The use of interstems in cherry orchards could revolutionize California’s cherry industry by expanding the area where cherries can be grown, increasing tree precocity and fruit size, and controlling tree size. Fruit breeder Floyd Zaiger of Zaiger Genetics, Inc., in Modesto, California, spent years developing hybrid interstems from different stone fruit species that would allow cherries to be grown on a wider variety of rootstocks—including Citation, his patented interspecific rootstock developed from peach and plum parentage.
After several years of testing different interstems, Zaiger finally settled on the Zee Stem, a patented and trademarked interspecific combination of peach and almond rootstocks. Zee Stem, commercially available for the last three years, has sparked the interest of California cherry growers who are looking for ways to bring trees into production faster, grow smaller trees, and improve fruit quality.
Leith Gardner, Zaiger’s daughter, who works in the family business, said that while they are looking for new dwarfing rootstocks for soft fruit that will help eliminate ladders, the Zee Stem is “strictly for cherries.” The University of California is also interested in cherry interstocks and is working to develop additional interstem combinations. An interstem or interstock is an intermediate piece of rootstock that is grafted between the scion and rootstock.
“We’ve tried the interstem with a lot of different rootstocks like Citation, Nemaguard, Lovell, Viking, and Atlas, and found that it’s compatible with most any rootstock that you put it on,” she said, adding that the interstem allows cherries to be grown on rootstocks normally used for other stone fruit. “We’ve found good compatibility with just about every cherry variety that we’ve tried—Bing, Van, Lapins, Stella, and our own cherry varieties.”
She added that because it improves the compatibility of cherry varieties and rootstocks, growers have rootstock options beyond the standard Mazzard, Mahaleb, and Colt cherry rootstocks and can plant in a wider variety of soil conditions. “Growers won’t be as limited in their rootstock choices. It will increase the cherry growing areas.” For example, what started as a cherry test plot in southeastern Kern County is now a commercial cherry orchard, with other cherry orchards also being established in the area.
Kern County, traditionally an area where table grapes, stone fruit, and citrus crops were grown, is rapidly expanding into cherries. While Zaiger Genetics hasn’t conducted research to understand the physiological response of putting interstems on cherry trees, Gardner explained that the interstem principle in cherries works the opposite of interstems in apples. Interstems have been used in apples to control tree size in sites where dwarfing rootstocks don’t perform well.
In cherries, the shorter the interstem, the shorter the tree; in apples, longer interstems result in shorter trees. She believes that in cherries, short interstems bring the scion wood closer to incompatible rootstocks, and the interstem acts as a girdle. Longer cherry interstems result in taller trees. “In my opinion, the cherry interstem combination with Citation rootstock is the best of all rootstocks,” Gardner said.
“You get a more precocious and smaller tree than