Thanks All!
It starts with all the stuff I mention in the article. I hate deadheading and simply don't do it, ever. All the varieties of petunias that I grow are "free-flowering" meaning continuous. They are Daylight neutral and/or close to. Many petunias that are NOT daylight neutral will stop producing new flower buds once the days start to become noticeable shorter than 13 hours. Daylight Neutral guys keep going with flower bud production with as little as 10 hours daylight.
In addition to the slow release, I fertilize every 2 weeks with a water soluble (sprayer) From late July 'til they freeze. We often don't really get good growing degrees days until late June. OUr days are over 16-1/2 hours then and the plants grow fast. By end of July they need that extra water soluble jolt every few weeks. Keep an eye out for a new tip slated for publication on ATP for Sept 16 called Flowerpower. You may find it helpful. I use a high middle number, that promotes flowers, for fertilizing the annuals with the sprayer. The better ones often have micro-nutrients and the container should state that.
If the plants are big, the roots are big and these guys are always hungry and thirsty. Bigger pots are easier to maintain and to keep hydrated. Right now, I water the baskets every 2nd or 3rd day. Our night have been near or freezing, so they don't dry out as quickly. Hope that helps.
