“When people will not weed their own minds, they are apt to be overrun by nettles.”

Speech dissolving the First Protectorate Parliament (22 January 1655)

Section 10
The True Believer (1951), Part One: The Appeal of Mass Movements

Source: Atlas Shrugged

The Usurpation Of Language (1910)
Context: The human mind loves the bondage of words and is apt, when freed from one form of their tyranny, to set up another more oppressive than the last.
The highest function of philosophy is to enforce the attitude of meditation and therewithal restrain the excessive volubility of the tongue. To us it seems that the reflective thinker wins his greatest victories when by what he says he compels us to recognise the relative insignificance of anything he can say. His task is not to capture Reality, but to free it from captivity.

Unknown source, attributed by Life Education and Resource Network (LEARN) http://www.blackgenocide.org/planned.html and by Roger L. Roberson, Jr, The Bible & the Black Man: Breaking the Chains of Prejudice (2007), p. 18.
Seems to take "human weeds" from "a garden of children instead of a disorderly back lot overrun with human weeds" or from "the gradual suppression, elimination and eventual extirpation of defective stocks– those human weeds which threaten the blooming of the finest flowers of American civilization" and "exterminated" from "we do not want word to go out that we want to exterminate the Negro population, and the minister is the man who can straighten out that idea" (see above).
Misattributed

1900s, Address at Providence (1901)
Context: We are passing through a period of great commercial prosperity, and such a period is as sure as adversity itself to bring mutterings of discontent. At a time when most men prosper somewhat some men always prosper greatly; and it is as true now as when the tower of Siloam fell upon all alike, that good fortune does not come solely to the just, nor bad fortune solely to the unjust. When the weather is good for crops it is also good for weeds.