I've noticed that since I put this cutting outside, the part that was actually cut from the original tree is starting to become a large hole.     

How should I treat it?   Do I need to put something in it to prevent rot and bugs eating it,  then seal it with something else?



   

It will be interesting to hear what others say.  That does look scary to me, but the tree looks healthy!  Maybe you should do a couple air layers for back up.

Suzi

This is a pretty normal occurrence when a limb is removed.  The bottom of the hole will heal itself.  I normally will leave it until I am bare-rooting the tree.  Then I will take a razor knife and shave away at the dried wood until I see hints of green (healthy wood).  More often than not, it is pretty flush at this point.  In your case, you wont have to remove very much of the material.  Just make sure it is smooth.  

The best way to avoid it is when the limb is being pruned.  Make sure the pruning shears are sharp and the cut is clean and at the proper spot.  Also, prune as soon as you know you want the material removed.

Some people paint or fill holes like that -- but when you seal it away from air, you are creating perfect conditions for rot underneath.

Yes, it looks scary -- but personally, I'd leave it alone.

As it turns out, I got a couple of rooted cuttings in the mail last night.  They spent two weeks in transit and went across the country twice.  Much of the growing mix had been knocked out of the containers, so I decided to bare-root them and pot them up.  One had a small nub where a branch was removed.  If left the center of the nub would sink in and the outside wood would dry up... so I removed it.

Here are the before and after photos:

    

I tried using an older razor and messed up.  By the time I got it cleaned up with a fresh knife, I had taken off a bit more than I would have liked.  It will take a year or two, but eventually it won't be visible.  I would not recommend putting a sealer on it for the reasons stated above.  Also, there are many reports on sealers interfering with trees healing their own wounds.

Interesting, James.   Looks like you are in the same boat as me with those pics, except the white core of mine has receded and left a hole.   

Are you saying that the hole will some how just heal up?   If so, that would be great, because my concern was that rain water would collect and just rot the thing from the inside out. 

But sounds like sealing is a bad idea so I will forgo that option.  

By the way, the little mini green tree growing next to the cutting...is that what's known as a "root sucker", or just a bud from the cutting below dirt?

Hi figlegacy,
up high on a stem, I would rub dirt against and let it be.
At that height, I would mound some dirt to allow the stems to root themselves.
The problem is that that hole is a weak point and the two stems might snap at some point if the leaves up above get too heavy.
I would put a stack at each stem to keep them up right as well .

The little green stem is indeed a root-sucker. You can use it to propagate your tree, just let it age for now. That stem will set back the other stems as being closer to the roots, it will use more sap as being closer to the source.
If you don't need it, just break it under the dirt to leave all the energy flow to the two older stems.
You could still let that tree as is. I would rub some dirt to seal that hole so that if an insect does want to go in, it will have to open the gate and you'll get the info .
I have clay dirt in the garden and that is a nice sealing material .

In a grafting show, the guy used to put two scion on each trunk in an opposite position - sort of, just like your two stems.
He said : when both scion take, I'll cut one. If both are left that will make a hole in the middle of both and that will not heal and both will tare apart and fall at some point.
If one is left and the other cut, then the wound will seal correctly as both callus will join and seal the top of the cut - he used young tree cut flat at some height.
So airlayering one stem, then cutting it, and leaving only one stem should allow for a nice sealing too.
My preference still is to bury as this allows for propagation at the same time.

I'm not suggesting the tree will fill the hole.  Rather, the hole is where the plumbing to another limb used to be.  The tree is redirecting the plumbing to the two remaining branches and will heal itself inside of the hole.  As the trunk gets thicker over time, the tree will heal the back of the hole and the wood around the hole will start to die off.  This is what I suggest gets shaved down just until you see some green poking out.  The splintered wood leaves the tree vulnerable to bug invasion.  It is hard to gauge how deep the hole is from the pictures.  I do not think the branches are necessarily at risk for breaking at this joint.