If the damage is severe, trim it back to about 4-5 inches above the pot. Water at least once a day. Every 7-10 days for about the next month, fertilize with a liquid fertilizer. This should snap in into shape.

Petunias are sturdy and will withstand all types of issues. Pulling off worn flowers and pinching off seed heads also helps them. This prevents the plant from going to seed, which takes huge amounts of energy.

Good luck!

SG

I wouldn't be adding any fertilizer to a very stressed plant.
I would submerge the pot up to the rim in a tub of water and let it soak until all is good and wet at least over night. What isn't brown and dead should perk up, cut the brown stuff off....and water water water!! You let the poor thing die of thirst!
Hanging baskets often have to be watered twice a day and if they are in the sun and it's hot and windy....3 times a day.
Linda c

The shriveling "within days" makes me think your plant is dying of thirst. Sometimes when a pot's soil dries out, you can water the pot "thoroughly" and excess water will run out the bottom. However, if you put your finger into the soil, it will be bone dry.

If you find this is the case with your petunias, then place the petunia basket in another container and fill the container with water to the depth of the petunia basket. At first the basket will float. Slowly the soil will accept the water. When the petunias have bottomed in the container and the soil is thoroughly soaked, remove the basket and let the water drain.

Container plants dry out quickly compared to plants in the ground. You don't want to keep the soil perpetually wet, but you also don't want to allow it to become so dry that it won't take water.

Hope this helps - June