Ruling concerning root vegetables (those which grow under the ground)
Question

                Asalam mu alay koem
Dear Brother,
 Please let me know if this is true or not, a lady whom i go to Talim
  Classes with has told me that vegetables grown under ground are makroo, and to cook many
  makrooh things in one pot is Haram.
 We are very used to making stew with more than 5 or 6 vegetables which are
  grown underground all in one pot. Is she telling the truth or is she mistaken.
  Wasalaam,
Sha
Praise be to Allah.Allaah says (interpretation of the meaning):
“He it is Who created for you all that 
  is on earth…” [al-Baqarah 2:29]
“O mankind! Eat of that which is lawful 
  and good on the earth…” [al-Baqarah 2:168]
“Say: ‘Who has forbidden the adornment 
  with clothes given by Allaah, which He has produced for His slaves, and al-Tayyibaat 
  [all kinds of halaal (lawful) things] of food?’…” [al-A’raaf 
  7:32]
From this, we know that everything that grows from 
  the earth is in principle halaal (permitted), and no-one should decree it to 
  be haraam (forbidden) or makrooh (disliked) without sound Islamic evidence 
  (daleel). What evidence is there that things that grow beneath the ground, 
  like potatoes, carrots and onions, etc., are makrooh? What is the strange 
  principle on the basis of which this lady – about whose background we know 
  nothing – states that cooking a number of “makrooh” things 
  in one pot results in haraam food?
If the resulting dish was intoxicating or poisonous, or was 
  cooked with something haraam such as pork, then we would say that it is haraam 
  to eat it. But the cooking of five or six kinds of root vegetables together, 
  as you describe, is something permissible, and there is nothing wrong with it. 
  We advise you not to give any weight to the words of those who have no knowledge. 
  Allaah is the One Whom we ask to guide us to the truth and to help us understand 
  our religion. May Allaah bless our Prophet Muhammad.