1)    A person should not pray in an open space like a field. Instead, he should pray in a building, as when he is inside a building the fear of the King falls upon him and his heart will be broken. (Shulchan Aruch 90)

2)    Travelers may pray in a field (or another similar place). Nevertheless, if there are trees in the area, it is better that he should pray between the trees, as it is a place which is a little private and this is better than a place which is completely open. (ibid and Mishne Berurah ibid)