It's very thirsty and probably also hungry. You need to water it, feed it and dead head it ie cut off the spent flowers to force it to produce more blooms. In fact you can cut it back a good bit further since it's got a bit gangly.

Its hard to see the specks. They could be aphids or caterpillar droppings. Have a close look at them if they are not insects then I'd guess droppings. In which case its time to hunt the caterpillar(s) and pick them off.

there are mature opening seed pods .. could your black things be seed????

take on.. and try to smear it on a piece of white paper ... if it smears green.. its frass ... .. if so.. then start looking for a bug the exact color of the plant ... they have to be hiding there .... especially on that balcony ....

https://duckduckgo.com/?q=frass&t=ffcm&ia=web

if its hard.. and just crushes... like a poppy seed.. then its seed ...

i think its time for a severe cut back ..... see if one of these videos guides you

https://duckduckgo.com/?q=pruning+petunia+hanging+baskets&t=ffcm&iax=videos&ia=videos

and for next time.. i would not repot a large mature plant ... if you want to make your own pot... start earlier .. and smaller .... or just buy it and enjoy it ... two different projects.. if you will .....

good luck .. ken

Ken, I don't see anywhere the OP said they'd repotted.

A hornworm shouldn't be too hard to find on that plant. There is a great number of bits that need deadheading.

Looks like aphids to me. Infestations are usually worse if the plant is stressed. Make sure you’re watering well and fertilizing regularly. Are you sure they’re getting enough sun? I’d cut back as suggested and some TLC should help. Wash off the aphids with a strong stream of water from hose or tap.

When I had them on Petunias they were small and green, so hard to see on the plant. I was advised to use BT as a remedy. I tried to no success. I had many plants and the BT has to be sprayed at a certain time in their reproduction for it to be most effective. I have never grown Petunias since then.

'Youtube has videos on the subject.

Ken, I don't see anywhere the OP said they'd repotted.

==>>> must have been too much coffee this early morn .. because i dont see it now either ... lol ... and i was sober.. just fyi .. lol .. [unless the post was edited .... ]

i would still try the smear test on the black things .... i find it a bit hard to believe that the thing is so infested with bugs.. that this could happen in 3 weeks .... and if it was.. i would be calling the county Ag office about the seller selling bug infested stock ...

ken

Thank you to everyone for your responses. This is the first time I am caring for plants so I really appreciate this. Will act on your advice. Hopefully I have a positive update to share soon. <3

The little brown specks are seeds. When you don't deadhead the flowers the spent blooms will form seed pods, which will pop when they are ripe hence little tiny seeds go everywhere. In order to get more flowers instead of seed pods you need to deadhead the spent flowers.

If I were you I would cut all the stems back to about 4 inches long and fertilize the plant with a liquid fertilizer where the middle number is the highest number. The middle number is phosphorus which affects blooming. When you use a fertilizer where the first number is the highest number that causes a lot of foliage growth instead of flowers. Make sure the plant isn't dry when you fertilize it.

The plant will look dismal after you cut it back but never fear it will grow back out especially with the addition of the fertilizer. You should fertilize petunias with a liquid fertilizer once a week, but never fertilize them when the soil is dry.

Deanna

They're moth worms and the little black things are partly seeds ((because they're eating the pods), rest is worm castings.

I just came from the backyard, I'm a sweaty mess and livid! I have 2 main infestation nemesis, those worms and Japanese Beetles. I filled a bucket with soapy, hot water, grabbed a chopstick, garden hose and spray bottle. Six or so went to a watery grave. This is the 2nd time I've had them and I haven't planted petunias in 3 years! If you completely saturate the soil, they will come up for higher ground. Seems they only eat petunias here in SW Ohio(?)

Oakley, you were right about the 'budworm'. Moths lay the budworm eggs. 3 videos in a row recommend using something called Captain Jack's dead worm. Sounds like it works pretty quick and is a 'natural' product. Back to Lowe's for me ☹️.

Glad I was right about something! lol. Good luck with them. Grasshoppers are doing major damage to all my flowers.