A trough garden is an excellent way to grow alpine plants. The potting mix, soil fertility and moisture level as well as sun exposure can be easily controlled and plants tend to be healthy with excellent form. The photo above shows a grouping of troughs at Wild Ginger Farm. Note the empty trough in the foreground. It is a concrete trough built using plywood forms and one bag of concrete. Using concrete allows us to have a larger and more structurally sound container than can easily be achieved with hypertufa. In the above photo, the smaller troughs are hypertufa and the large, low troughs are concrete.
The first step in making a slate crevice garden is to break the slates into irregularly shaped pieces. We use the larger pieces initially to form the main rock structure in the trough and add smaller pieces to fill in after planting.
The next step is to position the larger pieces of slate, starting at one end of the container. Create and fill the narrow planting voids between the slates with potting mix as you position the slate.
We use a gritty, very well drained potting mix consisting of at least 50% inorganic material, a combination of pumice and crushed rock. We use less than 20% peat to prevent the problem of the mix becoming too dry, seizing up and refusing to accept water. Once this has happened in a container, it is very difficult to rehydrate the soil, even with regular watering, and plants you think you are watering can die because the water can not reach the roots.
The slate tiles are all positioned at the same slight pitch, rather than being placed vertically. The resulting effect looks naturalistic to us, like upthrusting rock formations we have seen in nature.
This photo shows the slate positioned mostly submerged beneath the surface and with pieces extending at variable heights above the soil. The pitched angle of the slate also helps create sunny and shady microclimates within the trough garden. Even this small amount of protection from direct rays of the sun can create a favorable microclimate for alpines. The slate beneath the surface of the soil also provides heat protection to root zones.
We chose a planting theme of sun loving Western native alpines for this trough and included Physaria alpina, Petrophytum caespitosum, Erysium amoenum, Penstemon whippleanus, Eriogoum umbellatum, Heuchera pulchella, Draba cusickii, Penstemon rupica, Silene acaulis and Lesquerella arizonica.
Any combination of small scale alpine plants will work in a trough. Just make sure you select plants with similar cultural needs such as amount of sun, water and soil fertility. Also pay attention to the size of the foliage, growth rates and each plant's growth habit. If you select plants that grow quickly, you may need to trim and thin periodically to keep plant size in proportion and to keep them from overcrowding one another. You can view a selection of plants for troughs and in other containers in our mail order catalog. Another plant group to consider for trough planting are the dwarf and miniature conifers.
We finish off our planting with a 1/4" crushed rock mulch which will help retain moisture in the soil. This photo angle really shows off the beauty of the slate.
By mid-summer, the summer blooming plants are in flower and the plants have filled in nicely.
In the low light of fall, you can view a full season's growth in the slate crevice trough garden.