פירוש על עבודה זרה 97:15

The Talmud resolves the matter by saying that R. Yose does hold that when there are two causes the product is permitted, as we saw in the case of planting a nut of orlah—it is permitted to use what grows because there were two causes, nut and ground. So why then does he say that it is problematic to plant under an asherah tree because the leaves will fertilize the plant? This, the Talmud answers, was not his own opinion. He was only responding to the rabbis—you rabbis who hold that when there are two causes the product is prohibited should hold that planting under an asherah should be prohibited even in the winter.
And the rabbis would say it is permitted because while there is a gain in fertilization, there is also a loss because the shade prevents them from getting sun.