I would like to install an underground water line to a small garden about 200' from the house to wash tools and provide some irrigation in dry spells. Would 1/2 flexible pipe be OK? What kind of pipe should I ask for at Home Depot?

Thanks in advance for your time.

Too much friction loss in 1/2 " at 200', particularly if you want to do some irrigation. Minimum 3/4 ", preferably 1". Use Polyethylene or PVC.

Where do you live and is there a frost line?

Too much friction loss in 1/2 " at 200', particularly if you want to do some irrigation. Minimum 3/4 ", preferably 1". Use Polyethylene or PVC.

Great advise. I was looking at 1" buried 24" in Atlanta area. Thank you very much.

I would not use PVC, it is cheap but brittle and you will have over 20 fittings. PEX and other tubing can be continuous. Your frost depth is 24" so if you bury it at 24 you will need to blow out the line every winter or bury deeper. I would use PEX at 6" to 12" and blow out each fall.