Roots

At the beginning of Ch. 12 the Torah tells us when the Hebrews enter Eretz Yisroel they are to eradicate any trace of idolatry from the land.  Rashi says you have to go so far as to לשרש אחריה.
From the context of this possuk it is ruled (see Taz Yoreh Deah 146:12) that this law of לשרש אחריה only applies in Eretz Yisroel.  In other words, its a law in settling Eretz Yisroel, not a law in removing avodah zarah.
Many of the sifrei mussar and chassidus use this law as a metaphor for one's own conduct.  When one's אל זר has taken root, it is not enough to just remove the bad deeds, one must search for the source of why such conduct is happening to get rid of its very source to guarantee success for the future.  As the Gemorah in Shabbos (105b) says כך היא אומנתו של יצר הרע: היום אומר לו עשה כך, ולמחר אומר לו עשה כך, עד שאומר לו עבוד עבודה זרה, והולך ועושה.  Its not instantaneous that a person will come to such level of sin, it starts off as a small shoot and grows bigger and bigger.
In light of the previous halacha though, we have an additional twist on this.  It is only through the merit of being in Eretz Yisroel that one is able to fully eradicate the אל זר בגופו של אדם.  It is the great kedusha of Eretz Yisoel that shines a bright light even in under the dirt of a persons skin and allows one to uproot the roots of evil.  This idea is reflected in the Gemorah Arachin (32b) that understands that Moshe couldn't remove the yetzer harah for avodah zarah because he didn't have the merit of Eretz Yisroel, but the later generations, although not as great as Moshe could.  It is the merit of the kedusha of Eretz Yisroel that allows one to eradicate avodah zarah (based on Pnenei Harav and Emrei Emes 5670.)
In the ספר זכרון of Rav Hutner in the first section (written by his daughter) it seems he hold that uprooting all forms of avodah zarah is a prerequisite for yishuv Eretz Yisroel :