In Masechet (tractate) Avodah Zarah, Perek (chapter) Gimel (three), Mishnah Chet (eight), the Chachamim (rabbis) made a decree that the offerings of the Avodah Zarah worshippers to the idols transmit Tum'ah (ritual impurity) by Ohel, by being under the same roof, much like the Tum'ah of a corpse. If a person, a utensil, or food is under the same roof as a corpse, it becomes Tameh (ritually impure). The same applies to an offering to Avodah Zarah: that person becomes Tameh. Where an idol is, we assume there are remains of an offering to Avodah Zarah. For example, under an Asherah tree (tree associated with idolatry), we assume there are offerings at that spot. Here, we’ll talk about a person who goes underneath an Asherah tree and the other laws that apply to this type of tree.
The Mishnah says that a person is not allowed to sit in the shade of an Asherah tree, even if he is not sitting directly beneath the tree. Since he is prohibited from benefiting from the idol, we say that he cannot even sit under that tree that has the idol under it. But if he sat under it in an area that is not directly beneath the tree, but is in the shade, he is Tahor (pure). Even though there is an offering under the tree, he does not become Tameh as long as he did not go under the tree. You are not allowed to pass under the tree. You have to distance yourself from the idol, so you are not allowed to go under it. If he did pass under the tree, he is Tameh, because we assume there are offerings under the tree, and those offerings transmit Tum'ah through Tumat Ohel (impurity transmitted under a roof).
If an Asherah tree was creeping into the public road, robbing the public in a way by hanging over an area meant for public use, and a person really had to go through there and went under it, he is Tahor; he is considered to be pure. The rabbis decree that Tum'ah only happens if the tree does not interfere with the public’s right-of-way. A person is allowed to plant vegetables under an Asherah tree during the winter, when the shade would be problematic for the vegetables, so he is not benefiting from the shade, but not during the summer, when the shade would be beneficial to the vegetables. Lettuce cannot be planted in the summer or the winter, because shade benefits it all year. But Rabbi Yose says a person is not allowed to plant vegetables under an Asherah tree even during the winter, because leaves fall on them from the tree and become fertilizer, helping the vegetables grow. So, according to Rabbi Yose, you are gaining some benefit no matter what, even during the months when you do not need the shade, when the shade is harmful in the winter months. You are still gaining some benefit from the tree, and therefore Rabbi Yose says that planting under an Asherah tree is problematic the entire year.