By Ken Lain, the mountain gardener
This is the time to turn your garden soils, plan crop rotation, and dream while turning the pages of that new seed catalog. The soil is so easy to work right now that adding compost, manure, and plant food into your soil is almost fun for the winter gardener pent up indoors.
The next few weeks is the ideal time to create new raised beds, or put up a new retaining wall. Keep these new garden beds, or retaining walls less than three feet high and they are ideal winter garden projects.
This tutorial is designed for beginners interested in building stone retaining walls using a “dry-stack” method, but it also works for retaining block and flagstone walls. Other materials that could be used for such a project include landscape timbers, railroad ties, and cinder blocks. These step-by-step instructions are meant specifically for walls built of stone.
The dry-stack method is not appropriate for walls that are to be higher than 3 feet tall. For taller structures, use mortared walls. After reading this column beginners can expect to spend two days to construct ten feet of wall.
In the planning stages, you will be working out all the logistics, taking measurements, gathering materials, etc. If your slope is too large for a 3-foot-high structure, consider terracing the slope by building separate retaining walls in two or more places, rather than trying to do the whole job with just a single wall.
The greatest advantage for beginners is that a shorter wall doesn’t require engineered footers beneath the frost line. A shorter length is far easier to build.
How to Build a Stone Retaining Wall
Head to your favorite rock yard and choose stones that have at least two sides that are flat, which become the “top” and the “bottom” once in place. The heavier the stones, the more stability you have, but also the harder the work will be. As a general rule, bigger is better especially at the foundation.
In addition to stones, assemble the following supplies ahead of time:
- Line level and string or garden hose
- Mason’s hammer
- Carpenter’s level
- Dig a trench about 8 -10 inches deep, so that the first course of stone will be fully or mostly submerged. This will help your retaining wall withstand the pressure exerted by the garden soil it is holding.
- Plot where the retaining wall will sit at the bottom of the slope, using stakes and string for a straight terrace; a garden hose is also good for visual placement. You will want the first course of stone to be level and a line level is the best tool for the job.
- To calculate the necessary width of the trench, just remember the base of the structure should be half the wall’s height. Angle the trench so that it inclines back slightly into the slope. A good rule of thumb is 2 inches back into the slope for every 1 foot of height. This provides the stability needed.
- Terraces of natural stone are laid one horizontal row at a time. The top row of stone consists of your largest, widest, longest, flattest stones, so hold some of these in reserve for the final row, or use capstones. The bottom row should be your most stable stones. Take the time to fit this row as closely together and as level as possible. Building terraces with natural stone is like fitting together pieces of a puzzle!
- Check to ensure that the stones run level left to right. But because you have built a slightly backward slope into the trench’s base, your stones will slope slightly from front to back. After completing this first course, or foundation, backfill with some of your excavated soil and any stones too small to use for building the retaining wall, and tamp down the backfill.
- When laying the next courses of stones, backfill and tamp down after completing each row. Tuck soil in between gaps in the terrace. When the wall is complete, you can root plants into the larger gaps of the wall that bring life to the structure. Cascading plants, such as creeping thyme, alyssum, ivy, creeping rosemary, and vinca minor are very attractive spilling down the sides of stone retaining walls.
- Make sure there is as little wobble as possible between each stone. To counteract wobbling stones use small flat rocks as shims for stabilization. Use a mason’s hammer to knick off stone fragments so as to achieve a better fit where needed.
- Continue in the same manner with succeeding courses. By the time your terrace is half its planned height, incorporate what are known as “deadmen.” In the case of stone retaining walls, the term refers to long stones laid perpendicularly across the wall, rather than parallel to all the other stones. The idea behind deadmen stones is to tie the structure into the slope in back of it for greater stability. These longer stones reach back into the slope so the weight of your garden soil helps stabilize the wall. The longer the stones you can find to serve as your deadmen, the better. A good rule of thumb is to provide at least one deadman per 16 square feet of exposed wall face.
- As you reach the desired height of the wall, place the capstones on top. Capstones are similar to the stones used in your top row, in that they should be flat and have significant mass. They serve both to hold the stones under them in place and to provide a finished look, thus the importance of their being flat.
A Planting Tip
Garden plants help retain and solidify your new wall in place, and bring it alive. Plants anchor and prevent erosion. We have a lot of plant choices at the garden center right now, but three that come to mind for sunny locations are blue rug juniper, native creeping mahonia, and coral beauty cotoneaster.
Until next week, I’ll see you at the garden center.