North American Fruit Explorers mailing list at ibiblio

[Charles Paradise <machelp@attglobal.net>]

My two cents:

some plants like HYBRID PLUMS and those PEARS with less attractive nectar 
should be considered for multiple trees per hole. 

Multiple authors like Ken Parr and John Bunker have reported greater hybrid 
plum set when branches overlap.  It follows that a direct way to attain 
overlapping branches is multiple trees per hole.

4 per hole?  Well frankly they all 4 probably won't survive:  because 
there'll be so much difference in vigor between different varieties some will 
overgrow the other.   Plant 4:  end up with 2:  2 may increase pollination on 
hard to pollinate trees.

Charlie Paradise
in Massachusetts we figure its just easier to dig 1 hole than 4