One circle of greens is a wreath (rhymes with “teeth”). The plural is “wreaths” (rhymes with “heaths”). In both cases the TH is unvoiced (like the TH in “both”).
To decorate something with wreaths is to wreathe it (rhymes with “breathe” with a voiced TH like the one at the end of “bathe”). He or she wreathes it (also with a voiced TH).