The Chabad custom (Sefer Haminhagim Chabad, Haggada im Likkutey Tammim Uminhagim, see also Aruch Hashulchan 473:14) is to use a combination of ground horseradish and Romaine lettuce. We put the ground horseradish inside the lettuce and dip that into the charoses. Shulchan Aruch (473:5) rules that one may combine the different types of Maror, and this custom has the additional advantage of including Romaine lettuce, which Shulchan Aruch considers to be the most preferable type of Maror.