Outdoor fill flash

I went to Botanical gardens with my family this weekend. Usually I shoot without flash when outside, but this time I decided to use the flash as fill in.
I shot at aperture priority and all of my pics were overexposed. I had to turn off the flash to get the exposure correct. Why is that? I thought that the flash was going to fill in the shadows, but mine overexposed.
I am using 580EX with 30D.
Should I dial down the exposure comp on the flash? If so, I thought the automated system adjusts for the amt of light available.
Any help would be appreciated.

davudvl wrote in post #4213333
I shot at aperture priority and all of my pics were overexposed.

Most likely you were using an aperture that required a shutter speed faster than 1/250. Without high speed sync enabled, the camera defaulted to its maximum flash sync shutter speed (1/250). The result is images overexposed from ambient light.

Look at your pictures again. If the background is overexposed as well as the subject, then that's what happened.

There are two remedies. The first is to keep your shutter at 1/250 and stop down the aperture to properly expose the ambient. Tv mode at 1/250 will accomplish this, probably P mode will too, and M is always an option. Also, if you enable custom function 16 (safety shift) on your camera, it will save your butt when you decide to use Av mode and flash outdoors.

The second remedy is to enable high speed sync on your flash unit. This will allow larger apertures and faster shutter speeds at the expense of reduced range and flash efficiency.

Thank you Curtis,
I will go back and look at the data of my pics.

In addition to Curtis' suggestions, check your FEC settings.

If you used you flash, as I do, indoors you would use some +FEC. Outdoors, I find that -FEC is required.