I tried my hand at wave petunias this season. I heard petunias were slow to grow, and my basement is cool and dry (ambient temps in the low to mid 60's). Everything is grown under T5 lamps, and germination occurs over a heat mat.

So, over the last couple of weeks, the waves have done a little too swimmingly. I just went down to water and found several flower buds.
I am still at least 4-5 weeks away from planting outdoors.

Should I prune these back? If so, where? The wave petunias have runners, so I don't know if I should pinch those, etc. I am afraid of over-pinching them, and not sure exactly what location on the plant should it pinching occur?

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

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.

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

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.