Anytime really. From before they are in full bloom thru full bloom is a milder, I guess you could say normal, dill flavor. But some don't care for all those little yellow blooms floating in their pickles.

When they go to seed you can use the seed heads too but all those seeds give a much stronger (some feel bitter) flavor.

Dave

I can many pints and quarts of pickles every summer and always use the entire "head". One large head per jar, or two small ones. I use the heads before they go to seed and have never had any problem with any blooms floating around.

I just bought a bunch tonight since mine never came up (too dry). Most are green seed heads. The flowers seem to be wilted. Some have turned to dark seed - are just the dark ones strong (bitter) or are the green ones too?

Not sure if I should look for yellow flowers next time instead of letting the farmer give me whatever he pulls out of the cooler.

I like to use mine after it goes to seed and just when it's starting to turn brown. I also use it when it's green and the stem also because it has a lot of flavor. I've also picked it and dried it and used it the next year and it seems to work fine.