2016 Almond Bloom Considerations

Adequate chill and recent warm temperatures have accelerated almond bloom. Timing is close to “normal” in southern and southwestern areas of the State, but are 1-3 weeks ahead further north. Below are some thoughts to keep in mind with the anticipation of rain this week.

Authors note (2-16-2015): There has been some concern about the wording around the time of nutrient/boron sprays and bloom. Research has shown that boron applied postharvest to 10% bloom can increase yield, while applied later than 10% bloom will not have an impact. Nutrient sprays-with the exception of boron as described in the previous sentence- have not been found by researchers to increase yield when applied at bloom. Sorry for the confusion.

Almond trees in bloom.

Almond trees in bloom.

  1. Fungicides are usually effective for about 14 days after an application. This period of protection is decreased with rain events. Furthermore, as surface areas expands – whether it is the flowers or leaves – newly exposed tissues are unprotected from infection. This means that spraying too early may not provide the wanted protection. If planning to make a fungicide application, apply the material as close to the rain event as possible. Remember that it needs between 30-60 minutes to dry.
  2. Depending on the pre- and post-rain conditions, and the periods of leaf wetness, a fungicide spray may not be needed. Keep in mind that fungal diseases require several hour periods of leaf wetness – which comes from fog, dew, and rain.
  3. Be cautious with adding additional surfactants. Most fungicides have surfactants mixed with the active ingredient. Adding additional surfactants may impact the efficacy of the fungicide, damage the flower’s tender tissues, or impact bees. The label should indicate if a surfactant is needed.
  4. Avoid tank mixing in boric acid with sprays made during bloom when bees are active(1). Researchers at the University of California as well as around the world have not found an increase in yield with boron applications made at full bloom (1). Furthermore, boric acid is a known insecticide, and can impact bees.
  5. Be careful with bees. Do not tank mix in any insecticides for PTB besides Bt. Pesticides applied at this time do not control Navel Orangeworm. Do what is possible to reduce pollinator exposure to pesticides. More on this topic can be found here.
  6. Although nobody believes it, several replicated research plots testing foliar nutrition applications at full bloom have not shown an increase in yield. These trials were in Kern, Stanislaus, and Colusa County. Granted, not all of the products on the market have been tested (there are only several 100), but nearly all of them have a high mark-up. Feel free to tank-mix in what you want, but consider asking questions regarding what you are applying and why.
  7. Do not irrigate too early. Nutrient uptake at this time of the year is minimal, but the ability to kill roots from saturated soil is great. If the tree was fertilized in the post-harvest, it should have enough nitrogen stores to make it through bloom and into full leaf-out. Killing roots now will lead to increased stress levels in June/July.
  8. Never-the-less, don’t ignore the irrigation system altogether. Make sure to perform the required maintenance in case it is needed for frost protection.

Good luck!

(1) – Edit to clarify the timing of boric acid application made on 2/15/2016.

Print Friendly, PDF & Email

8 thoughts on “2016 Almond Bloom Considerations

  1. David,

    What about the study done by C. Sotomayor, H. Silva, J. Castro, “Prunus dulcis, almonds, boron, zinc, fruit set, foliar sprays”?

    Fruit set is crucial in almond trees (Prunus dulcis) and it has been proven that micronutrients, zinc (Zn) and especially boron (B), have a significant impact on fruit set, as well as on fruitlet abscission. In several fruit trees it has been reported that even foliar spraying of one or both elements has improved productivity. As far as almond trees are concerned there is very little information in this respect; therefore 2 experiments were carried out with almond trees cv. ‘Nonpareil’ and ‘Carmel’.

    “In the first experiment ‘Nonpareil’ trees were sprayed at full blossom with boric acid at 0.2%, Zn-50 at 0.3% and with the combination of these micronutrients, compared with a non-sprayed control. Fruit set of the latter was 23.4%, similar to the B treatment (27.7%) and Zn (22.2%). However the B + Zn combination produced a significantly higher fruit set (38.1%).”

    This seems to contradict what you wrote, that “Researchers at the University of California as well as around the world have not found an increase in yield with boron applications made at bloom.”

    Furthermore, on the boric acid on bees study, the amounts used in that study were exponentially higher than those used in foliar sprays on almonds. 1 mg/g boric acid solution = 1000 ppm boric acid solution. To achieve that concentration of boric acid (not B, but boric acid) in a 100 gallon spray tank, it would take almost 32 quarts of an industry standard 10% B foliar product made from boric acid in a 100 gallon spray tank, which is about 64 times higher than the typical rate per acre (1 pint 10% B). I have regularly recommended to my growers if they have been struggling with low B levels reflected in leaf and hull samples, to apply 1 quart of 2.5% Boron made from boric acid and organic acids, in 100 gallons water, which equates to 78.65 ppm elemental B or 7.8 ppm 10% boric acid equivalent. Not only have we not seen bee damage (though that is always difficult to ever really know beyond their own time at your orchard), but we have seen corresponding yield increases in controlled test plots (randomized and replicated with P=<0.1 Duncan's New MRT).

    In talking with Dr. Patrick Brown about this some time back, he referenced the study done in Holland or Denmark on agapanthus where high concentration of boric acid damaged the pollen tube (I have the photocopy but the quality is poor). That study referenced other work (Dickinson, 1978; Kandasamy and Kristen, 1987; Rosen, 1961; Dugger, 1983; Robertse et. al) where applications of boron to petunia and avocado were actually beneficial since boron tended to mostly be bound to cell walls during bloom and therefore not available to the growing pollen tube tips. The conclusion Dr. Brown made was that in light of the damage boron did to the pollen tubes in the agapanthus study (although again, at high concentrations), that caution should be used when spraying almonds in bloom, but I note that no actual work has been done in prunis dulcis (almond) to verify this in our crops. From an agronomic point of view, if I know there is a shortage of B historically in the orchard demonstrated in valid samples, a low rate of B applied when bees aren't flying seems to be the proper thing to do. However, if the B levels are good in the orchard, and since we have this potential problem at high rates, I do agree with being cautious and not applying B at bloom in those particular situations.

    1. Dear Jerel,
      Thanks for the comment. Yes, I was aware of that study and even referenced it in a recent column. The research does not contradict what I wrote. I stated that foliar nutrients applied at bloom does not impact yield. The abstract (and full text article) indicates that yields were not taken. The abstract continues by highlighting the methods of a second study that tested 0.15% and 0.3% boric acid applied at postharvest or during bloom and concluding with boric acid at 0.15% applied either in postharvest or bloom, or 0.3% applied at postharvest increased fruit set. The 0.3% treatment did not outperform the control. The authors speculated that the 0.3% boric acid applied at bloom may have impacted flower fertility.

      Studies done by Duncan (Stanislaus County), Niederholzer (Colusa County), Viveros (Kern County), Ellis (Graduate student at Fresno State) across replicated studies and multiple years have not found any increases in YIELD with the application of foliar nutrients at bloom. The only exception has been with boron and that has been if it was the first boron spray of the new crop (meaning no boron applied as a foliar during postharvest or dormant, or at pinkbud). These studies were within California, not Chile.

      Applications of boron in postharvest, dormant, pink bud, or at early bloom impacting yield is nothing new. The studies used Solubor at 2 pounds/100 gallon/acre of water.

      Re: studies with bee health and boric acid: yes, the lab studies used high rates. Yes, this may not be 100% applicable to the field. The study does suggest, however, that there is a negative interaction between boric acid and fipronil (which we dont use in almonds). This suggests that there could be a similar response with other pesticides. Boric acid is classified as a pesticide and is used for ant, termite, and cockroach control. Ants are in the same order as bees, suggesting further that too high of an accumulation may be toxic to bees. More research is needed to see if pesticides and boric acid concentrations that are used in perennial cropping systems may have the same effect.

      Perhaps there is no impact – but I dont see how expressing caution when using boric acid nutrient sprays, and suggesting that applications should occur when bees aren’t active is bad thing. I did see how my statement could be interpreted differently and I edited it and provided a notation.


      1. Thanks David for the reply.

        Re: the Sotomayer et. al study, yes you are correct that was an increase of flower set not actual yield. Of course in hindsight one can look back and wonder if there was inadequate nutrition later on in development to sustain the increase of flower set.

        It’s interesting that you mention Ellis (grad student). He now works with me. We are aware of the University research that has not shown yield increases from foliar applications on almonds. But there is also private research, conducted to University standards, which do demonstrate yield increases in some cases. As Dr. Brown would say, it always comes down to the 4 R’s: Right Rate, Right Timing, Right Source, Right Place.

        I look forward to more research re: boron on bees that would more accurately reflect what actually happens in the field, at the field use rates rather than 10x those rates, and not with a known pesticide that damages bees. I am concerned about bee health, and would never want to recommend something that could damage bees.

        I also look forward to more research as it relates to boron applications specifically in almonds at bloom. Much of the work Dr. Brown referenced last decade showed benefits to B applications just prior to flowering. I think that is probably the best approach to take (apply just prior to flowering), or move to post-harvest. I appreciate that you edited your original blog to reflect that. There seemed to be some benefit to B application later in bloom in that older research, but not always, and that might have to do with later timing of B not influencing pollination, or it could be that the flower already had sufficient B and the foliar spray was high in rate and had an adverse effect. I understand the University must take a much more conservative route and advice caution, but CCA’s and other advisors in the field who take in the full picture might see trends in the orchards they watch, that the University misses. But your work and help is still appreciated.

        1. I agree, Jerel. There is still a lot we have to determine when it comes to the 4 Rs of foliar applications within almonds. As you pointed out, this article focuses on applications at bloom.

          Thanks for the comments and discussion.

  2. Firstly I must emphasize that it is really important in this discussion to separate ‘pre-bloom (bud swell through <5% open flowers), from 'during bloom' (a significant percentage of flowers open and receptive to bees') spray timing. This is a distinction that may have been lost.

    The results of the work we conducted on B timing in the mid-late 90's (Nyomora et al, 1997, 1999) showed a frequent benefit to B sprays and occasional no benefit (predominantly boric acid) when applied just prior to flowering. We found occasional benefit when sprays overlap the earliest stage of flowering <10% bloom and on occasions we also observed occasional significant negative effects when sprayed with a majority of flowers open – this was never observed with pre-bloom sprays. Work in Olive showed that the benefit of the B spray was partly due to changes in floral integrity (Perica et al 2001 HortScience 34:714) when applied 3 weeks prior to bloom. The results from olive suggest the benefit of B occurred during the rapid cell development that occurred as flowers developed but before they opened. Niederholzer also showed (in agreement with our work) that B sprays in late summer early fall were frequently as effective as pre-bloom sprays, further suggesting the benefit of B was not to pollen (though that clearly occurs) but to the developing flower. This in conjunction with the Robbertse work that showed B sprayed directly on flowers could disrupt pollen tube directional growth (their ability to grow toward the ovary) led to the conclusion that sprays pre-bloom (and summer/early fall) were the ideal choice. You are correct however that work on the negative effects of B when sprayed onto open flowers of Almond has not been published, this was because the negative impact of during bloom sprays was extremely inconsistent and as such did not meet the scientific standard of proof.

    With regard toxicity of Boric acid to bees it should be noted that the lowest boric acid concentration used in the experiments of de la Cruz et al (2010) of 1 mg/g (1000ppm) of food material was significantly toxic to bees and hence the lower limit of toxicity was not established and was almost certainly lower than 1mg/g (1000ppm). It is also somewhat difficult to reconcile artificial feed supplementation concentrations with in-vivo nectar concentrations. Boron concentrations in sprays commonly used in almond range from 100 to 400 ppm, which is close enough to be of concern. Having said that, direct toxicity of B sprays to bees in almond has never been demonstrated.

    The general principle of not spraying active bees with any substance is a sound precaution. Given the efficacy of sprays applied pre-bloom and indications of negative effects on flower health when sprayed during bloom it certainly seems prudent to avoid a 'during bloom' spray application of boron.

    I hope this has helped.


    1. I am unsure on what you mean – From a single flower or spur? Sometimes spurs can have multiple flower buds. I have seen double embryos/kernels, but I am unsure on what you mean by multi-fruit.


Leave a Reply

Your email address will not be published. Required fields are marked *