National Nut Grower National Nut Grower

Jul 25, 2022
Top 5 things to consider at hullsplit

Hull split is a time when growers’ various orchard practices – proper irrigation, nutrient management, pest control and orchard monitoring – come together in an effort to maximize almond quality and yield. To assist during this busy season, we’ve pulled together a list of the top five things you should consider before hull split.

1. Measuring hull split progress

Proper detection is the first step in effectively managing the potential for myriad pest and rot issues that may occur following hull split.

  • Hull split typically begins in early July, starting in the early growing regions and spreading to later growing regions accordingly. The splitting of blank nuts (as much as 3–5% of the almonds) will occur about a week ahead of hull split and serve as a “heads up” that split will soon occur.
  • Tip: A key to identifying hull split is to look for an initial split in the upper and outer portions of the canopy, particularly the southwest quadrant. Once the fruit splits, hull split has begun.

2. Spraying to target navel orangeworm (NOW)

The most effective time to spray for NOW is during its second flight, which often coincides with the initiation of hull split. As a confirmation that flight activity is coincident with the beginning of hull split, check to see that eggs are being laid on egg or pheromone traps (note: mating disruption may affect trapping). Then, when spraying, make your application no later than at 1% hull split.

If your orchard experiences high NOW populations, a second insecticide application may be needed 2-3 weeks after the initiation of hull split, and a rotating of chemistries is advised to reduce resistance development pressure. As noted in the University of California (UC) Integrated Pest Management (IPM) guidelines, be sure to consider the following before conducting a second application:

  • varieties in the orchard, particularly late-harvested varieties
  • anticipated harvest dates
  • pest pressure noted from egg and pheromone traps
  • overwintering mummy load
  • proximity to an external NOW source, such as neighboring pistachio or almond orchards managed by a different entity

As indicated in the UC IPM guidelines, Intrepid (methoxyfenozide) and Altacor (chlorantraniliprole), are preferred materials for hull split sprays. Pyrethroids have been used in the past, but their use should be considered with caution as NOW resistance to these materials has emerged, and pyrethroids are toxic to beneficials. If pyrethroids are used, they should be applied only once per season. It’s imperative that you also make sure your sprayer(s) are well calibrated to improve spray efficacy and minimize drift.

3. Manage hull rot from day one

Almond hulls are susceptible to several hull rot fungi from the beginning of hull split until the hulls dry, typically a month-long period of time. This period may vary, however, depending on fertilization and irrigation. Successful hull rot control is based upon effective strategic deficit irrigation and nitrogen management.

Strategic deficit irrigation (SDI) 

SDI, also known as regulated deficit irrigation, or RDI, is the practice of mildly drought-stressing almond trees at hull split. By managing SDI, growers can reduce hull rot by 60–90% and experience a more uniform hull split and timely harvest. These factors may contribute to less crop damage from weather and late-season NOW flights, leading to less aflatoxin contamination, as well. What’s more, SDI reduces water use for 2-3 weeks without impacting crop productivity.

The target period of when growers should stress trees is from the start of hull split through 90% hull split, a period of about three weeks. The most accurate way to practice SDI is to use a pressure chamber to track and maintain slight tree water stress levels, between -14 bars and -18 bars, depending on the weather. Information about the pressure chamber is available on the UC Davis Fruit and Nut Research and Information webpage.

Growers who do not rely on pressure chambers, particularly those with a history of hull rot and a visual baseline from which plant stress can be observed, may achieve similar results by irrigating at 50% of normal tree demand, using crop evapotranspiration (ETc) calculations during hull split.

The onset of hull split will vary according to orchard conditions; stress levels vary with soil type and other factors. In shallow soils where trees may dry quickly, growers should initiate stress when blanks start to split, usually about a week before the onset of hull split. On deeper, well-drained soils, it can take up to 20-to-30 days to reach mild-to-moderate stress levels. In this case, growers may want to reference UC Agriculture and Natural Resources Publication 8515 to determine when to initiate water stress.

Nitrogen management 

Almond Board-funded research has found that excessive nitrogen increases incidences of hull rot. It is critical that a proper nitrogen budgeting plan, which is a function of crop load, is in place for the growing season to ensure that only the necessary amount of nitrogen is applied. Growers should also follow their nitrogen budgeting plans throughout the whole season and update their plans as actual yields become more evident. Regular updating will also allow growers to focus their nitrogen applications to the key times of nitrogen demand in almonds: For instance, application(s) during kernel fill should typically occur the during end of May through early June.

You can find more information on nitrogen budgeting and management at

4. If applying AF36 Prevail, aim for 1-2 weeks before hull split

Recently approved for use in almond orchards, the Aspergillus flavus biocontrol agent AF36 Prevail offers a supplementary approach to reducing aflatoxin contamination in the orchard.

If you plan to apply AF36 Prevail this season, plan to do so around early July and make the application on or close to the berms so that following irrigations can wet the Aspergillus-carrying seed. Learn more about AF36 Prevail and how it should be applied in this article from the Almond Board: “AF36, Biocontrol Agent for Aflatoxins, Now Approved for Almonds.”

5. Aim for timely harvest

Growers should keep in mind that a third-generation NOW flight poses one of the greatest threats to the orchard around harvest time, so almonds should be harvested as soon as possible. Timely harvest is when there is 95–100% hull split at eye level in the canopy and will avoid NOW infestations and subsequent aflatoxin contamination. Early-harvested varieties, such as Nonpareil, are much less likely to attract egg laying once they’re on the ground, reducing the risk to later harvest varieties remaining in the trees.

– Almond Board of California

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