Extreme weather at bloom can have damaging short and long term effect on prune growers in California and the entire California industry. Excessive heat (2004, 2005, 2007 and 2014) or prolonged cool, wet weather (2016) can dramatically reduce prune production. These losses harm growers’ bottom line as well as the local economy, and force the industry to struggle to recover market share for years afterwards. The following are the conditions of most concern to prune growers at bloom and what, so far, are options to manage extreme weather at prune bloom.
Prune fruit set is vulnerable to a short duration(s) of high temperatures (greater than 80oF) at full bloom or extended periods of low temperatures (less than 60oF) throughout bloom.
What can growers do if flowers are beginning to swell and hot weather (>80oF) is forecast?
Run water. Running micro-irrigation sprinklers reduces orchard temperature 1-2oF. This might be enough to allow for better fruit set if high temperatures don’t exceed 82-83oF. Deep watering is not necessary. Just the surface foot of soil needs to be wet. If temperatures for the day are forecast to come near 80oF, run sprinklers only when temperatures reach 70-75oF and shut off when they drop below those temps later in the day. Since the goal of running sprinklers at bloom is to drop orchard temperatures by evaporation of irrigation water and evaporation is greatest under warm temperatures, there is no benefit to running water once temperatures cool off in the evening. Don’t wait too long (when temps are past 75oF) to start water, as temperatures may change faster than it takes for maximum cooling benefit from sprinklers.
What can growers try if extended cool (<60oF) weather is forecast during bloom?
Include boron (equivalent of 2 lbs Solubor®/acre) in green bud spray. This practice improves set in almonds during extended, wet bloom weather. No trials with prunes and boron have been done during wet, cool springs, but boron is cheap and will not harm bloom if applied at green bud. Avoid spraying boron at full bloom.
Closely mow any weeds or cover crop. A closely mowed orchard floor is warmer than one with tall weeds/cover crop. But don’t disk. Freshly disked soil will not hold and re-radiate warmth.
What could growers consider if hot (>80oF) or cool (<60oF) weather is forecast during bloom?
Spraying the orchard with a heavy oil rate (for example, 4 gallons 440 oil per acre at first flower) can delay opening of some flowers a day or two. This might, depending on the timing of bloom and bad weather, extend some bloom beyond the period of bad weather. If hot or cool weather extends longer than late oil delays bloom, there will be no benefit. With a good quality oil and proper spray tank agitation, no flower damage should occur with his rate of oil.