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Rock Gardens Rock!


Published: 06/19/2012


Fall is a good time to deposit earth for a future rock garden, as it will settle well over the winter.

A rock garden can be a useful addition to many yards, by creating visual interest and allowing us to grow plants that are well-suited to our climate. The combination of such dissimilar textures as rocks and plants generally creates a pleasing effect, and it's a place where we can let our hair down and leave those fussier formal plants behind us.

A few words of explanation first. By rock garden in the context of this article, we mean a berm or a rockery. It is usually a mound a few feet higher than the surrounding ground in which are interspersed an assortment of plants with rocks. In a broader context, a rock garden can also refer to a landscape setting with rocks throughout, including moraines, screes, rock streams and other features, but we'll leave that subject for another day.

In the rock garden, or berm, discussed here, the majority of the area will be of plant material, though at least a quarter of it should be of rock. For visual appeal, the rocks should be of different sizes though generally of the same material. In other words, it is best not to mix, say, limestone with granite. Natural mounds and areas that are suddenly exposed to huge chunks of daylight (such as might happen through the felling of old trees) are excellent candidates for rock gardens, or you might like to consider erecting one just to break up a boring section of lawn.

A rock garden is generally, though not always, positioned so as to enjoy full sun for most of the day. In summer it will really bake, helped by sun-heated stone, and it offers the opportunity to grow those sun-loving plants that may not thrive elsewhere. As well, the contouring and exposure will allow the formation of mini-climate zones that may be a notch or two higher than the local area, allowing you to experiment with more exotic plants.

The layout of the rock garden should be such that you have ready access to all the plants, all the time. That's because they are almost certain to thrive, and you will have to frequently go in and thin out or cut back the more aggressive cultivars, lest they take over. On larger berms, you can achieve access through pebble pathways. Watering should not be a problem, since most of the plants will be drought-tolerant anyway.

Generally, you will not wish to locate tall plants on the berm, for they will create shade and make life difficult for the other denizens, though you should try to vary the contour through a variety of medium, small and ground-covering plants.

To create the berm in the first place is the hardest part, since you will need many tons of soil. You can of course turn the total job over to a landscaping company or you can follow the lead of my friend Gwen Norton-Wilks at her delightful North York garden, where she persuaded a local contractor to dump a couple of truckloads of clean fill in her frontyard, then let it settle over the winter -- an important point, since it will diminish by as much as one-third. The excavation from a newly constructed swimming pool would be ideal, since you are looking largely for a clay base on which to deposit a layer of triple mix or topsoil, then some composted material for the final few inches.

To get the rocks in place will call for professional help, since each one is likely to weigh a half-ton or more, depending on the size of the berm. Obviously the rocks should go in before the good soil does, otherwise you will waste a lot of it, plus the rocks will sit on top of the berm instead of having that natural partially buried look. If you are dealing with a considerably smaller berm, with rocks of up to 40 pounds each, then you might manage placement yourself, but at least get someone to help you.

The range of plants that you can grow in a rock garden allows you to enjoy colour all the way from early spring to late fall, and it can still look interesting in wintertime, especially if you make sure to include some evergreen foliage. In fact, it would probably be easier to list what won't work than what will (and yes, rhododendrons top the list of non-starters!).

Blue-carpet juniper, Japanese garden juniper, cotoneasters and sedums will make an excellent base selection: the sedums alone offer an enormous range that includes Autumn Joy, Dragon's Blood, Sedum anacampseros with its lovely purple flowers, and the family's close relative, Sempervivum (hen and chicks). One of best qualities of sedums is that most are late flowering and will attract bees and butterflies to your garden.

Other plants worth considering are the saxifrages, artemesia, dianthus, columbine, aubrieta (rockcress), iberis, miniature roses, creeping thyme, ornamental grasses, lamb's ears and Spanish broom (though the last-named is not reliably hardy). In those spots where you need a little extra height and bushiness, potentilla can fill the need quite nicely. There are so many plants to choose from that you must be careful not to over-plant.

The berm creates a good environment for herbs, so you might like to plant a sprig or two of your favourites along the edges, such as coriander, parsley and dill. Remember too that many annuals which thrive in a rock garden will self-seed, including portulaca, alyssum, snapdragon and calendula. And in that corner that gets the most sun, be sure to sprinkle the seeds of night-scented stocks, for they will reward you with their gorgeous aroma on many a summer's eve.

Richard Rix lives and gardens in Toronto and is working on a book on rock gardens.

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