For best results, plant your wall in spring on an overcast day, so sun does not burn or dry out the plantings. Fall planting can work, too, but only if tender starts have enough time to become established before a hard freeze.
A stone wall planted with herbs makes a spectacular yet practical landscape feature. You can use the wall as a freestanding fence, a retainer for a slope or as a low border for a raised bed. If you plan to grow large herbs, a retaining wall is best because roots can grow into the soil behind the wall.
Select a variety of arching, creeping and upright herbs; a 2½-inch size pot is best. To build the wall, you’ll need either a garden hose or stakes and string to lay out the shape; a spade or shovel; a tamper; and a small trowel or five-in-one knife. Then assemble your tools, rocks and plants, roll up your sleeves, and dig in …
Step 1: Make a soil mix using equal parts natural topsoil, high-quality compost and crushed gravel, which will provide good drainage. Avoid using manure; it can burn plants if it isn’t completely rotted. Remember: Rock wall plants like a lean and mean soil!
Step 2: Build the wall. To start, dig a trench 4 inches deep, then fill it with gravel or sand. Tamp the gravel firmly in place. Next, lay the rocks, fitting them together as you would a jigsaw puzzle, with gaps no more than 1 to 1½ inches between them. Build your wall one layer at a time, covering each rock layer evenly with a 1- to 1¼-inch layer of the soil mix. Stagger rocks so that the gaps are not all in a row, leaving occasional spaces for larger plants. As you build, step back each layer a half inch to ensure stability. (For a retaining wall, fill in the back with gravel as you go.) Finish wall by mounding some soil on top. Do not tamp this down; you don’t want to compact the soil because plants will grow here.
Step 3: Prepare to plant. Although it might seem easier to plant as you stack, tucking the herbs in after the wall is finished allows more artistic control. Before inserting your herbs, prepare a planting mix of equal parts clay and sphagnum moss, along with a little water to make a slurry. Squeeze out excess water.
Step 4: Tuck herb into crevice. Using a trowel or knife, dig a hole just big enough to accommodate your plant. Push in the plant so that it fits snugly and touches the surrounding soil. Mash and cut roots, if necessary, to make them fit.
Step 5: Fill in with slurry, being careful not to cover the plant’s crown. If there’s a gap below the plant, use your fingers to fill that in, too: You don’t want any air pockets.
Step 6: Settle plants in place. If you’ve cut off any roots, also trim back the top of the plant to reduce stress on the plant’s roots. Water your plants with a gentle stream trickled down from the top of the wall.
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