Because many dwarfing rootstocks have less than the desired level of cold resistance and trees on all dwarfing rootstocks require support, dwarf rootstocks are sometimes used as interstems to combine the desirable characteristics of a vigorous understock and the dwarfing interstem.
Interstem trees consist of three parts:
- A vigorous rootstock (usually MM.111 or MM.106)
- A dwarfing (usually M.9 or M.27) interstem is grafted to the rootstock. The interstem is usually about 6” long, but additional dwarfing can be obtained by increasing the length of the interstem piece.
- The scion cultivar is grafted onto the interstem.
Although this approach seems well conceived, results are usually disappointing.
- Interstem trees are generally more expensive that trees on a rootstock because an additional year is required in the nursery.
- In a rootstock trial in Virginia spur- and non-spur ‘Delicious’ and ‘Golden Delicious’ trees were grown on several rootstocks including two interstem combinations (M.9/MM.106 and M.9/MM.111).
- In general, tree survival was similar to trees on M.26 EMLA; in a few cases non-spur strains leaned or broke at the union between the rootstock and the interstem.
- The M.9/MM.106 trees were slightly smaller than trees on M.9 and the M.9/MM111 trees were similar in size to trees on M.26 EMLA.
- Interstem trees, especially M.9/MM.111, produced many more rootsuckers than trees on M.9 or MM.111.
- Cumulative yield per tree and yield efficiency over a 17 year period was similar for trees on M.9, M.26, M.9/MM.106 and M.9/MM.111.
- In a Massachusetts rootstock experiment a spur and non-spur strain of ‘Delicious’ were grown on three rootstocks and two interstem combinations.
- Depending on the scion cultivar, tree size for trees on the two interstems (M.9/MM.106 and M.9/MM.111) was slightly larger or similar than trees on M.26.
- Precocity was similar for interstem trees and trees on M.26.
- After 5 seasons, cumulative yield was greatest for trees on M.9/MM106, intermediate for trees on M.26 and lowest for trees on M.9/MM.111.
- Following a hurricane during the fifth growing season tree leaning was similar for trees on the two interstems and for M.26.
Rich Marini, Penn State University