A slight problem in Linking Verbs.

As we know that linking verbs never express an action instead provide additional information about the subject. So by the virtue of this definition, the following example is good;

1. As he became old, he grew slow at remembering dates.

Here "grew" is the linking verb and "slow" is the predicate adjective. Now consider the following example;

2. The tree grew slowly.

Here the "grew" is a non-linking verb while "slowly" is an adverb. Within the exing context of this sentence (where no previous information is given about the tree), we cannot us a predicate adjective (or can we?). But if we modify this sentence by providing some addition information, so can we use an adjective, rather than an adverb, in this sentence as shown in the examples below;

3. After the 50th anniversary of the tree, which was planted by the President of the United States in 1945, it grew slow.
4. After the 50th anniversary of the tree, which was planted by the President of the United States in 1945, it grew slow in height.

Are both of the above examples acceptable or not?
Can we still use slowly in the above sentences?

GB