In cases where mums are too short or compact late in the season is there any remedy? You might try the growth regulator Fascination. The following is more information.
Some growers have asked about using Fascination [a cocktail of two plant hormones, gibberellic acid (GA4+7) and benzyladenine (BA) in equalproportions] to increase plant height. Fascination increases height by stretching the distance between the leaves. Trials using Fascination on poinsettias have shown that it will stretch soft tissue about an inch per application. Response is quick, within one week of application you will know how much additional growth the treatment produced. Application can be repeated as needed but results are best when shoot growth is still soft and several of the upper internodes have not yet expanded. Once stems harden they will be less responsive to treatment and late treatments may just cause the flower buds to stretch above the foliage. Trials using Fascination on garden mums has been minimal at this time but preliminary results show increased stem length with a 10ppm spray application.
For more information (and photo) of Fascination on mums: GroLink July Newsletter
Information from the current New England Greenhouse Update posting http://www.negreenhouseupdate.info/index.php/updates