I've grown my outdoor cucumbers in a growbag for years. The usually B&Q ones, at that! They've always don't very well. As to watering, I always use a method my Dad showed me. When planting, cut the bottom out of a 4" flower pot, and plant the cucumber in that, in the growbag. So when you water, the pot keeps the water away from the stem at ground level.( you water outside the pot, of course!).