Questions of competing shade between s’chach and different types of trees (including vines) are complicated. If you have reasonable alternatives, we recommend that you set up your sukka in a place where these questions, whose answers can change from year to year, do not arise.[2] In any case, after presenting some background, we will give you instructions that allow for a kosher sukka according to the accepted halachic opinions.
The mishna[3] says that a sukka situated under a tree is invalid.[4] However, the gemara points out that if the tree lets more sunlight through than it gives shade (chamata merubah mitzilata), then the sukka is valid. The gemara then says that since material that is attached to the ground is not valid as s’chach,[5] a sukka that has even a minority of its shade resulting from a tree is valid only if “chavatan.” Many Rishonim explain that chavatan means that the branches are lowered until they are intermingled with the s’chach. In this way, they are batel[6] and no longer cause a problem. Based on this gemara, one could say, at first glance, that in the case that your grapevines are intermingled with kosher s’chach, they will not cause a problem.
However, this leniency has several caveats to which one must attend. Firstly, the permissibility of combining the vines with the s’chach presumes that the gemara’s first condition is met, that is, the vines must be chamata merubah mitzilata.[7] Also, in order to use bitul, there has to be more of the kosher s’chach than of the pasul.[8]
It is also not clear exactly how inconspicuous the attached branches have to be among the majority of kosher s’chach.[9] The Shulchan Aruch’s language is that they “are not recognizable,” and the Bi’ur Halacha[10] says this means that one cannot tell what is attached and what is detached.
What should one do if the above criteria for the vines to be batel to the s’chach are not met? The first opinion of the Shulchan Aruch is that it suffices if the kosher s’chach is thick enough to provide more shade than sun without the help of that which is pasul as s’chach (and that the pasul s’chach is chamata merubah mitzilata).[11] However, he also cites and apparently prefers[12] the Avi HaEzri’s stringent opinion that the areas that are covered by both kosher s’chach and the tree are considered to be covered by invalid s’chach. Thus, one has to confirm that after subtracting the area that is covered by the tree or vine, there still is a majority of shade from the s’chach that remains. This can be difficult to determine and raises serious doubts about any sukka that stands in large part under a tree. If there are significant areas that have thick tree cover and other areas where it is sparse, then it is similar to a sukka that is partially covered by a balcony and partially exposed. The halacha in those cases requires independent study, beyond the scope of this response.
One matter that alleviates the problem significantly is that the Avi HaEzri’s stringency may not apply when the kosher s’chach lies on the branches.[13] Thus, if the density of the vines is chamata merubah mitzilata, the s’chach lies on them, and the s’chach is thick enough without the help of the vines to give more shade than sun, then one’s bases are covered according to the major accepted opinions.