Two of the first unfamiliar terms a newcomer to the Brugmansia world finds are the mysterious terms "Y" and sub equal leaf.These are just part of the jargon that long time Angel Trumpet fanciers have come up with to describe two of the indicators that a plant is about to bloom.Generally speaking,a brug must form a "Y" before it begins to flower.The exception to this is a cutting taken from the flowering region which already has buds,no matter how tiny,in the leaf axis.These will flower without forming a "Y".The sub equal leaf signals that the plant has formed a bud or buds.
Notice in the above picture the leaf in the forefront.At the junction of the stem to the leaf midrib,notice that the left side of the leaf is much shorter than the right.Hence sub equal.Now look to the point where the leaf stem joins the main plant.In that leaf juncture you can see a tiny flower bud,just formed,lying along the main stem. Now look just above the top right of the subequal leaf,and deeper into the plant,and you can see a good example of a "Y".Don't confuse this with a side branch.The "Y" shape is distinctive.
THE following was posted by Bettydee in an answer to someones question in the help section.She has given me permission to use it here.
This is as good as it gets.
"Seedlings and cuttings taken from below the mother plant's "Y" will have to go through the entire vegetative growth cycle and form their own first "Y" before they switch over to flowering growth and start producing flowers. If your cuttings came from vegetative branches, chances are you probably won't see flowers before winter. If your cutting came from above the "Y", then the chances are very good that you will see some flowers by the fall. In either case, Brugs are heavy feeders and need to be fed with a complete liquid fertilizer at least once or twice a week. That together with keeping them healthy and as pest free as possible gives you the best chance of seeing flowers.
How do you tell what part of the mother plant the cutting came from? Vegetative growth cuttings tend to grow very straight and without branching until they "Y". The leaves are symmetrical where they attach to the leaf stem. For an example of a symmetrical leaf, take a look at the enclosed photo. Note that both sides along the main rib are the same and are even at the point of attachment to the leaf stem. Cuttings that exhibit this leaf shape will have to go through the vegetative cycle. How long it takes depends on how tall that variety gets before it produces a "Y". Height varies between ~ 5' and 12'. Obviously, the shorter varieties will bloom sooner. Another bit of information: Don't pinch the top of a seedling or the vegetative growth or you will prolong the vegetative cycle. Yes, you will get more branching, but each will have to go through the complete vegetative growth cycle before producing a "Y" taking longer to bloom.
If the cutting came from above the mother plant's "Y", the growth tends to be more zig zag and the leaves will be asymmetrical. The 2 sides of the leaves where they attach to the leaf stem will be uneven.This type of cutting behaves as if it were still attached to the mother plant. It will continue to produce more "Y"s and buds after each growth spurt. Some varieties have very short spaces between the nodes and "Y"s. I have one that has very long spaces. The branches look like grasshopper feelers. They are so long. This type of grower seems to take longer to produce each flush.
Brugs bloom in flushes. So even though you may start to see the formation of buds, it could be weeks or a few months before the buds get large enough and the plant accumulates enough buds to open as a flush.
When you get ready to take your Brug inside, if you can overwinter the entire plant, do so. You can prune in the spring. This helps prevent die-back. If you have to prune, spray cut surfaces with a fungicide. If your plant has produced a "Y" don't cut it off or your Brug will have to start the vegetative growth all over again. If you don't want to take the entire pot indoors, take long large cuttings that include "Y"s, place them in a bucket filled with several inches of water and overwinter them in a cool dark place. A Brug will not go dormant unless it is consistently under 45ºF."