Sort of depends on the size of the trellis and what you want grow on it. For small to mid size trellises, would be easier to drive wood stakes into the ground, leaving a foot or so above ground. Then strap or screw the legs of the trellis to these stakes. For VERY large trellises, you could pour cement footings (with or without a pvc pipe) and that would certainly be very secure. But i can't imagine why you would need need so much strength. Some vines can be quite large and heavy, it's true, but anything that big (trumpet vine or wisteria, for example) would be trained up over a pergola, typically, or perhaps on a trellis bolted to a wall.