Wednesday, May 27, 2020
This is the time to follow-up on your grafts and strip the sprouts off the rootstock.
You may have done it when you grafted, but that rootstock wants to live.
It is likely that you stored your scion in a refrigerator or other cool place and then waited for the weather to warm up before grafting. That gives the rootstock a headstart.
When a bud breaks dormancy it puts out growth regulators that suppress surrounding buds from breaking. The biologists call it "apical dominance". From the organism's perspective, the extra buds are insurance. It is a waste of resources to have more buds start growing than there is sunlight to capture.
Since buds on the rootstock are nearly always exposed to more heat units than the scion, they push buds first and that suppresses the scion.
The key is to strip the growing buds off the rootstock until the scion takes off and grows. For some shy scionwood, you might have to come back two or three times before the buds start growing.
Too kind-hearted to do that? It is a matter of priorities. A concert pianist shouldn't break bricks with karate chops. You have to harden your heart and convince the rootstock that supporting that scion is the only way it will survive.
Rootstock that produce optimal branch angles
I was not a believer. I did not see how it was possible for the rootstock to change the angle of the branches springing from the scion.
I now see how that could be possible.
When you look at last year's growth on an apple tree, you will notice that the branches (shoots, really) near the top of the tree tend to be far more vertical than the branches lower on the stem.
Some apple varieties (Northern Spy, Granny Smith and Fuji for instance) are notorious for producing "blind wood". Blind wood are lengths of stem bereft of any side shoots. Blind wood typically occurs on the older part of last years growth. The few side shoots that grew tend to be closer to the apex. For whatever reason, many of the buds did not break dormancy and start growing.
If the rootstock produced some kind of growth regulator that stimulated "feathering" on the portions of stem that would otherwise be "blind", then those feathers would likely be at near right angles from the stem. So it would be more precise to say desirable rootstocks enhance the bud-break along the entire length of the previous year's growth than to say they produce "optimum branch angles".
The optimum branch angles are a by-product of the feathering. The apple grower has far more branches to choose from when pruning. He is not forced to take the vertical ones near the top because those are the only ones available. He has a wealth of branches along the length of the stem including an abundance of shoots at "good" angles.
An alternative hypothesis is that the desirable rootstock "eats" or destroys the growth regulator produced by the growing points. The lower level of the dormancy extending growth regulator would encourage buds to break dormancy and grow.