Well, I found it!
This is from the Blue Book of Grammar and Punctuation.
Sometimes the pronoun who, that, or which is the subject of a verb in the middle of the sentence. The pronouns who, that, and which become singular or plural according to the noun directly in front of them.
So, if that noun is singular, use a singular verb. If it is plural, use a plural verb.
So, in your sentence,
Eatable flowers are a subject that comes up in almost all cooking classes eventually and I am a big fan of them.
Short answer: Don't think too much about it and choose is, because every and each work the same way. If it helps, read every as every single.
Every (single) man in this line is required to sign the forms.
Every (single) woman in this line is required to sign the forms.
Every (single) child in this line is required to sign the forms.
A. Every single [man, woman, and child] in this line is required to sign the forms.
B. Every single man, every single woman, and every single child in this line is required to sign the forms.
C. Every man, every woman, and every child in this line is required to sign the forms.
Since the subject in A, B, and C is equivalent, is is required. In each sentence, you are talking about one person at a time.
Also we would not write:
*Every man, every woman, and every child are each required to sign the forms.
Another example:
Every pen on this desk is broken.
Every pencil on this desk is broken.
Every eraser on this desk is broken.
Every single pen, every single pencil, and every single eraser on this desk is broken.
Every pen, every pencil, and every eraser on this desk is broken.
And certainly not:
*Every pen, every pencil, and every eraser on this desk are each broken.
The above explanation covers standard written English. In spoken English, it's every person to their own device(s).
Best Answer
I had originally provided a comment to an answer (which is now deleted) saying that all can be either singular or plural. But that's not really correct. As a comment, I didn't have space to explain it properly.
All is used along with a subject that is either singular or plural.
Sometimes, all is used in a sentence with a subject that's considered singular, and sometimes in a sentence with a subject that's considered plural.
According to Merriam-Webster, all can be an adjective, adverb, or pronoun, and it can be used in sentences that involve singular nouns or plural nouns.
It can also be a noun itself. When it is, it's considered to be singular.
Whether it's used in a sentence with a singular subject or a plural subject is contextual.
Here, it's used in a singular construction. It's being used in the same sense as everything would be:
On the other hand, this would also be acceptable, if not common:
In this case (assuming of them is not actually present in the sentence), all is being used as a pronoun in place of something like:
Or, depending on how you look at it, it's not a pronoun but an adjective for something that simply isn't present:
The verb form really has nothing to do with a singular or plural subject in this sentence.
What this sentence is actually saying is:
While the sentence is implicitly talking about the plural you, hail is the conjugation used for both the singular and plural form of you.