If the arugah is planted amoung other arugot, there are a number of solutions depending whether you think a 1, 1.5 or 3 tephach separation is necessary. In general, it is assumed that each arugah has a one tephach wide border, so there is a minimum of two tephachim between the arugot. (Pictures redrawn from Dafyomi Advancement Forum of Kollel Iyun Hadaf, http://www.dafyomi.co.il are marked kollel.)