Why does Mr. Nathan Radley fill the knothole of the tree in with cement in To Kill a Mockingbird? Is his stated reason to Jem the truth?

Mr. Nathan Radley tells Jem that he filled the knothole of the tree with cement because it was dying. Nathan explains to Jem that you are supposed to "plug ‘em with cement when they're sick." Jem is suspicious of Nathan's reasoning and consults his father. Atticus says the tree looks perfectly healthy, and Jem discovers that Nathan was lying. The real reason Nathan fills the knothole with cement is to prevent Boo from communicating with the children.

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Nathan Radley is Boo Radley's older brother, and he continues their father's tradition of keeping Boo isolated from the community. Boo knows, however, that Jem and Scout have taken a great interest in him. He reaches out to them by leaving the two children small gifts in the hollow a tree.

When Nathan realizes what is going on, he fills up the knothole with cement. He explains to Jem when confronted that he did it because the tree is dying. However, the tree is healthy. It is evident that Nathan wanted to cut off the communication between his brother and the children.

This help makes it clear that Boo is more sinned against than sinning. He doesn't wish to be an isolated figure that the children fear. Miss Maudie explains to Scout that Boo's father had the wrong kind of religious conversion experience, one that left him judgmental and punitive and caused him to isolate his family. Nathan is continuing along that path.

Luckily for the children, Boo's spirit can't be quenched, and he continues to take an interest in and look out for them. He saves their lives when Bob Ewell tries to kill them in retaliation for Atticus humiliating him at Tom Robinson's trial.

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Nathan Radley tells Jem that he filled the knothole of the tree with cement because it was dying. Jem is wary of Nathan's reasoning and asks his father about the matter. Jem knows that Atticus won't lie to him, and Atticus's assessment of the tree's health contradicts Nathan's reasoning. After speaking with Atticus, Jem realizes that he has been lied to and his chance to form a friendship with Boo Radley is over.

The reader likely recognizes that Nathan filled the knothole with cement to prevent his brother from communicating with Jem and Scout. For some unknown reason, Nathan wants to keep Boo Radley from having a friendship with the Finch children. He is obviously aware that Boo has been leaving small gifts in the knothole of the tree and wants to put an end to their friendly interaction.

Once Jem discovers that Nathan lied to him, he stands alone on the front porch and cries to himself. Jem is heartbroken by the fact that he will never have another opportunity to communicate with Boo Radley. Despite the numerous rumors surrounding Boo, Jem understands that his neighbor is a friendly, compassionate person. Scout is too naïve to understand the gravity of the situation and doesn't understand why her brother is so upset.

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The nature of Nathan Radley's character betrays a degree of weirdness and unfriendliness toward the children that leaves us no choice but to not take anything he says to them at face value. That being said, it is more than likely he was lying about the reason why he poured cement in the knothole of the tree, which served as the primary medium of unspoken communication between Boo and the kids.  Leaving "gifts" for the kids was tantamount to making contact with the outside world. Other than that, his brother, Nathan, keeps him inside the house, perhaps as a way to protect him but potentially as a way to control him. We never really know. 

What we do know is that Nathan decided to eliminate the tree stump, thus eliminating the contact with the kids.

"I filled it up."

"Why'd you do it, sir?"

"Tree's dying. You plug 'em with cement when they're sick. You ought to know that, Jem." (Ch. 7)

Whatever his reasons truly were, there is nothing in the annals of any field or study on common knowledge that has ever indicated that you plug a knothole with cement when a tree is sick.

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In Chapter 7, Jem and Scout write a thank you note to the anonymous individual who has been leaving them gifts in the knothole of the Radley tree. When Jem runs out to put the note inside the tree, he discovers that the knothole has been filled in with cement. Jem then patiently waits for Nathan Radley to arrive the next day and asks Nathan if he put cement in the knothole of the tree. Nathan tells Jem that he put the cement in the tree because it was dying. Nathan said,

"You plug 'em with cement when they're sick" (Lee 39).

Later that day, Jem asks Atticus if he thinks the tree is dying. When Atticus tells Jem that the tree looks healthy, Jem realizes that Nathan Radley lied to him. In my opinion, Nathan Radley told Jem a lie and the real reason as to why he put cement in the knothole was to stop Boo from communicating with the children.

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This happens in Chapter 7 of the book.  When Nathan Radley fills the knot hole in the tree with cement, he says that it is because the tree is sick.

I really do not believe that Mr. Radley is telling the truth when he says that he did it because the tree was sick.  Cement would not be very likely to help a tree stop being sick.  In addition, Atticus tells Jem that the tree was not sick.  Instead, I believe that Nathan does it because he does not want Boo to keep leaving gifts for the kids -- just out of meanness, really.

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When Nathan Radley fills the tree knothole, it distances Boo from Scout and Jem.

Boo Radley is not a normal person.  He is very reclusive, and never even comes out of his house.  Scout, Jem, and Dill begin to bring him out of his shell with their efforts to contact him.  He strikes up a friendship with them through the simple means of leaving presents in the knothole of his tree.

The children do not know who the gifts are from at first, but Boo keeps leaving them.  The little trinkets are his way of connecting with them.  He gives them gum, pennies, twine, and even a watch.  The best present and the most personal is the soap dolls.  They are perfect miniatures of Scout and Jem.

Nathan Radley, Boo’s older brother, is almost as quiet as Boo.  However, he does not seem to want Boo to interact with the neighborhood kids.  Maybe he feels it is inappropriate.  Nathan tells Scout and Jem that the tree was sick.

“Mr. Radley, ah—did you put cement in that hole in that tree down yonder?”

“Yes,” he said. “I filled it up.”

“Why’d you do it, sir?”

“Tree’s dying. You plug ‘em with cement when they’re sick. You ought to know that, Jem.” (Ch. 7)

The children ask Atticus if the tree is sick and he doesn’t think it is.  Jem in particular feels very badly about this.  He knows that Boo Radley meant no harm, and that now he has no connection to the outside world.  When Boo leaves a blanket on Scout’s shoulders, Jem insists that Atticus not return it.  He doesn’t want Nathan to know that Boo was there.

“…Mr. Nathan put cement in that tree, Atticus, an‘ he did it to stop us findin’ things—he’s crazy, I reckon, like they say, but Atticus, I swear to God he ain’t ever harmed us, he ain’t ever hurt us, he coulda cut my throat from ear to ear that night but he tried to mend my pants instead… he ain’t ever hurt us, Atticus—” (Ch. 8)

Jem and Atticus both understand that Boo needs his connection to the kids.  He has no one else.  Cementing the tree was cruel, in Jem’s mind, because Boo did nothing wrong. He thinks Boo is harmless, but Nathan must not have thought so.  Still, cementing the hole made poor Boo even lonelier.

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Even though the reader never explicitly learns why Mr. Radley filled in the knothole, we can infer at least two explanations for his actions. When Atticus discounts Mr. Radley’s initial explanation – that the tree is dying – Jem comes to some understanding that the reader is not privy to. It can easily be inferred that Mr. Radley is purposely keeping Boo away from the kids, or any outside influence. Jem is starting to understand that the rumors about Boo Radley are false and that he is not the monster many people make him out to be.  This theory leads the reader to see Radley’s actions as malicious. Another explanation, but less plausible, could be that he is trying to protect Boo from the outside world. This inference would require the reader to see Mr. Radley as more compassionate and caring than Harper Lee makes him out to be, which is why the first reason is the most believable.  

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