Rock plants are normally referred to as “alpine”, but there is a difference. Alpines grow on mountaintops above the timber line, where as rock plants grow at any elevation among the rocks. Alpines first received their name on the Alps of Europe, but now from any mountain peak on any continent. They are small plants, usually tufted, are unable to compete with the vigor of plants of larger growth.
True alpines become difficult to grow in garden soils and at sea levels, and for the average rock garden where most of us live, it is often best to avoid these types of plants. Yet after Arabis, Iberis, and Phlox have taken over the rock garden, alpines may seem like a nice change in pace. The construction of an alpine rock garden takes a lot of determination and creative means. The soil areas must be narrow and perfectly drained below and on the surface.
For an alpine garden, freely insert gravel and rock chips into the soil mixture, and provide a rooting depth of at least three feet. The soil in the crevices should be the poorest of quality and reach far back into the bank. This is the first and greatest requirement. Surface drainage is the next issue, so be cautious with any water features or fountains. If you notice water standing for hours about the crowns of alpines, they will die. The pockets must be raised above the adjacent soil or given a steep slope.
It is hard to create a perfect surface drainage, and you will be able to tell if it fails because the plants will die. The next provision is to make up for the lower altitude of their new home. Few of us have gardens at an altitude of two or three miles, where the air is cool and moist. On flat plains, in the dust and heat of cities, it is nearly impossible for these plants to survive.
There are quite a few things you can do to make these plants comfortable. If you live near the sea, or in a location close to the water, where moist air comes in helps, but in hot, dry summers the alpine garden should be watered with the sprinkler in the evening if there are no heavy night dews. The morning shade should be kept as long as possible, to keep the benefit of night dew for the greatest number of hours.
Alpines need full skylight and free air circulation, and should not be in the shade of trees or buildings. Alpines should not, however, feel the full force of midday sun. Thus, the greatest possible use must be made of the low angle of slope to the north, with plenty of upstanding rocks to cut the rays of the sun. A bank facing directly to the north is a good alpine possibility, but best of all is a deep cut like the sides of a sunken drive running some- what east-west, with a cool breeze drawing through from woodland or lake.
Natural ravines are frequently found, and with a little expense, can make perfect alpine gardens. By keeping the alpines out of baking sun and in a cool air, you can keep them alive. This is especially true of all the smaller Saxifrages. One more detail, the most essential, is surface drainage close about the crown. An alpine will not deal with wetness among the foliage and should have a pack of stone chips holding the crown from the soil below.
These plants will need fed, each year, with chips of rock the size of corn grains. This will also help with winterkilling and frost-heaving when the chip blanket is present. As for winter protection, the North Slope and chip blanket will take care of the mild winters, and in the cold winters with plenty of snow, the alpines will take a new interest into their new home.