the filter.
The most common type of filter is a screen of perforated metal varying from coarse to fine. Often two are used together, the coarse to remove the larger particles and the fine to remove the silt and algae. Filters must be removed from the filter tank and manually cleaned with a brush and a jet of water.
If water contains large amounts of trash and algae, it is often necessary to filter it using a sand filter first. The sand used in these filters is of uniform particle size. Sand filters are used in pairs but are connected so that either one or both may be used separately or together. These filters are back-flushed to remove the foreign particles by passing a stream of water through the filter in a reverse direction. The cleaned water from one filter is used to back flush the second filter and then the process is reversed to clean the first filter. A screen filter is often used in conjunction with the sand filters. This screen filter acts as a fuse does in an electrical circuit to assure that no trash enters the system in the case of failure of the sand filters. In newer filter systems, back flushing can be automated, but although this option is available to the grower, most systems are back-flushed manually. Some provision must be made to dispose of the water used in the back-flushing operation.
Where well water is used, it may be necessary to use a coarse screen filter as a sand trap to remove the larger sand particles from entering the system and clogging the sprinkler tips or causing excessive wear in the tips. The latter tends to enlarge the orifices, causing them to deliver too much water. The sand trap is installed at the well head as the primary filter. The accumulated sand must be periodically removed by hand from the trap.
With drip irrigation, it is mandatory to have a reliable filter system to clean the water so that it will not plug the small orifices in the dripper. When a sprinkler system is used, a filter system may or may not be essential depending on the cleanliness of the water source.
Fertilizer Application
When any type of liquid fertilizer is to be applied using the irrigation system, a holding tank must be provided either by the grower or the supplier of the fertilizer. When dry materials are used, a covered storage shed adjacent to the fertilizer injector is a necessary convenience. After the fertilizing materials are placed in the injector tank, a bypass system creates a pressure differential to actuate the injector with its associated valving, including a double check valve to prevent the fertilizer from flowing back into the potable water supply.
After the fertilizing cycle is completed, the irrigation lines should be thoroughly flushed with clean water to remove any trace of fertilizer. Nitrogen fertilizer residuals form nitric acid, which is extremely corrosive to the metal parts of the irrigation system.
Hot Line
Every ranch is likely to benefit from a hot line system in the grove. A hot line is a separate water line with risers and valves at strategic locations through the planted ownership and even into undeveloped areas. Such a line should be pressurized at all times so that water is available on demand. Usually these lines are part of the original irrigation system design and serve best when placed along internal ranch roads and in work or fruit storage areas.
Some beneficial uses of hot lines are:
1. Source of water for spray rigs.
2. Fire control.
3. Special application of irrigation water for problem areas.
4. Source of drinking water for field workers to avoid tampering with a functioning irrigation system by untrained persons.
5. Source of fogging water to precool harvested fruit awaiting delivery to the packer. In the summer and early fall, field heat that the fruit retain adversely affects its quality, but packers can ship precooled fruit much further away from the California market with confidence. The more fruit that can be moved away from local markets, the stronger grower returns are apt to be.
Tensiometers
If tensiometers are to be used in the grove, arrangements should be made at this time for their purchase. The locations where they are to be placed should be marked on the layout map. The use of tensiometers is discussed under the heading DETERMINING SOIL MOISTURE in Chapter III.
Soil Percolation Testing
Before actual tree planting begins, growers should check the new irrigation systems as well as the soil. With planting holes, testing water should be allowed to fill up the holes both to preirrigate and to determine if there are any clay pockets in the planting zone. If the water cannot percolate out of the hole in less than 24 hours under dry soil conditions, it had best be left empty or be used by some other plant such as citrus.
Botanically, the avocado belongs to the Lauraceae family, which is widely distributed and includes camphor, cinnamon, the California laurel or bay tree, and the eastern sassafras. It is in the genus Persea and the species americana, or Persea americana Miller. The species is divided into three distinct races; the Guatemalan, Mexican, and West Indian.
Brief description of the races of the avocado are as follows.
West Indian
Moderately thin, leathery skin, watery flesh with low oil content, fruits large to very large.
Guatemalan
Thick skin often with grit cells, medium to high oil content, fruits medium to large.
Mexican
Very thin skin, flesh buttery with high oil content, anise odor in leaves, fruits small to medium in size.
