A s long as there have been rocks, there have been rock walls. Rocks are plentiful, inexpensive or even free, and strong enough to last centuries. We gardeners simply cannot resist bringing rocks into our gardens and fashioning them into walls of all sizes, shapes and colors.
Rock walls can be "dry" or held together with mortar. Dry-stacked walls use friction and gravity to hold them together. Besides looking more natural, water drains through the cracks instead of building up against a mortared wall. They are notoriously hard to build, since the rocks must fit closely together. When it's done correctly, the builder gains as much integrity as the wall.
The art of making a real dry-stack rock wall is as old as civilization. Thousands of them still exist in Scotland, Britain, Italy, and Ireland where they were constructed centuries ago to hold livestock, surround castles and enclose fields. Built in the Middle Ages, a dry-stone bridge strong enough for horse and carriage traffic is still standing in Alby, Sweden. Mayan and Inca ruins in Peru contain standing rock walls dating back to the 15th century.
Immigrants brought the knowledge to New England, where "rock fences" are still seen on farms in Kentucky, Pennsylvania and New York, and as far west as the Napa Valley in California.
Sadly, this ancient craft is being lost as fences are more quickly and inexpensively constructed with wire and wood. "Most landscapers we deal with now are building rock walls with square concrete blocks or wood and then facing them with rock veneer or artificial rock," says Charlie Mott, sales representative for Cascade Block.
Fortunately, there is much appreciation for the landscape value and heritage of old rock walls. Many are protected treasures that will remain standing into the future.
"The selection of your rock is critical," says Pete Cislo, owner of Leave Your Mark Pavers & Landscape Supply. Square or angular rocks are easier to stack than round river-type rocks. Cislo suggests you open your pallet of rocks and separate them into sizes and shapes first. You don't want to place all small or all large rocks at the same location in a wall.
Perhaps the most important thing to do is to stagger your rocks so that one rock sits on top of two rocks in the row below. This spreads out the empty spaces between the rocks so they don't become a weak point in your wall and allows for drainage through the wall. If your wall is a retaining wall, "the secret is in the way you lean it back," says Cislo. For each foot of height, you want to lean the wall back one inch into the slope you are holding. Long rocks called "deadheads" anchor the wall to the slope behind it, creating greater stability.
To add strength, and make it easier to build, you can "sneak" some mortar into your wall that won't be seen when it's done. Make sure your mortar mix is not too runny and your rocks are clean and free of debris, says Cislo. He suggests washing them off with a hose before you start. It's a good idea to determine the placement of your rocks before you start because it will give you more time to change them around and make them fit when you're using mortar.
After you've placed them, simply mortar the back half of the wall, he says. Set your rocks and carefully scrape out any mortar visible from the front. Don't wait until the mortar is completely dry or you will break or crack it and damage the integrity of your wall. The mortar will set right away to hold your rocks, but will take about 24 hours to dry completely. Make sure to allow for drainage.
"Most decorative walls are generally not more than 2 feet high, mortared ones up to 3 feet high," says Charlie Mott, sales representative for Cascade Block in Medford.
Higher walls should be engineered for stability and strength. "Often walls higher than 4 feet need to be inspected by a city or county building department," adds Mott.
Rock wall crevices and pockets make wonderful planters. Let a rambling rose climb up and over or tuck a trailing rosemary herb into a pocket to cascade down the face. Succulents like hens and chicks will soften, but not cover your beautiful stones. Brighten things up with a colorful trailing lobelia or a re-seeding Mexican daisy (Erigeron karvinskianus). Drought-tolerant plants are a good choice because walls are usually fast-draining, or a drip or spray line can easily be hidden among your boulders.
Rock walls add a timeless beauty and romance to your garden. And, if you're careful, no-one need know about the mortar.