Portulaca grandiflora, also called moss rose, tends to self-seed, but harvesting seeds from this annual plant helps keep it where you want it. Moss rose quickly spreads in full-sun, dry areas when seed capsules are left on the plant. Growing 4 to 8 inches tall and 6 to 24 inches wide, moss rose varieties include the early-flowering Margarita series, "Sundial," which produces early, double flowers in sunset colors, and "Calypso," which features white, yellow, orange, pink or purple double flowers.

Moss-rose (Portulaca grandiflora)
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Moss rose grows well in rock gardens and in containers.

Moss Rose Varieties

Suppliers sell moss rose cultivars, hybrids, strains, heirlooms and mixes, and only some of these come true from seed. Cultivars and hybrids are specially bred to have certain characteristics, such as flower color, shape or performance, but they don't pass on these characteristics to their offspring. Some cultivar and hybrid seeds may not even sprout. Strains and heirloom plants are often older varieties, and these are more reliable in producing plants that are similar to themselves. Mixes are different varieties mixed together, and these may cross-fertilize, producing unpredictable results. Moss rose cultivars and hybrids are often more expensive than other varieties. If you aren't sure what type of moss rose you're growing, ask the garden center where you bought it.

Harvesting Seed

Moss rose seeds are fine as dust and develop inside seed capsules, which you can harvest. Moss rose flowers in summer. The seed capsules are 1/8 to 1/4 inch long, and they develop after the flowers are pollinated. When the seed capsules are ripe, they turn dry and papery and split open. Inside are many tiny, blue-gray seeds.

Harvest ripe moss rose seed capsules on a dry, sunny day. If you aren't sure whether the capsules are ripe, gently rub one between your thumb and forefinger. To avoid spilling the seeds on the soil, where they may sprout the following year, take a seed capsule indoors before testing it. Pinch off the ripe capsules and put them in a paper bag.

Separating Seed

Moss rose seeds should be separated from the seed capsules and dried before storing. Crush the seed capsules over a sheet of newspaper in a draft-free place. Place a colander on another sheet of newspaper, and pour the the crushed seed capsules into the colander. Gently shake the colander so that the seeds fall through but the capsule debris is left behind.

Moss rose seeds must be stored in an airtight container. Lift the newspaper and close it along its crease. Place one edge of the crease at the lip of an open airtight container, and lift the newspaper so the seeds slide into the container. Flick the newspaper once or twice with your finger to loosen any remaining seeds, put the newspaper down, then seal the container.

Storing Seed

A refrigerator is the best place for storing moss rose seeds, where they will remain fresh for sowing the following year. Put moss rose seeds in their sealed, airtight container in a refrigerator or similar cool, dry place such as a basement or cellar. Don't place the container in direct light, and don't open the container to check on the seeds until it's time to sow them. Direct light warms up the inside of the container, damaging the seeds, and opening the container allows moisture inside, which encourages the seeds to rot.