You will need to amend your soil for rock garden plants, because many of them will not survive the winter, especially if they are planted in typical St. Lawrence valley clay soil. This is because in winter, a critical period for these plants, clay soil freezes very deeply and is slow to thaw in spring, smothering the roots. The small air pockets in more granular soil help avoid this problem.
Alpine plants need few nutrients and often colonize unstable, disturbed and poor soil. Instead of concentrating on soil quality, as one needs to do for other types of plants, here it is more important to improve its texture to promote drainage.
Drainage is key for any new rock garden. You can spread a layer of coarse material (gravel, bricks, etc.) to trap air under any areas where water is apt to accumulate because it cannot run off. Another option is to raise the bed so that rainwater will run off quickly.
Another way to avoid collar rot, a common problem with rock garden plants, is to spread a layer of 3/8" to 1/2", or 5-10 mm, gravel around their base. This will encourage water to drain quickly and keep their collars dry. Finally, it is a very good idea to add mulch, as it will keep the soil cooler, prevent mud and soil from splattering onto the foliage and help control weeds.
You absolutely must amend the soil with coarse sand, gravel and peat moss. If you wish, you can add extra organic matter in the form of compost, digging it in rather than spreading it on the surface, to avoid the collar rot that afflicts alpine plants.
Because they often grow on slopes where surface water runs off quickly, alpine plants need deep roots to reach water far underground. They are adapted to this situation in the wild, and have developed quick-growing roots that also anchor them firmly to slopes, scree, cracks and cliffs. In a rock garden, it is important to force your plants to grow the deep roots they will need to survive the intense heat and droughts of our summers.
The first summer is key for young alpine plants. You must expose them to some degree of stress by alternating deep waterings with periods of heat and drought, while keeping a close eye on them. It is also important to carefully gauge the degree of water stress you allow (when a plant starts to wilt, it should be watered), because too much water will be just as disastrous as too little. In subsequent years, your plants should be able to survive with regular care.
You cannot successfully grow alpine plants without careful weeding. Alpine plants grow on mountainsides, far from opportunistic plants like weeds, and usually colonize virgin sites, with the priority going to anchoring themselves, avoiding dehydration and reproducing quickly. Because all their energy is channelled into meeting these basic needs, they are poorly adapted to competing for sunlight and nutrients, and weeds will usually get the upper hand.
Rock garden plants are not all equally susceptible to invasive plants. Those that grow at lower altitudes on mountainsides and in meadows, in more clement habitats, are better adapted to sharing their space with a large number of plants. This makes them better choices for rock gardens in weed-ridden sites.
You need to be strategic and put alpine plants where you can keep a close eye on them and act quickly at the crucial time in summer when weeds are at their worst. For instance, it makes no sense to try to grow moss campion (Silene acaulis) or purple saxifrage (Saxifraga oppositifolia), both of which are small, slow-growing, mat-forming plants, in a site that is invaded every year by common groundsel, wood sorrel or horsetail. It is better to choose a site with few weeds for your rock garden.
When we talk about pruning rock garden plants, we are referring to limiting those plants that tend to spread quickly and seem to want to reseed themselves everywhere. Alliums are the key culprits here, along with penstemons, bellflowers, asters, columbines and rockcresses. For the same reasons as those mentioned above for weeds, it is essential that you control the spread of some rock garden plants by pruning their seed-bearing capsules.
Snow offers the best plant protection, by insulating them. But since snow cover isn’t reliable from one winter to the next, we have to install other forms of protection. If you are not using geotextile cloth, you can spread evergreen branches or a thick layer of straw or wood shavings in spots that are exposed to the wind, to encourage snow to accumulate there. You can also use protective cloth instead of organic mulch, but you must first wait until the ground freezes.
Based on an article by René Giguère in Quatre-Temps magazine, Vol. 22, No.1.