Re: Why do my lilac bushes bloom fully some years and not others?

Why do my lilac bushes bloom fully some years and not others?

The answer to this question is really a tough one.   I guess a silly 
statement from W. C. Fields comes to mind.  "What a woman wants you are 
fresh out of."  It is really very hard to determine exactly why some things 
happen and the blooming of a flower is about that hard.  

Plants behave in many curious ways.  Their behavior is not well understood 
yet.  This behavior of trees and plants having alternating years of big 
blooms and production of seed is well known but really not understood.  
Pecan trees are notorious for having lots of good fruit one year and little 
poor fruit the next year.  Apple trees as they get older tend to become 
more and more cyclic.  

Grapes require frequent and severe pruning in order to force them to bear 
regularly.  The factors here appear to be the reason the plant seeks to 
bloom.

Many plants are severely affected by temperature and day length cycles.   
Chrysanthemums are tricked into blooming by shading them and fooling them 
into believing the days are getting shorter.  Soya beans only bloom when 
the days are getting shorter.    Clearly the appearance of clouds can 
affect this.

In the strong tree and shrub classes of plants the issues include soil 
chemistry, soil temperature and even the presence of other plants.   Also 
many plants seem to store up from one year and only put on their best every 
other year.   

So trimming may affect a plant by telling it that it needs to reproduce due 
to an injury.  
Warming it may make it bloom like as in a corn plant.  Many factors 
determine blooming but frankly I cannot tell you exactly which influence 
your specific plant.  
http://elroy.nmsu.edu/cahe/redtops/_h/h-401.html

The behavior of trees and shrubs is dramatically different at different 
latitudes.  For example Oak trees of the exact same variety grow very tall 
and straight in Kentucky with a fine strong stalk (bole) and a fine ball 
shape on top.  The same tree in north Florida grows low and have no tall 
bole but may spread to cover several acres.    The same breed of pine tree 
grows a thick tuft of nearly 8 inch needles in north Florida and grows very 
tall with a tiny tuft of branches at the top and yet makes the classic 
conical form in Kentucky.   In north Alabama it does mid way between.  The 
same tree called the Southern Yellow Pine with its coarse grain in Alabama 
grows fine and stable grain in Arizona and is called the Ponderosa Pine. 

Blooming is different too.  Dogwood trees bloom only once in Alabama but 
there are spots in the country where they bloom two times a year.  

This is a worthy subject to study and of great commercial interest because 
the reproduction of plants is of great commercial value.    Measuring the 
factors is very difficult.  

As to a personal observation,  Lilac bushes seem to not respond very well 
to pruning.  They seem to be more affected by the nature of the winter 
before the bloom.  Remember that a plant is not dead in the winter.  Peach 
Trees are similarly affected.  They require a minimum of cold to bloom.
 http://www.cciw.ca/eman-temp/ecowatch/GREENWAV/CH1.HTM

I am sure there are books on Lilac culture but remember that much is still 
not known.  
Sincerely 
Paul Noel