I believe you could dig up a part of this plant remove foliage and use the rhizome fresh. It most certainly is edible fresh or dormant. If the plant continues to grow I would let it. As long as it is growing it will produce more rhizomes and give you more to use in cooking. You didn't mention if the pot was indoors or outside? How long has it been growing? If you dig part up dig the oldest part out and use it first and let the remainder continue to grow or eventually go dormant.If you truly want it to all go dormant remove from pot cut off all top growth and place in a sunny place to dry then wash really well and begin to use as needed. But I would save some for future planting.
James Maloy

Thanks for the info. Right now it's outside but when it gets really cold I'll move it inside the garage for the winter. I've had this plant several years and it has never gone dormant. For some reason I lost part of it this summer so have replanted a rhizome. I love to use fresh ginger in cooking so it's nice having it to use at will without having to go to the store!

We've been having such a warm fall (temps in the 80's) that my Z. officinale is going dormant at one end and putting up new growth at the other. Yes, you can use it any time. In fact, it "keeps" better while it's growing, less chance of moldy tubers.