The usage of singular and plural verbs in English is sometimes more complicated than in other languages. One example of this phenomenon is the phrase “a lot of”.
Usually, when something has an indefinite article, i.e. “a” or “an”, it is followed by a singular verb, for example “a tree is”, not “a tree are”. However, “a lot of” is used in a way similar to collective nouns—when we talk about several objects, e.g. “a lot of trees”, we use plural verbs, e.g. “a lot of trees are”, not “is”:
Another way to look at this is that in English, unlike many other languages, the subject doesn’t have to be in the nominative (grammatically, “of trees” is in the genitive). In sentences like the above, you should ask yourself: “what has been planted?” Since the answer is “new trees” and you would say “new trees have”, that’s the verb form you should use, regardless of what precedes the trees.
“A lot of” can be used also for non-countable nouns, i.e. nouns describing a substance or a material, such as “water”, “sand”, “iron” etc. In this case, since the noun is in the singular, so is the verb: