Hi, Wimpy -
Working with the lens aperture wide open gives the best differential focus - there's the greatest difference between things in focus and things out of focus (which most people call "bokeh" - though this is, I think, a term for the quality of the blurred image, not the fact that it's blurred).
Aperture isn't the only consideration, though. If, for this shot, you'd gone closer, the relative distances between the camera and the woman, and the camera and the background would be greater, and the background would be more out of focus at any given aperture.
There are disadvantages of working with the aperture wide open. First, it makes focus absolutely critical. You need to get it right to the millimetre: if you sway slightly between the moment you focus and the moment you expose, you lose sharp focus on the part of the subject you wanted sharp. This makes it a technique that's best for a subject that doesn't move, when you cna work very slowly and carefully.
I love wide-aperture differential focus effects, and I've been using them for twenty or thirty years. There's a problem, though: for every shot that you get just as you want it, you get several that are not quite right. I therefore only use the technique when it wil lbe really effective: that is, when the main subject is much closer to the camera than the background.
Here, I reckon that the woman was around 12 or 15 feet away, and the fence 30 feet beyond her - the background is 3 times as far away. If you'd been closer - say 5 feet from the woman, the background would have been 7 times as far from the camera, and you'd have had a much greater differential effect.
The amount of softness in the background is very predictable - there are tables on the internet to calculate it, and you can get a preview with the depth of field button on your camera. Or, come to that, you can take a shot, and magnify the image on screen to look at the details.
Having said all of that, for this particular picture, there isn't much wrong with having everythign sharper - and the pattern in the fence is too strong to lose it completely through sending it out of focus. In such cases, it's often best either to accept the background, or to move round the subject to find an angle that gives yo ua plain background.
I hope this helps. Please come back to me if you need more explanation, or if I've misunderstood.