In spite of being a hardy tree, pistachios can be picky about bloom time temperatures.

Mood swings aside, pistachio growers may want to consider using supplemental male pollinizers in addition to standard males when planting a new orchard. UCCE Farm Advisor Craig Kallsen, Kern County, made this suggestion in his presentation for the annual Statewide Pistachio Day.

Kallsen also noted that ensuring bloom synchronization does not guarantee adequate pollination and yield potential in years with inadequate chill.

Lack of adequate chill hours during the winter dormant period is forecast to occur more frequently in coming years. Katherine Jarvis-Shean, UCCE farm advisor in Yolo, Solano and Sacramento counties, said in her Pistachio Day presentation that climate models predict winters in the future will be warmer than in the past. Models also predict winter temperatures will continue to vary from year to year. Winter-to-winter variability will be two times the expected shift in temperature, Jarvis-Shean said, and there will be some cold winters and winters that would now be considered average.

With more low chill winters predicted, understanding pistachio tree response will be critical to achieving adequate nut yield.

Dormant Season Influence

How well pistachio trees leaf out and/or bloom in the spring partially depends on what happens during their dormant period. Dormancy, Kallsen explained, is closely associated with environmental conditions. During dormancy, growth and development is reduced, there is decreased metabolic activity and carbohydrate resources are conserved.

Plants that go dormant in the winter are generally adapted to cold climates, and going dormant reduces their chances that new growth and flower buds will push when temperatures are freezing. Pistachio trees grown in the San Joaquin Valley have been able to adapt, Kallsen noted, but some rootstocks do not appear to go dormant.

There are two types of dormancy, Kallsen explained. Endodormancy is broken by chill accumulations while ectodormancy is broken by heat accumulations. For leaf out and bloom to occur, the plants endodormancy requirement for an adequate fall/winter cold period first must be met. That means the weather during fall and winter must provide the trees with a certain amount of ‘cold’ before trees will come out of dormancy.

Kallsen said pistachio trees appear to be ‘switched off or on’ to enter or exit dormancy through genes and associated proteins. These genes are fully integrated in pathways leading from ‘sensing’ and ‘measuring cold’ to resuming growth after the endodormancy period. Gene-encoded proteins exist in plants that measure the length of day and may also be involved in the tree entering and breaking endodormancy.

Once endodormancy or chill requirement is met, the plant is primed to get growth started in the spring, but not until temperatures warm. The time period between when a plant is ‘switched on’ and to when temperatures are warm enough for growth is ectodormancy.

Kallsen said pistachio trees need a certain amount of heat to push buds and begin flower development. When that level is reached, the tree is no longer in ectodormancy and the growing season begins with initiation of flowering and bloom. The warmer it is, Kallsen noted, the faster these things take place.

Time for Bloom

Once growth commences, the available carbohydrate is critical for bloom, nut set and yield as well as next year’s flower bud retention.

Adequate flower development and pollination appear to have a window of success, Kallsen said. Hot temperatures (one study noted over 80 degrees F) during this time can cause reduced flower weight, premature senescence of pollen and ovaries, and poor flower quality. If cool temperatures prevail during bloom, enough heat will be available for slow but normal flower development, but Kallsen said that there will be a threshold where temperatures are too cold for fertile male and female flowers to develop. Flower development must occur within a given time frame, and flowers cannot wait indefinitely for warmer temperatures.

“Trees adapt to a certain time frame, and if this time frame is missed, there won’t be much nut production,” Kallsen said.

In the San Joaquin Valley, pistachio trees that experienced insufficient fall and winter cooling, compounded by fall and winter solar bud warming, will have the following symptoms: Male and female trees not blooming together; North side of tree blooms before south and top of tree resulting in late or prolonged bloom − or no bloom on south side; Abnormal flower development; Flagging of shoots; and Leaves only pushing at ends of branches.

Leaf-out on the Valley floor in the cool and rainy spring of 2020 was slow, Kallsen noted, but it was uniform and there was male and female bloom synchrony.

At higher elevations, tree showed some unusual symptoms, where both sides of trees experienced late leaf-out. Kallsen attributed this to low winter chill and low temperatures during bloom. This should not be confused with winter juvenile tree dieback, he noted, as the branches were not dead and eventually did leaf out.

Picking Male Pollinizers

In recent years, the pistachio industry has assigned a standard pairing for pollinizing males and female cultivars. Those are Kerman-Peters, Golden Hills-Randy, Lost Hills-Randy and Gumdrop-Tejon. However, Kallsen noted that in the 1970s and 80s, it was common to plant two different male pollinizers in orchards.

The industry may be coming back to that practice, he said, in an effort to compensate for erratic bloom.

Average full bloom date for Gumdrop is March 28. For Golden Hills and Lost Hills, it is April 7. Kerman is last at April 12. At a given location, and for similarly aged trees, Gumdrop will be in full bloom about 11 days before Kerman.

“If you are planting a new orchard with Kerman, Golden Hills or Lost Hills, it might be good to have more than one male,” Kallsen said.

For Kerman plantings, Peters would work in good chill years and Randy in low chill. For Golden Hills, Randy would work in moderate chill, but Tejon would help with low chill. Kallsen said that as the trajectory for chill decreases, there is no need for a male pollinizer in high chill years.

However, Kallsen warned that male and female trees blooming at the same time does not necessarily mean that pollination and nut production will happen.