Oasis square risers are a block of florist foam with a smaller square of Styrofoam welded to the top. This unique block of foam allows florists to "raise up" flower accessories in order to keep them dry. This may be something like a book, stuffed animal, framed pictures and other items that you do not want to get wet.
It also provides a firm base in which to insert a dry topiary stake. This is essential if the top of the topiary contains wet foam, thus making heavy to balance. The counterbalance of the wet foam below filled with flowers is a perfect solution to the problem.
The square riser fits perfectly in the Essentials Large container by Oasis.