I have an opportunity to get two English Walnut trees and three Plum trees but I think they are too large to dig up so I am thinking air layers.  They both have 4-6" caliber trunks.  If anyone has any experience on these please let me know.

Danny,

No clue on the walnut but aren't plum trees usually grafted? I know my Japanese plums are.   Do you know what type of plums they are?

I think they are Methley Plums, might be Santa Rosa or Morris, or Bruce, or Ozark Premier.  These are the only 5 plums that are recommended for Texas.  These are all hybrids off of the Japanese/Asia Plums.  There is no mention of grafting Plums on the Texas A&M site.

I found this article with a search.  I don't know how well it works.
http://homeguides.sfgate.com/root-plum-tree-limb-51332.html

Danny,

I think all the commercially sold plums are grafted?   I hope Fignutty sees the thread he will be able to tell you for certain.

Most fruit tree are grafted to rootstock. This does not mean that an airlayer will not work. Most plums, apricots, peaches and nectarines are grafted on to a variety of plum.
Depending on the charcteristics of the tree whether it is a dwarf, semi dwarf or a standard tree they usually use different types of plums. I received some native plums from a forum member in Colorado that I will use to graft peaches onto. The native plums have a very hardy root system that will live in many soil types making it a very good choice for rootstock. With that being said, almost all fruit trees can be rooted and live from its own root system. It just may not do as well in your soil as your rootstock. There are several native plums that grow in Texas. You could use the Chickasaw, Munson or the Mexican plum as a rootstock and graft on to them.