The longer your focal length, the tighter the DOF will be. And the larger your aperture (smaller f/number), the tighter the DOF will be. And the closer you are to the subject, the tighter the DOF will be also.
Since the 18-55 doesn't have either a very long focal length or a large aperture, it's pretty difficult to get subject isolation with it, and even when you do, the results aren't exactly going to be creamy smooth either. But on that particular lens, reframe your flower so that you're zoomed all the way to 55mm, get closer to the flower, and make sure the lens aperture is set wide-open, which is f/5.6 on that particular lens. That'll probably give you a tighter depth of field than when framed at 18mm and f/3.5.
I'll try to post some examples later.