Rose seeds (also known as achenes) are carried inside the rose hip fruits that develop and ripen on the plant in the summer and early fall. The ovary that contains the rose plant's DNA material resides just under the flower bloom. After the flower itself dies back, the ovary swells and deepens in color to become a mature rose hip. Mature rose hips are round, red, orange or golden fruits, with each containing numerous rose seeds that can be harvested for propagation.
Refrain from dead heading or harvesting all of your roses from the plant as this removes the rose's ability to develop rose hips and seeds. Simply leave some flower heads in place to die back on the plant, and over the course of a month or two, seed bearing rose hips will develop.
Allow the rose hips to reach peak maturity while on the plant, as this will increase the health and vigor of the seeds inside. Wait until the late summer or fall when the rose hips are swollen, deep in color and are soft enough to be dented with your finger and then harvest the hips.
Cut the bunches of rose hips off the stem. Rinse the bunches lightly in clean water to remove any soil or applied pesticides. Cut each hip open in half to reveal the white seeds inside.
Scoop the seeds out of the hips with the tip of your knife and into a small, clean glass jar filled halfway with water. Once all the seeds are in the jar, screw the lid in place and agitate the seeds in the water to wash any ovarian residue off them.
Pour the water and seeds from the jar through a fine mesh strainer; the seeds are now ready to be planted.