Sorry, I do not grow Rose F. Kennedy.
However, if anyone wants to test for the rebloom ability of a cultivar, the first step is patience. The second step is perseverance.
In plants, in general, there is a relationship between the size of the plant and its flowering ability. Typically plants do not flower until they become a certain size or larger. Typically the larger the plant the more flowers. The size of the plant in this respect does not mean the size of the clump but rather the size of the crown of one fan.
To check for rebloom, or for maximum number of buds per scape, height of the scape, etc. one must have as large a crown as possible for your location (weather). Depending on your location and the average size of the cultivar's crown at the start of the treatment it may take more than one year.
So patience is required because the change in flowering may take two years or possibly more.
Perseverance is required because the grower must provide as close to optimum growing conditions as possible for a lengthy time. When I run this sort of test I use liquid fertilizer with high nitrogen.
If a package of soluble fertilizer indicates that x teaspoons of fertilizer in y gallons of water are used every four weeks then that is what would be needed throughout the growing season for up to several years (or until one sees the best response).
