100 Seeds of Rose Moss to Grow. Moss rose plants (Portulaca grandiflora) are popular flowering annuals . The medium green, cylindrical, succulent leaves of moss rose are another clue to the plant's hardiness, namely its tolerance of drought conditions. These plants grow three to nine inches tall and spread to create a dense mat, making moss rose a good option as a ground cover. The flowers come in several bright colors and often have ruffled petals, looking very similar to miniature roses. The blooms grow in clusters on reddish stems and typically don't open on cloudy days or at night. Moss rose is easy and quick to grow and is best planted in the spring after the danger of frost has passed.