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Following a bad chill hour winter (2014-15) and an “okay” winter in 2015-16, California pistachio growers are looking for a larger accumulation of chill hours by the end of February to bring in a top quality crop.

“We are in better shape at this time than we were last year,” said Setton Pistachio grower services rep. Mike Smith estimated in mid January. “The rain and fog are adding more hours, but we still have a long ways to go this season.”

Chill hours, a summation of hours below 45 degrees F, are counted from November 1-February 28.

Pistachio trees in the northern growing areas of the state are ahead in chill hours, said Bob Klein with the Pistachio Research Board. In the south, where 95 percent of the state’s pistachio trees are growing, the outlook is that chill will be sufficient, but not great, Klein said. With the memory of the warm winter of 2014-15 and subsequent poor yields, growers have experienced what very low chill hours can bring.

“When it is marginal, it’s hard to say,” Klein said, noting there are still several weeks to rack up more chill to give trees a better dormant season. Insufficient chill hours can result in delayed or irregular bloom, late vegetative development, altered leaf morphology, poor pollen production, death of stigma, reduced fruit set, increased proportion of blanks and unsplit nuts, late maturation and general reduction of crop yield.

Using 900 hours of temperatures below 45 degrees F as the benchmark for desired chill hours, UCCE farm advisor Elizabeth Ficthner notes that some pistachio production areas in the state did attain enough chill hours in 2015-16 but other areas fell short.

The amount of time California pistachio trees experience temperatures below 45 degrees F may not tell the entire chilling story. There is some heat and chill interdependence according to UC researcher Louise Ferguson. Excess chill hours means fewer heat hours are necessary in the spring for good bloom and fruit set. Spring heat compensates slightly for winter chill deficiency.  Very warm spring temperatures can adversely affect flower quality.

Ambient air temperatures may not reflect what trees are experiencing, Ferguson said. Other environmental conditions such as fog or bright sunlight may influence tree response. For example, less fog or wetness decreases evaporative cooling. More sunlight hours may warm even the shaded parts of a tree. Another method for measuring chill hour accumulations, called chill portions, places weight on intermittent warm temperatures during the November 1-February 28 time period. The chill portion formula subtracts from the total hours accumulated when temperatures reach 55 degrees. There is no correlation between the two chill measuring methods.

UCCE farm advisor emeritus Bob Beede, in his 2017 task list for pistachios, noted that in mid December, chill portion accumulation was about 40 percent less than last year at that time. Chill portions- the temperature measuring method that is weighted, were counted from September 1-December 13. Shafter, Delano and Blackwell’s Corner areas in Kern County had accumulated only 12, 15 and 18 chill portions respectively compared to 24, 25 and 24 last year. Madera and Durham areas were closer to the 2015 mark with 21 and 22 chill portions. The only cold weather during that time occurred early in December and the state experienced unseasonably warm temperatures later in the month.

Growers can check on their local chill accumulation at the UC Fruits and Nuts Center web site, clicking on the “Weather—related Models and Services and selecting chilling accumulations models, then cumulative chilling portions. The site shows chill portion accumulation for every CIMIS station in the state and also provides historical data. Beede cautioned that the stations were designed for irrigation scheduling, not for chill portion measurement, and the absence of fog at the stations can cause temperature differences up to 20 degrees F between ambient air and the buds.

Beede noted that research he and Ferguson did found that Kerman and Peters varieties do not grow normally when they do not receive adequate winter rest. Beede said the research suggests that Kerman requires 750 hours below 45 degrees F and Peters 850 hours to leaf out and bloom adequately in the spring.

In addition to shortening the rest period for the trees, Beede said warm temperatures also elevate the bud respiration rate which consumes carbohydrates critical for spring growth. This finding by UC Plant Sciences professor Maciej Zwieniecki, may explain why oil applied to pistachio trees to enhance rest breaking cause poor production in 2015, Beede said. In response to the applications, the trees increased respiration rates to metabolize the oil and depleted their carbohydrate stores. At bud break, the combined deficiencies in available sugars and low chill caused poor leaf out and fruit set. Beede said due to the uncertainty of this winter’s weather pattern, oil application is not being suggested.

Some growers, Beede reported, took the precaution of applying kaolin clay or calcium carbonate in early December to reflect or diffuse solar radiation. The use rates for the clay vary from 30-50 pounds per acre, he said, while the liquid calcium is typically applied at four gallons per acre. After significant rainfall, the products must be re-applied. Costs per application is in the $80-$90 per acre range.

The effects of winter applied kaolin clay or calcium carbonate to pistachio trees

continue to be studied. Each has a different mode of action. The clay products, which are finely ground powder, work by reflecting light to reduce the absorption of solar heat. Calcium carbonate crystals modify the incoming light, reducing its energy and dispersing it in multiple directions. These actions reduce energy absorption by the plant, resulting in lower temperatures.

Merced County farm advisor David Doll’s research showed a 200-250 pound increase in CPC yield over untreated trees when kaolin clay was applied prior to the 2015 season. Beede said the data suggests that the treatment to mitigate the negative effects of warm winter temperatures does not ensure a normal crop, but may prevent extremely poor yields.

Beede noted unreplicated field trials with calcium carbonate in Kern County last year were able to increase chill portion accumulation by about 13 percent due to lower bud temperatures. The trials included single and double applications January 12 and February 12. Flower bud temperatures were monitored in treated and untreated trees. The data showed bud temperatures were reduced by as much as 10 percent and the rate of heating during the morning hours was slower. Beede said the January treatment increased chill portions accumulation by about 13 percent. The treated trees also leafed out more evenly and the second treatment in February delayed development by about four to five days compared to untreated trees. Beede that the trial was done where the chill requirement for pistachio was satisfied during the previous winter and the outcome for trees that did not have sufficient rest could be different.

Yield improvements were found in the trial using the calcium carbonate product Diffusion and the return on the cost of application was positive. Without replication, Beede stressed, they were not reporting the actual yield increase.


Cecilia Parsons

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 I am not an engineer. Neither are most growers, but a lot of engineering has gone into the nozzles used for weed control, especially in the last ten years. It was a lot different when I sprayed my first herbicide trial in 1983; I used brass TeeJet 8002 nozzles. If I wanted more ‘drift reduction’ I could use a larger nozzle, such as an 8004, or choose an ‘LP’ nozzle; remember those? Those nozzles were designed for use at lower pressures. I could use a 11002 nozzle. It didn’t reduce the number of small (driftable) droplets but I could hold the boom closer to the target so it wouldn’t be as affected by the wind. The nozzles available were brass, stainless steel or ceramic.   

Nozzle Revolution

                  A few years later I found the TeeJet XR (Extended Range) nozzle and like many used this nozzle almost exclusively for the next 15 years. They were basically a plastic (polymer) nozzle with a stainless steel core and best of all color coded-02’s (as in 8002) are yellow, 04’s (as in 8004) red and so on. We could adjust the pressure from 20-60 psi and still get a pretty consistent pattern.

                  Then the nozzle “revolution” started. TeeJet introduced venturi nozzles like the “AI” or air Induction and the “chamber nozzles” like Turbo and Turbo TwinJet nozzles and now the Turbo TwinJet Induction TTI; GreenLeaf had the “Turbodrop” and others. Each of these nozzles had some kind of chamber where the spray wasn’t forced directly out of the nozzle tip. Many growers began to use these nozzles to get good coverage and reduce drift potential.

                  Today we have a wide range of nozzles available for use when making herbicide applications in orchards and vineyards. But have we really examined these new innovations in nozzle technology? In my travels I have seen many, not all, growers using these new nozzles (if it ain’t broke why fix it?). Unfortunately, many aren’t using them properly. So what’s different about these new nozzles? It all comes down to droplet size (see Figure 1.).

                  The following quote is taken from a Pesticide Environmental Stewardship document titled “Understanding Droplet Size:” “The Volume Median Diameter (VMD) refers to the midpoint droplet size (mean), where half of the volume of spray is in droplets smaller, and half of the volume is in droplets larger than the mean. A VMD (DV0.5) of 400, for example, indicates that half of the volume is in droplet sizes smaller than 400 microns, and half the volume is in droplet sizes larger than 400 microns. Some pesticide labels specify a recommended droplet size. An example would be: “Use nozzles producing a coarse (VMD of 400 or greater) droplet category.” 

Herbicide Applications and Coverage

                  So how does that effect herbicide applications? In a word: coverage. The amount of coverage needed is determined by the type and size of target weed (grass or broadleaf, large or small), application timing (preemergence or postemergence) and the activity of the herbicide (contact or systemic). The smaller the droplet size the better the coverage. Unless the droplets are too small and actually dry up before hitting the target, which is a subject for an entirely different article. In general, herbicide applications in orchards should be made with nozzles delivering a droplet size (VMD) of 250-500 microns (Medium to Very Coarse).

                   An example of when it would be better to use a larger droplet is when using a systemic herbicide, like glyphosate, on a plant with large leaves, like a mustard plant; a target that is easy to hit and a systemic herbicide that should be able to move within the plant to control it. But what about using a contact herbicide, like paraquat, on small grass plants? Larger droplets may not provide the coverage needed to hit the small targets. In this scenario you may have to increase your spray volume (more droplets), or use a nozzle that is delivering a smaller droplet size. The trick when selecting nozzles is to be at the intersection of good coverage and drift reduction.

                  Figure 1.

                  droplet size

(Copyright Pesticide Environmental Stewardship. Initial compilation courtesy of Jim Wilson, PhD. South Dakota Cooperative Extension).

                  Figure 2.     

                    Picture1 xr     

                       Picture1 aixr                     

                  I am concerned that we are not using these “new” nozzles properly and longterm weed control may suffer. Figure 2 shows the chart of two types of nozzles from TeeJet. First the nozzles on this chart that correspond to the colors ( Blue 03, Red 04, etc) both deliver the same volume of water at the standard (40 psi) pressure. An 02 (11002 or 8002) delivers 0.2 gallons of spray solution per minute and the 04 (11004 or 8004) 0.4 gallons of spray solution per minute at 40 psi. But as you would expect the droplet size changes, quite a bit in some cases. Looking again at Figure 2, the XR8004 nozzle at 40 psi correponds to an  ‘M’ for medium droplet size or a VMD range of 226-325 microns. Staying at 40 psi but changing to the AIXR11004 nozzle will produce a droplet size ‘XC’ or extra coarse (EC on color code chart) with a VMD range of 501-650 microns that is twice as big as the XR8004. A Turbo Teejet TT11004 nozzle at 40 psi (not shown) produces an UC (ultra coarse) droplet size which is defined as VMD more than 650 microns. These larger droplet sizes will provide good drift control but may not be the best in every situation, as described above.

Drift Control

                  If you need drift control but have determined that a ‘Coarse’ or ‘Very Coarse’ droplet size would be more appropriate for your target weeds you have a couple of choices and both will require a new calibration. (Note:  calibration, or at least a volume test, should be done every time the sprayer is used). If you use the AIXR11004 nozzle you can increase the system pressure to 70 psi, or keep the pressure at 35 or 40 and change to a AIXR11002 nozzle. They will both produce about the same sized droplet profile—but the AIXR11004 now will be delivering 0.55 gallons per minute (chart not shown) and the AIXR 11002 should be delivering 0.2 gallons of spray solution per minute at 40 psi. So you will either have to speed up, or slow down, to stay at the same gallons per acre. It would be better to recalibrate your application using these new parameters.

Off-Center Nozzle

                  The standard practice in many orchards and vineyards is to spray down one side of the row and then come back on the other side. The ‘OC’ or Off-Center nozzle, like the one pictured below is often used at the “tree- end” of the spray boom. The nozzle that is most often used is the same design that has been used for 50 years and can produce a lot of “driftable” (small) droplets. Newer designs are available such as the TeeJet AIUB nozzle. The problem is the same for any off-center nozzle design when it comes to uniform coverage. Why is an OC nozzle needed? Standard flat fan nozzles are designed to be used in an overlapping configuration and are often positioned at 20-30 inches apart. If you don’t use an OC nozzle and were to spray close enough to achieve this overlap you would need to be 20 inches from the other side or 10 inches from the middle of the tree. The OC nozzle can be used to replace the “missing nozzle.” They can be set 40 inches apart or a safer 20 inches from the middle of the tree. However, this nozzle is often farther, in some cases much farther, out than 20 inches from the tree. To get the same coverage as the rest of the boom the output from the OC nozzles must overlap 80-100 percent when you come back around the other side of the tree; a fact surprising to many growers. Many growers try and overcome this by putting a flat-fan nozzle and OC nozzle in tandem at the end of the boom. At this point the OC nozzle is directed at a higher angle which may or may not overlap. I am amazed that for every other nozzle the height and spacing are very specific. Try to find any literature on the use of an OC nozzle that says more than “use on the end of orchard herbicide spray booms.”

Uniform Spray Pattern

                  Having a uniform spray pattern is important for even coverage and weed control. I use the TeeJet Pattern Check to collect the spray solution and determine the coverage. When sprayed from both sides the red ball indicators should be almost flat reflecting that the same amount of water is hitting each area. My experience is that in most cases the middle of the tree row receives less spray solution, therefore a lighter rate of herbicide than areas farther out yet the middle of the tree row is the area that often gets more irrigation with micro-sprinkler or especially drip irrigation and where weeds are more likely to grow. I recently learned more about boomless flat fan nozzles. These nozzles, such as the TeeJet BoomJet or Hypro Boom X Tender and others, have been used in the roadside and turf industries successfully for years. As yet they have not been used widely in orchards and more research is needed to determine if they can be successful there as well.

                  The take-home message is to choose the right nozzle for each situation while attaining the best drift reduction possible and still getting the performance needed for good weed control and checking to make sure that the nozzles you are using are getting the herbicide where it needs to go.

(Note: My reference to TeeJet nozzles is not a recommendation, but simply nozzles with which I am familiar. HyPro, GreenLeaf and others also manufacture similar nozzles) 


Every year, hungry hives are placed in orchards before the dawn of almond bloom. In growers’ experience, hives that forage on cover crops early (before bloom) are stronger in the second week of February, when almond bloom usually occurs. What’s more, cover crops planted in orchards provide bees with nutrients, allowing growers to boost their pollination potential.

In addition to providing better bee health, cover crops improve soil health in the orchard. Orchards will have better water infiltration, earlier field access, reduced compaction and better nitrogen contribution if the right mix is used. All cover crops excel at increasing organic matter, an important, yet often overlooked, aspect of soil health. Organic matter holds 18-20 times its weight in water. In fact, a 1% increase of organic matter in soil can hold up to 19,000 gallons of water per acre!

Here at Project Apis m. (PAm), understanding cover crop impact on orchards is part of our persistent tracking and funding of honey-bee-related research. Located at the nexus of two complimentary industries – almond production and pollination – PAm develops programs, such as Seeds for Bees, that fit the needs of both beekeepers and growers.

Launched by our experts in 2013, Seeds for Bees is a free bee forage cover crop program that almond growers with orchards of all sizes use to take advantage of the many mentioned benefits of cover crops. Demand for Seeds for Bees has grown steadily. Last year, the program provided 6,200 acres of cover crops to California growers, up significantly from 2,100 acres the first year. Each grower may receive a set amount of seed via Seeds for Bees funding. However, many growers with large acreage will buy additional seed outside the program to supplement coverage of their entire orchard.

Planting Cover Crops

With the aim of ensuring an early bloom, the ideal time to plant PAm cover crops is by October 5, before the first winter rains. However, some orchards with late harvesting varieties may not be ready by then. Cover crops planted after October 5 will still germinate, though the hive-strengthening aspects will be diminished. Synchronizing cover crop bloom with the bees’ arrival is the best way to take full advantage of the Seeds for Bees program.

There are three Seeds for Bees options from which to choose:

  1. PAm Mustard Mix is a mixture of Canola, ‘Bracco’ White Mustard, ‘Nemfix’ Mustard, Common Yellow Mustard and Daikon Radish. This mixture is great for adding organic matter and alleviating soil compaction in the orchard and requires the least amount of water among the three options.
  2. PAm Clover Mix is a mixture of six different species including Crimson Clover, ‘Hykon’ Rose Clover, Nitro Persian Clover, Frontier Balansa Clover, Berseem Clover and Annual Medic. Unlike the rapid fall growth of the Mustard Mix, this mix grows slowly over the winter. Clovers are nitrogen fixing plants, adding up to 84 lbs. N/acre.
  3. ‘Lana’ Woollypod Vetch is not a mixture, but a single species. Vetch, like clover, has nitrogen-fixation properties and should be planted early.

The best method for planting is direct seeding with drill equipment. We recommend an orchard/compact drill, equipment sold by companies such as Schmeiser or Great Plains. If broadcast seeding is the only option, a fine seed bed is desirable, since most of the seeds are small, like alfalfa. Ideally, the soil should be disked, cultipacked with a ring roller, planted and rolled a second time. The cover crop can be mowed or disked any time after almond bloom. If reseeding is desired, leaving plants intact until June may be necessary, however, this date will vary depending on the planting date and local climate. Reseeding of the PAm Clover Mix and ‘Lana’ Woollypod Vetch is encouraged, but the PAm Mustard Mix can be aggressive and is not a good candidate for reseeding.

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A new item has been posted at "The Almond Doctor" titled "Almond Postharvest Management: Nitrogen considerations." This article highlights our understanding and some of the research regarding nitrogen management practices within almond. An excerpt:

"Interestingly, recent research suggests that late postharvest fertilization (October) can be skipped if mid-July leaf nitrogen levels are adequate (over 2.5% nitrogen). This work by Franz Niederholzer conducted at the Nickels Soil Lab in Colusa County has found no negative impact on yields when skipping postharvest nitrogen applications within orchards over the past two years..."

A link to the article can be found here:   David