The growth of plants can be modified by changing the growth conditions or using chemical growth regulators. The purpose of this thesis was to examine influences of chemical growth regulators on the plant height of the New York Aster (Aster novi-belgii). The aim was to enhance the efficiency of transport and storage by producing shorter plants. The study was ordered by Taimistoviljelijät ry, in order to get more Finnish research of growth regulators on tall perennials.

The experimental part of the study was carried out in the summer of 2010 on the nursery of Lepaa. All the plants in the study were a cultivar of New York Aster called ´Herttoniemi´ and the two growth regulators were B-Nine SP and Korrensääde 5C Limit. To compare the treatments more effectively, there were also two other treatments, water spraying and control sample in the test. At the beginning of the experiment all the plants were cut down and planted to bigger pots. Most of the plants were outside on the field during the whole study. To compare if the results varied depending on growth environment there were also a few experimental plants in the greenhouse. Some of the plants in the greenhouse were control plants and others were sprayed with Korrensääde 5C Limit. The growth regulators were sprayed to plants two times within the examination. The information collected from the plants contained measurements of plant height, time and the amount of flowering, fresh weight and damages.

There were no major differences between the treatments. Plants that were sprayed with B-Nine SP had a lower growth habit but on the other hand the compound reduced and delayed flowering. Korrensääde 5C Limit did not cause shorter plants but it did cause chlorosis and reduced flowering. Spraying with water proved to be worse than the control sample because it lowered flowering. Plants that were sprayed with water had also smaller fresh weight than control sample. There is need for further studies comparing different concentrations and applications to find the best options to use growth regulators in cultivating perennials.