"If I use Preen in the Spring - will it also stop the Bishops Weed from emerging?"

No...bishop's weed spreads by runners, not by seed.

Thank goodness/:

Thank you very much for your prompt reply. I'm sure you think I'm crazy. Debbie

No Debbie, otherwise I'd have to think that my best friend is crazy. She loves it too.

Please tell me how does one kill Bishop's Weed as I detest it and want to get rid of it from my flower garden.

Start pulling, and be prepared to keep at it for a while. Pernicious stuff.

I tried pulling BW semi-weekly for 2 years at one house and gave up. That just seemed to make it grow more. The only way to save existing perennials from an infestation of BW is to dig them up and completely rinse their roots, making sure no BW roots remain tangled into the mix. If you try to pull these away from existing plants, you better be strong enough to pick up huge areas entangled in the root mass. The leaves usually break away from the roots too easily also, making it impossible to remove these by pulling. All it takes to start a new colony of BW is a little piece of root.

After desirable plants have been moved, you can smother the area under cardboard or plastic, preferably until next spring to make sure it's really dead. If it's been allowed to make seeds in previous years, be vigilant about pulling new sprouts immediately whenever you start using this area again.

I too have BW and have tried to get rid of it. It is on both sides of the back yard and the neighbors on each side have it too. I have finally decided (since I have a small yard) that I will put potted plants amoungest the BW and put a raised bed (with a solid bottom) for flowers or vegs.
I was told by a landscaper to use commercial Round-up but mix 5-7 oz to a gal of water, spray, wait 10 days and spray again.

If you're going to use a smothering method, why add the round-up step? I wouldn't spray that stuff where I plan to grow veggies.

Totaly amazing to me that the manufacturers of plant poisons very often have one strength product they sell, at great cost, to homeowners, another strength they sell for "commercial" application, ans still another strength they sell to Licensed Pesticide Applicators. And then someone tells someone else to mix it in proportions that violate the law.
If the product actually does what it is supposed to there should be no need for different strengths and there should never be a time someone would need to mix the product at any strength other than that listed on the label.