Fruits trees with multiple grafts allow gardeners to enjoy a variety of fruits in the space of a single tree. For this reason, these trees are desirable for people who have little room for multiple trees. Multi-grafted trees are normally all fruits of the same type, such as pears or apples, but sometimes different, but compatible, fruits can be grafted on the same rootstock. Even though multi-grafted apple trees (Malus spp.) are all the same species, there can still be a number of challenges in the care and maintenance of such trees. Apples are reliably hardy in U.S. Department of Agriculture plant hardiness zones 4 through 9. However, apples are grown in all 50 states, and some varieties can withstand the cold of USDA zones 2 to 3 or the heat of USDA zone 10.
Different Growth Rates
The rootstock affects the growth rate of apple trees to some degree. Dwarfing rootstocks, for example, will cause the grafts to remain smaller than they would have normally grown. However, each graft has a certain growth potential compared to the other grafts. One grafted variety may naturally grow more aggressively than other varieties, even when grafted onto a rootstock that controls growth. In this case, the more aggressive variety may require extra pruning to maintain balance.
Different Flowering Times
Apples require cross-pollination, in most cases, to produce fruit. Ideally, the varieties grafted onto a multi-grafted tree can pollinate each other. However, if they bloom at different times, one variety may finish flowering before the other begins, and therefore can't cross-pollinate. If there are other apple trees around, this may not be a problem. However, if there are few apple trees close by, individual grafts may produce few or no fruits for lack of available pollen. For example, "Liberty" or "Yellow Transparent," two early-blooming varieties, may not pair well with "Red Rome" or "Northern Spy," two late-blooming varieties.
Different Temperature Needs
All apples need a certain amount of time in cold temperature to produce fruits. This time period is called the chill time or chill requirement. Some varieties have a low chill requirement, while others have a high chill requirement. If one variety on a multi-grafted tree has a high chill requirement and the tree is grown in an area where the winter's temperatures do not satisfy that requirement, that variety may never produce fruit. If that graft doesn't produce fruit, likely it will also not produce flowers, and if other varieties on the tree depend on that variety for pollination, these dependent varieties may also not produce fruits.
Different Disease Resistance
Some varieties of apples have different disease resistance. This can be a problem if a disease attacks the tree. Even if certain varieties are disease-tolerant or disease-resistant, these varieties may not be as resistant when a neighboring graft has no resistance to the disease. Disease resistance in apples does not equal immunity. Under stress, excessive exposure to disease or extreme environmental conditions, even resistant varieties may succumb to disease. Select disease-resistant cultivars if possible.