After growing for a few years, some plants, including cushion-forming ones, tend to have bare, unattractive centres. This is entirely natural, for alpine and rock garden plants are often short-lived.
In the wild, the tips of growing stems break off from the parent plant, roll down the slope and take root if they happen to land in a suitable spot. It is easy to simulate this in a rock garden. If you fill the bare centre with organic matter and gravel, the plant will rejuvenate itself and new shoots will soon appear. The following year, you can take cuttings from around the rooted plants and grow new ones. This method works well for such genera as saxifrage, rockcress, aubretia, draba, pinks, phlox and thyme.
Root division is another propagation method used to rejuvenate some plants. Spring is the best time to divide species that flower later in the season, while it is better to do it with spring bloomers in early fall. You’ll need to plan accordingly.
Finally, you can produce additional young plants and add to your mass plantings by taking cuttings. It is easy to encourage many species to root by placing cuttings in a mix of sand, perlite and peat moss that you keep moist and, at first, out of direct sunlight.
Based on an article by René Giguère in Quatre-Temps magazine, Vol. 22, No.1.