The number of plants you put in the 30-inch planter is not necessarily going to be a problem, especially if you fertilize regularly and trim them whenever they need it. But the potting mix you used is likely to cause a lot of problems. Garden soil usually just becomes mud when wet and cement when dry. It is never suitable in a container. It compacts and smothers the roots. How hard would it be to replant them in straight store bought soilless mix? The core lining of the basket will help some with providing extra drainage, but better potting mix would do much more.

As for the potting mix you bought that says it feeds for 6 months, I wouldn't count on it. Petunias outdoors need a lot of fertilizer. They might be okay for a month, but if you want luxurious growth, you'll need to start fertilizing with something like MiracleGro 24-8-16 soluble fertilizer according to the instructions every couple weeks. Here's a 16-inch round core lined basket with two petunias, two million bells and two trailing bacopas about midsummer. I used the 5-1-1 potting mix discussed in this forum and fertilized regularly.

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Thanks for the response! Your post reminded me I was incorrect - now I remember it wasn't garden soil this time, it was actually potting soil I took from an older planter on our patio. I forgot since I had been doing other things with actual dirt dug from my yard, but not this time. The potting soil had perlite in it. But it is old, and it's mixed with the new 6 month fertilizing potting soil. Unfortunately I think for now I'll have to deal with it.

And thank you for the recs on fertilizer. My after-the-fact research mentioned that a lot but I was worried about spending money on more stuff without being sure what was best. I will plan to start regular fertilizing soon too.

Good to hear in your experience they should be okay with fertilizer and the occasional trim. :-)