Sorry, my answer is no.  Do not use floral cut flower preservative on living plants. The cut flower preservatives are made to cut down on bacteria growth in vase water. Living plants in pots need good bacteria in the soil to thrive. To make your potted rose blooms last longer do not let them dry out, keep them shaded from the hot afternoon sun and use a slow release plant food like Osmocote rather than a liquid plant food which can be delivered too quickly to the rose plant and cause premature flower ripening and opening. You can have your rose plants in full sun while the buds form, then move them to a cooler spot or shade them to encourage the blooms to open and mature more slowly. Keep growing, Marianne Binetti