Spring of Fall, just dig, pull or cut with a knife if they are too tight. They are tough, so just about any size plant will grow. The nodes on the roots are for food storage, so no harm if they fall off. If they have started growth in the Spring, cut the foliage back about half. Actually they can be divided any time, but the flowering might be affected.
Herman

if lifting the whole pile , i dig the whole thing up and take it to the water hose and spray it apart, the roots that are tight together i put them in a bucket for a few minutes and then spray again ,they come lose for me this way without ever cutting anything. holly