H. Lincoln Foster (see last week's book review on Rock Gardening) divides the saxifrages into 4 groups with a few leftovers. Let's examine each of these groups since they require different growing conditions.
Encrusted Saxifrages
These are silvery leaved plants with white encrustrations along the edges of their leaves. People often mistake them for sempervivums (hens and chicks) because of their growth habit. If in bloom, the open sprays of flowers quickly reveals that they are not hens and chicks. Encrusted saxifrages are easily propagated by dividing them into separate rosettes. Those rosettes without roots will root quickly in moist sand. They can also be grown from seed sown on grit covering a fertile, rocky soil mixture.
These saxifrages demand a rocky soil with plenty of lime and a slightly shaded spot on the rockwall or in the rock garden where they don't bear the brunt of the hot sun. When happy with their location, they quickly multiply along cracks of the rocks. Species examples include: Saxifraga aizoon, S. cochlearis, S. cotyledon, S. crustata, and S. Longifolia.
Mossy Saxifrages
These saxifrages form mats of soft, evergreen foliage. They need a shaded spot in moist, well-drained soil. It is important that the soil is moist year-round. Brown patches frequently develop if the weather is hot and humid. One way to prevent this is to add sand and leaf mold in the spring. You'll have to work it down into the matt of foliage. This will encourage the individual rosettes of leaves to form roots. Propagate these plants through division. Seeds will produce plants that may or may not be identical to the parent plant.
The species include Saxifraga caespitosa, S. decipiens and S. muscoides but you are more likely to find hybrids. The colors of the flowers, which are born high above the matt on wire-like stems range from whites to shades of pink and red.
Engleria Saxifrages
This group can be similar to the following group, but generally these plants have flat, gray leaves in a very neat rosette. Leaves toward the center of the rosette are smaller than those on the edges. The flower stalks come from the center of the rosettes. This grouping demands a rocky soil and north facing growing conditions--perfect for the shaded rock garden. Foster recommends using 1 part of stone chips/sand and 1 part of leaf mold packed into limestone rock crevices. Using limestone chips under the rosettes will help provide sufficient lime for the plants to do well.
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