It probably needs fertilizer. You can use a balanced fertilizer like 10-10-10 in a slow release form every two months.
https://www.gardeningknowhow.com/search?q=petunia%20wilted%20yellow
I started petunias from seeds in peat pellet "muffins".There is mold on some of them,but the plants look healthy.Are they still OK to use?
I'd treat them with some neem oil spray and get them into the ground or basket. They should be just fine. https://www.gardeningknowhow.com/ornamental/flowers/petunia/petunia-care.htm
I planted petunias in my deck railing baskets. I used Miracle Gro dirt. It says this will fertilize for 3 months. Knowing petunias are heavy feeders, can I add fertilize to this or is the fertilizer in the dirt enough?
I would wait about a month and add more. When plants are in containers, the fertilizer is flushed out frequently and you need to use it more often. Petunias will stop blooming when they need more. I also pinch them back or they go to seed.
I have grown petunia from seed, I put them into two hanging baskets two weeks ago the plants look very healthy they are six to eight inches high but no sign of flowers.
Your petunia probably needs some phosphorus. Heavy nitrogen fertilizers will encourage lots of green growth but no flowers. Here's some info: https://www.gardeningknowhow.com/ornamental/flowers/petunia/petunia-plant-no-flowers.htm
it might be worms, as i see holes on the leaves and on the flowers. though i have caught little brown birds nibbling on the edges. that gives us two invaders. any good ideas to prevent my petunias becoming a feast?
Birds can be tricky, but pesticides can take care of the insect problem! Here are some articles that will help:
https://www.gardeningknowhow.com/plant-problems/pests/pesticides
https://www.gardeningknowhow.com/plant-problems/pests/animals/controlling-birds-in-gardens.htm
It has killed healthy plants that I have tried to replant. The only one that are still alive is the petunias and a couple of bacopa. I don’t want to keep purchasing plants to replace them.
It looks like aphids and it takes some diligence to eradicate them. Here is how:
https://www.gardeningknowhow.com/plant-problems/pests/insects/homemade-aphid-control.htm
We keep hearing that we should dead head plants to encourage new growth, but is it best to just take the dead petals off or snip the stalk further down? Many thanks.
Snip or pinch off the plants further.
https://www.gardeningknowhow.com/ornamental/flowers/fgen/deadheading-flowers.htm