It is not an exact quote.
A related exact quote is:
Birth Control does not mean contraception indiscriminately practised. It means the release and cultivation of the better elements in our society, and the gradual suppression, elimination and eventual extinction, of defective stocks--those human weeds which threaten the blooming of the finest flowers of American civilization.
Margaret Sanger, "High Lights in the History of Birth Control," The Thinker, October 1923 pages 59-61.
Several years earlier, in The United States v. Margaret H. Sanger, she was charged under federal law for, in July 1914, using the mail to "incite murder and assassination", because she used the mail to distribute the following article, "A Defense of Assassination" printed on the front page of her newsletter The Woman Rebel, No Gods, No Masters, Vol. I, No. 5 (quoted in part):
It is generally agreed that lower forms of life must give place to higher types, and when the pioneer of civilization makes his way into the forest, he must of necessity destroy the man-killing animals living therein. Exterminating warefare is also waged against the savage members of the human race wherever they oppose the establishment of conditions necessary for the development of the more highly organized types. Of course, where improvement by instruction and subsequent cooperation is possible this extreme of annihilation need not be practiced, but unless it can be shown that there is room enough on earth for both savage and civilized, the savage must go
Full text of the article is available in Senate Documents, 64th congress, session 1, vol. 29, page 10891, made part of the record by Ohio Supreme Court Justice R. M. Wanamaker who testifies about the case.
Sanger's bias against people of color is illustrated by her statements about the Immigration Act of 1924, which banned the immigration of Asians, Arabs and Africans, in her speech MY WAY TO PEACE:
(c)keep the doors of Immigration closed to the entrance of certain aliens whose condition is known to be detrimental to the stamina of the race, such as feeble-minded, idiots, morons, insane, syphilitic, epileptic, criminal, professional prostitutes, and others in this class barred from entrance by the Immigration Laws of 1924 (d) apply a stern and rigid policy of sterilization, and segregation to that grade of population whose progeny is already tainted or whose inheritance is such that objectionable traits may be transmitted to offspring....Open the gates of the U.S.A. to those countries whose inhabitants have the inherent talents and national characteristics desirable, eliminating entirely those countries whose subjects have already been difficult to assimilate.
In What Every Girl Should Know: Sexual Impulses--Part II New York Call 29 December 1912 Sanger wrote:
The lower down in the scale of human development we go the less sexual control we find. It is said that the aboriginal Australian, the lowest known species of the human family, just a step higher than the chimpanzee in brain development, has so little sexual control that police authority alone prevents him from obtaining sexual satisfaction on the streets.
To further understand who exactly she considered to be the "human weeds" and who the "finest flowers of American civilization" one should read her works such as Woman and the New Race, for example starting in the charter "Materials For the New Race" at page 31:
Among our more than 100,000,000 population are Negroes, Indians, Chinese and other colored people to the number of 11,000,000. There are also 14,500,000 of foreign birth...Fifty percent are of the native white strain...the slums of Europe dumped their submerged immigrants into America...The 1920 census will in all probability tell a story of an even greater and more serious problem than did the last... Do these elements give promise of a better race? Are we doing anything genuinely constructive to overcome this situation?
She goes on to lament the lack of "unmixed native white parentage" and says:
These proportions are increasing rather than decreasing, owing to the
extraordinarily high birth rate of the foreign strains.
Toward the end of the book, page 229, she concludes:
In a word, natural law makes the female the expression and the conveyor of
racial efficiency. Birth control itself, often denounced as a violation of natural law, is nothing more
or less than the facilitation of the process of weeding out the unfit, of preventing
the birth of defectives or of those who will become defectives
Another use by Sanger of the term "human weeds" further illustrates:
So far we have not been gardeners. We have only been a sort of silly reception committee. A reception committee at the Grand Central Station of life. Trainload after trainload of children are coming in, day and night--nameless refugees arriving out of the Nowhere into the Here. Trainload after trainload--many unwelcome, unwanted, unprepared for, unknown, without baggage, without passports, most of them without pedigrees. These unlimited hordes of refugees arrive in such numbers that the reception committee is thrown into a panic--a panic of activity. The reception committee arouses itself heroically, establishes emergency measures: milk stations, maternity centers, settlement houses, playgrounds, orphanages, welfare leagues and every conceivable kind of charitable effort. But still trainloads of children keep on coming--human weeds crop up that spread so fast in this sinister struggle for existence, that the overworked committee becomes exhausted, inefficient and can think of no way out.
Margaret Sanger, "The Children's Era", Proceedings of the Sixth International Neo-Malthusian and Birth Control Conference, Volume IV New York, 1925, 53-58