Your article suggests pruning the geraniums back by one-third. Does that mean cut all the stalks so that they are only two-thirds as tall, or cut out one-third of all the stalks?
Winter Care for Geraniums.
Certified GKH Gardening Expert
Both methods are fine. The reason for this advice is that with geraniums and many other ornamentals, removing more than 1/3 of the plant's total size at one pruning session can harm the plant's health or reduce future flowering. (Some plants can tolerate harsher pruning, though.) Pruning back each stalk to 2/3 its size will encourage "bushier" growth, while removing 1/3 of the stalks will encourage taller, less bushy growth.