There are two options for fertilizing.
Option #1 fertilize once in mid April and once again in late June or early July. Do not fertilize after the first week of July otherwise the plant will continue to send out new growth and will not go dormant on time for winter. All that new growth will die and create an opening for freezing within the plant.
Option #2 fertilizer once in late October when the plant has gone dormant. This fertilizer will not act until next spring but will save you some time then. In spring you can fertilizer once in mid June. Don't fertilize any earlier than mid June if you fertilized in fall; the fertilizer from fall will still be acting in early spring and you can do burn damage to the plant if you add more fertilizer to early.