I assume you are asking because you want the dill seed.

To collect dill seed allow the seeds to turn pale brown in order to collect them and dry them for culinary or sowing purposes. Cut the tops from the plants with about a foot or so of the stalk intact and hang upside down over a screen or with the umbels in paper bags to catch the seeds. When dry, store in dark glass jars.

Or just go ahead and freeze the umbels after they have turned to a pale brown.

If you want the dill flower heads for pickling I find it's better to put them into a temp proof bowl and pour boiling vinegar over them, then put in large mouth jars or just put a tight cover over the bowl to keep out fruit flies. Dill is always ready before pickling cukes are and this is a way of storing them till the cukes are ready. When you want to pickle just use the vinegar the dill was in. Personally I like to have a small flower head in each jar of dill picles. It makes them look nice on the shelf. You can do the same trick on dill foliage. The dill vinegar is also nice on salads.

Sorry I should have been more specific. Hoodat is right, I wanted to preserve the dill head for pickles. I love to add the heads/flowers to the jar for a pretty touch. How long will it keep in the vinegar at room temp? Thanks!

The vinegar will keep it from spoiling indefinitely. I've kept some that way for over a year without spoiling. As I already noted you have to be careful to keep fruit flies out. They seem to find vinegar irresistable. They won't spoil it, they just get pickled themselves but dead fruit flies floating around in your dill pickles doesn't make them apetizing.

Thanks for the awesome advice. I harvested my dill heads and stored them like you recommended! Still waiting on cukes..... =0)

do you use cider vinegar or white?

I like to plant small patches of dill every few weeks throughout the summer, that way I have some ready whenever I have enough cukes on hand for pickling. I also plant enough extra that I do not mind when the colorful caterpillars of the black swallowtail butterfly arrive and start eating! :-)