Grafting usually increases vigor, however you can go about dwarfing them via graft, not sure how, just know it can be done. An idea you might want to try is growing them more horizontally than vertical. You can cut the apex of the trees and force lateral dominance, thus making them grow wider as apposed to taller.
Also you can graft a family tree, nam doc mai on one branch, glen on another, kitt on yet another. Combine these techniques and you'll have a compact orchard on one tree. Just some ideas.

Thanks Bob --

The problem here with my mangos, as well as citrus, is that I have to keep them pruned to about five feet so that I can winter-protect them. The condo mangos and citrus are fine with this, and I build cages around each tree that I can cover and heat when necessary.

The seedling mangos especially want to spread out and grow horizontally (as well as taller), but I just can't protect them at that size. I've also thought about grafting onto my dwarf mangos, but then the issue is managing additional branches when the total space I can allot to each tree is pretty limited -- about six feet in diameter.

I'll bet I'm the only person in the country growing mango trees in the ground in zone 8a!

-Bruce

It's not unherdof, but it is crazy! If you enjoy it that's all that matters, that's why we garden. I would say massive pruning is the best idea. I'm guessing that your limited to 6' dia X 12' h, and if you prune extensively, and continually cut the apex, it could be manageable.
You could also let the seedlings grow to about 6' and root prune, it will induce dwarfism, but your yeild will decrease too. Just a thought.

Thanks again, Bob.

I already do heavy pruning, at least twice a year on these seedlings so I can fit them in the cages from November through March. As you probably know, however, heavy pruning with mangos = no blooms, because the blooms are on the ends of the branches.

I think I understand correctly that grafted trees are smaller because of something to do with the hormones associated with branch tips, but I'm not sure. Or, is it more of a function of dwarf root stock? Or both?

-Bruce

Auxins are plant hormones that are synthesized in the buds, the force apical dominence and long roots, cytokinins are another group of hormones that are produced in the root tips of plants, the inhibit apical dominence and are responsible for shoot development. These two hormones regulate most of the growth of plants and are needed in balance, more bud growth means more auxins and more apical dominence and root growth, more root growth means more cytokinins and more lateral bud growth and previously formed buds begin to develop; balance. Grafts inhibit transport of these hormones and make grafted plants grow different.
As for pruning, I would pick two or three good branches, and get rid of everything else, it will keep growth restrained and let fruit set on the chosen branches. At the end of the season, cut those branches back and let them grow out in spring.

Have you considered root pruning or girdling to help control size? You can also top your seedlings very low, maybe around 1-2feet tall, pretty much heading the tree and then wait for new growth to form and train the new growth laterally. You will have to tip the newly forming branches to an outward facing but node because the new growth will still be very vertical. You can use wire to shape the branches and hold them in place.
Simon

Recently I was researching the new HD Mango plantations ( High Density ) were mango trees are planted in rows very close together ( around 10ft ) and only allowed to grow no higher than 12ft or so.

I found out they use a growth retardant on the mangoes. I lost all interest in this because I want to grow my mangoes organically, and frankly I do not trust anything that Monsanto makes on edible crops.

But I wonder if there are natural growth retardants, which do not affect production, taste etc...

Thanks everyone for all your suggestions. My winter-protection of the seedling trees isn't always perfect, and there is quite often freeze damage on the ends of the branches that are close to the tarps and frost blankets encircling the cages. This system works well for my grafted mangos, because branch length is relatively short, and they can bloom because the branch tips aren't so close to the cage.

But on seedling mangos, as you probably know, branches need to be...I don't know...at least six feet in length or more before they bloom. That just isn't possible for me to protect them at this size. Hence, my question about the self-grafting. If you take a branch tip, and move it to the trunk -- I thought that was part of the principle involved in getting a dwarfed tree, and also to get it to fruit sooner?

With so much pruning and die-back, my three trees are essentially topped. Main trunks are perhaps three feet tall, and the branches radiate outwards.

I know picturing this is difficult for those who have never had to winter-protect and heat a tropical tree!

-Bruce

When I read your post I had a nagging feeling I had come across something from the Phillipines. So I looked again and voila!

http://blog.agriculture.ph/low-bark-grafting-how-to-make-a-dwarf-mango-tree.html

Thanks Steve for the link! So, it looks like it might be somewhat successful, although the tree pictured is still "too big". Also, the bark grafting pictured is on small rootstock. I guess I could try cleft or awl grafting against the trunks?

I've never done any grafting before, but mango trees are impossible to kill with abuse. (Freezing, yes, abuse, no!)

-Bruce