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Forcing Deciduous Trees

T. Kimmerer populus at pc.jaring.my
Sat Dec 16 02:15:08 EST 1995

>  >Has anyone here had any experience with forcing deciduous trees to
>  grow 
>  >indoors in the winter?  I have an extra Oxydendrum arboreum (Sourwood)
>  tree, 
>  >aprox. 2 years old, that I am attempting to grow indoors.  I have
>  added a 
>  >light source that shortens the dark time to about 11 hours, and  I
>  have placed 
>  >the tree in from of a south facing window.  Anyone have any comments
>  or 
>  >suggestions?   Should I fertilize for better results?  

There are two controlling factors in bud dormancy in trees: temperature and photoperiod.
Short photoperiod commits trees to dormancy,  while chilling  breaks dormancy.  

So, you can do one of two things: 1) keep daylength long (>14hr, or give a middle-of-night light break);
or 2) once the tree has stopped growing, give it a chilling period.  Chilling periods for buds are not
cataloged, but those for seeds are cataloged in "Seeds of Woody Plants" by Young and Young. It turns
out that the chilling requirements for most seeds are similar to the requirements for buds of the same

Note the two complicating factors, here: 
1) daylength and chilling don't substitute for one another,
they are different kinds of signals.  Once short days have committed the plant to dormancy, only
chilling can subsitute.  So, once your sourwood has become dormant, chilling is usually required to
break dormancy.  Even this isn't strictly true: some trees can be fooled with extra long photoperiods or
light breaks.

2) Sourwood, like many trees, has a fixed growth habit, meaning that the shoot extension period is very
short.  In Kentucky, shoot  growth in sourwood is usually complete by the end of June, though
cambial growth continues.  In this case, long days may permit a second flush.  

For the hobbyist, the easiest treatment is cold - just leave the trees outdoors for a month or so, then
bring them in to force them.   I would be very surprised if you could keep sourwood growing for very long
without cold periods.

Fertilizer can stimulate growth, especially if applied after bud set but before true  dormancy ensues.  With
sourwood, you want to be sure to use an acidic fertilizer such as Miracid.  

All this is complicated, isn't it?   You will need to experiment for your particular conditions, because we
don't really understand all the rules.

Good Luck
Thomas W. Kimmerer
University of Kentucky
Kuala Lumpur, Malaysia  

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