Marian, you should be patient to develop a linguistic instinct in order to decide on such matters, because not everything is described by grammatical rules. Why should you translate with all those indefinite articles in the line, as if they were in a garden where everything was indifferent and faceless? In Greek, at any rate, use of the definite article is the norm, so that if it is not there, you should rather imply this than the indefinite.
Always be careful about the context of a phrase, the whole chapter. Since the reply of Eve to the Devil makes a distinction between the other fruits and the particular forbidden fruit, it is right to translate ξύλου in plural, i.e., trees.