How good is the internet. I remember when the our computer fan stopped working and You Tube showed me exactly how to put in a new one. I took the computer to the shop for them to do it. Then there was our new air con unit that started dripping inside. A You Tube video showed me exactly how to fix it. We called the man back to fix it. Our oven door handle is loose, which requires the door to be taken apart to fix it. The internet showed me an exploded view of the door and all the parts and how it fits together. We will get a man in to fix it. Truthfully, I have lost confidence in myself to fix things. I used to think nothing of pulling things apart, fixing them and putting them back together. Now, I prefer to pay.
Nevertheless, I was prepared to take on the petunia. A little net searching told me more than I needed to know about care of petunias. I deadhead it nearly everyday but I have been doing it wrong. Pulling off the dead flowers is not enough as within the area where the petal grows from are seeds. These need removing, so you pinch it off a bit down the stem. But the stem looks ugly, so what I now do is cut the stalk above closest leaf. I wouldn't expect anyone to go to such bother if they have a mass of petunias growing in their garden, but when you have two balcony plants, you can be quite fussy.
So to remedy the legginess of the plant, I gave it a haircut and a week or so later, it has responded very well. Photo is from when it was first trimmed. Kill or cure I reckon.