Furlongarmer,
A five in one apple
tree is an example of multiple-variety
trees that have 5 varieties of apples grafted into a single trunk and rootsock. A 3 in 1 or 4 in one or 5 in 1
tree gets you a selection of apple
tree (and pollination as well).
All apples require pollination although most will set adequate fruits on their own, but will set more fruits if pollinated by other variety.
If you have only one
tree and your neighbor has a pollinating variety, you don't need to
plant another one. If your
tree is not bearing, you can graft a branch from another variety into it, or place a fresh bouquets from another variety (in a can of water) at the base of your
tree. Don't use bouquets from a triploid.
Certain varieties (triploids) are pollen sterile (ex: Gravenstein or Winesap)and do not produce fertile pollens and will not fertilize their own
flowers or those of other apples.
What variety is your green apple?
Source: Sunset Western Garden Book