Pistachio trees in the southern San Joaquin Valley have had a good rest this winter, and should the recent warm weather continue, growers could begin seeing bud push 7 days to 10 days earlier than usual, says Bob Beede, University of California Cooperative Extension farm advisor for Kings County.
That would put the start sometime in the last week of March. Normally. pistachio buds start pushing around the first week of April.
For reasons not fully understood, pistachio trees require about 800 hours of temperatures below 45 degrees during dormancy for normal growth. This winter, in the warmer areas of the Valley along the foothills of the Tehachapi Mountains in Kern County, observers measured as much as 921 chillling hours through early February, Beede says.
Typically, those locations receive only about 400 chilling hours in winter. To the north, in Madera County, where a total of 800 chilling hours is the norm, some orchards have had more than 2,000 hours.
“Overall, it’s been a cold winter,” Beede says. “Although we’ve had some warm days without fog, which tends to negate some of the cold temperatures we’ve had, it still appears the trees’ need for rest has been well-satisfied this winter.”
But, the meager amount of rain that has fallen on orchards this winter in both the San Joaquin and Sacramento Valleys, is another matter. Without adequate irrigation, low soil moisture levels could limit early season root growth.
“We’re recommending that all nut growers assess the moisture content of their soils,” Beede says. “If you find the ground is dry, now’s the time to apply water.”
Nut trees, including pistachios, require about 4 acre-feet of water a year for optimal productivity, he notes. Providing that water this year could force some growers to revise their budgets
“Water costs this season may prove to be exceedingly high on the West Side,” Beede says. “Growers in the eastern part of the Valley with wells would be less affected. No question — if the current dry weather pattern continues, this will be a year when the ground water table is receding.”
On the upside, more dry weather would lower other production costs. For example, it would mean less natural vegetation in native areas, reducing migration of crop-damaging insects into orchards and the cost of treatment to control them. Dry conditions would also offer relief from fungal diseases, which require a certain amount of free moisture to spread innoculum. That includes botrytis and botryosphaeria,both of which can pose significant threats to pistachio growers in the southern San Joaquin Valley in wet seasons, such as 2010 and 2011.
Cold temperatures in November and early December have raised concerns about possible winter injury to pistachio trees in their 6th and 7th leaf.
“Symptoms of cold injury have been reported from all over the southern San Joaquin Valley,” Beede says. “They include trees with a black, sooty appearance and a white, milky substance exuding from the bark. It remains to be seen how much winter injury may have occurred.”
To help assess the extent of any such damage, he suggests growers delay pruning this year as long as possible.