The following is information on the bract splitting disorder of poinsettias.
The splitting disorder was first encountered with the variety 'Paul Mikkelsen', and it has been a problem with some other varieties since them. It can occur on stock plants or the finishing plants. It is especially frequent on stock plants if the shoots become long, with numerous leaves, before a pinch or removal of a cutting is made. It can occur even when the daylength is too long for floral initiation, but a floral primordium is produced and is then surrounded by lateral vegetative shoots. Splitting in stock plants is almost like an uncontrolled pinch, and growers should be wary of cuttings taken from such shoots. Cuttings taken early in the propagation season and grown too long as single-stem plants are more likely to split than those taken later.
Splitting can occur on plants which are to be sold as flowering plants, and can make the plants unacceptable. Again, this is more likely to occur on plants which are grown single-stem with a mature main axis, compared to plants of the same variety which are grown as pinched plants, with the younger lateral shoots.
Several ways have been recommended to reduce the changes of splitting. These methods are:
1. Select varieties which are known to be relatively free from splitting. (Unfortunately some of the longest lasting varieties and those that withstand shipping and handling the best are the ones most subject to splitting.)
2. Apply lights (extend the daylength or interrupt the dark period) to stock plants until mid-May, to keep plants vegetative.
3. Pinch shoots on stock plants regularly, so only short, young shoots are produced.
4. Try to avoid taking cuttings from the interior portions on the bench, where light intensity is low.
5. Do not propagate cuttings early for single-stem plant production.
6. If the variety is highly susceptible to splitting, apply lights until mid-September on finishing plants to make certain the shoots remain in the vegetative stage.
7. Make certain the dark period is not interrupted by lights, once initiation is desired, or splitting can be induced.
Information from the Poinsettia Problem Diagnostic Key on Physiological Disorders from North Carolina State University http://www.ces.ncsu.edu/depts/hort/poinsettia/corrective/a11.html