Answer: Geraniums tend normally to be a bit leggy as yours has become. To prevent it you would periodically pinch off just the growth tips as they form. This subsequently causes the plant to branch. By doing this as the plant develops you can control and direct the branching to keep the plant full and bushy.
When the plant is overgrown, it is too late for such delicate pinching. You will need to cut the plant back by about a third to a half to try to allow the lower shoots to catch up and encourage the older stems to send out new growth from the base. The start training the new growth and regrowth by pinching.
When you cut, cut just above a branch or growth bud. That branch or bud will then direct the new growth and become the new tip. Then watch and pinch the resulting new growth at the ends of the branches as needed.
It is not as bad as it seems because you can root tip cuttings using the pieces you have had to cut off -- and you'll have lots of plants that way. The best time to do this is in the spring or in early summer, but you should have no problem doing it now. When you cut the plant back you may also want to take the opportunity to repot it into fresh soil.
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