Yes you can cut them back and they will get bushier with more branches. Go down the branch until you find a small branch and clip it off below that stem. You can cut them back as much as you like. It is advisable to do this all summer cutting back the longest branch and working around the plant every couple of weeks or so. This will keep the plant bushy in the center where it gets bald if you don't trim it back. Use a sharp knife or scissors not your fingers.

How long are the branches though. If they are only a few inches long just leave them alone and trim them later.

When you do trim them back you can start those cuttings and make new plants. Trim it to a 4 or 5 inch piece cutting just below a leaf joint. All roots come from a leaf joint. Remove the bottom leaves. Prepare and water a small pot with soil. Dibble a hole in the center with your finger. Dip the cut end in rooting hormone--it comes in gel or powder and lasts a long time because you only use a little for each cutting. Too much is not better than just enough to cover the node or nodes if they are close together. Put it in the hole you dibbled and cover it in pressing slightly to secure it. Treat it like a seedling. You knoiw it's rooted when it starts to send out new growth

Dowlinggram pretty much covered it. You can pinch off all the blooms as they come or just enjoy them for a while. I can never resist leaving them for a bit. When it gets closer to time to pot them into containers, or plant them in beds, pinching back the flowers will send it into bloom mode again.

Snipping off the runners and rooting them is the reason I usually start them early. I don't use the rooting hormone. They seem to root fine without it. I use a clam shell box (from the deli or produce section) put in about 1 1/2 " of very wet pearlite, pop in 20-30 cuttings, and close it up. They root faster in a warmish place, so I place them on my light shelves, at the end of the shelf, not under the light. In about a week you should see them starting to root.

If you don't need more petunias and don't want to root them, snip off the runners anyway, so the the plant bushes out. Cut them right above a leaf node, and it will fork into two "branches". The first time I take cuttings, I take them close to the base of the plant, so it bushes out at the bottom, then when the grow out again, I cut them 4-5" out from the base.

What size pots do you have them growing in now? By the time mine are blooming, I have them in 9oz plastic cups. That seems to be big enough to keep them healthy until I move them into baskets. Even that size will get a bit root bound and need to be watered well every 2-3 days by the time you can plant them.

I started a little later this year. My Tidal Waves are the only petunias that are big enough to be shooting out runners so far, and they are just beginning. But I am a zone colder than you, with that much longer to wait for spring.

Thanks!
Each plant has probably 3-4 runners starting, and the runners are only a few inches long. If it's no big deal to have them blooming already, then I'll let them go for a bit.
mandolls, they are in 9 oz. solo cups now. I started them in those so I didn't have to pot up before planting outside.

timing was always the bane of my seed starting ...

the only question now.. is that you will remember in about 8 months or so ... to start next years... a month later ...

and often its not about remembering ... its more about having that itch ... and not being able to stop yourself.. because you need a 'fix' in january ...

i suspect you are having great fun ... so .. as you were ...

keep us posted on progress to the yard ... shall we presume you know all about hardening off to temps and sunshine???

ken

"and often its not about remembering ... its more about having that itch ... and not being able to stop yourself.. because you need a 'fix' in january â¦"

You hit the nail on the head !

I actually managed to hold off til mid Feb this year, but it hasn't slowed me down much. I have about 20 trays of seedlings growing, 4-5 of which haven't been divided yet, which will make it closer to 35.