May 16,2008 00:00
Q: I have some questions regarding my mortarless stone wall. All of the wall stones are the same size. For the 6-inch layer of rock (gravel) underneath the bottom stones, is it OK to use rock that is the size of a half-dollar, or is that too large?
How do you smooth out the layer of gravel before placing the first layer of stones on it? Do you put a layer of small rocks, or pebbles, on top of the larger rock? Is there any trick to making sure the bottom layer of stone is level?
Do you just wiggle the stone until it is level with the one next to it? Do you put landscape fabric in the bottom of the trench underneath the gravel, or should it be placed on the slope between the soil and the stone wall?
Fortunately, I don't have to be too careful with this project, as my wall is going to be only 1 foot tall.
A: Even though it is a short wall, it is still better to build it correctly to avoid having to rebuild it. You can use the larger gravel stones, but most people tend to use smaller gravel that packs together better. More importantly, fragmented, angular or cracked gravel, rather than smooth or round gravel, is preferred. The round gravel will roll and move around, but the sharp-edged gravel will pack together and lock into place, supporting the wall stones better. Choosing gravel with the "fines" or dustier particles also helps keep the stones together.
The smaller gravel will smooth out better when a trowel or board is moved across the surface to level it. A 1-inch layer of smaller gravel or sand is often used with larger gravel, so the surface can be smoothed out and the wall stones will stick into place. Small gravel or sand used on top of large gravel may settle between the larger stones as it is spread or leveled, meaning a lot may be necessary to fill the gaps. This upper layer to the gravel base must be compacted enough to not allow the wall stones to settle in the future. Extra handfuls may need to be tossed into place under some stones to make them level.
If the wall stones are similar or the same size, (as square cut stones often are) then the bottom wall stone layer needs to be settled into the gravel - their tops are nearly the same height and level to make the wall look right. If the wall stones are a variety of sizes or thicknesses, then the bottom layer can settle by wiggling them in. Don't worry about how much they sink into the gravel, but be concerned about the stones being level. The bottoms of either kind of stone will be hidden by the backfilled soil at the base of the wall; therefore, it doesn't matter how they look.
It is a good idea to use a layer of landscape fabric in the trench before adding the gravel. This keeps the gravel pore spaces open, not filling them with eroded soil. The pore spaces will help water to drain away from the base of the wall.
I would suggest lining all three sides of the trench and putting the cloth between the soil and the wall stones. Put any gravel behind the wall to keep dirt out of all stones and gravel, preventing any of the spaces from being filled with eroded soil.
Q: We planted some ornamental grasses. Some are staying in neat bundles while others are taking over huge areas. What can we do to stop them?
A: Grass has two types of growth patterns. First there is the clump or mound forming style - the clump may be only a few inches tall to more than 10 feet tall. The plant could be an evergreen or may become dormant in the fall.
Over the years the clump will get wider. In the future, the edges may have all the new growth surrounding a dead center. When this becomes unsightly, go ahead and divide the clump and eliminate the center. Replant a portion of the outer ring and reuse the rest in another location.
Seeds from clump-forming grasses can spread around the landscape. As they are noticed, they should be removed as quickly as possible. The longer they grow, the harder they will be to remove. One of the pretty aspects of these grasses is the ornamental seed stalks, but the longer they remain on the plant, the more seeds fall all over the garden. Some species never seem to spread in some locations, while the same species can become a weed for other people. If your seeds become a weed, pull all the small ones, and either cut off the seed heads or remove the whole parent plant.
Other ornamental grass species can also form tall stalks that look like a clump-forming grass; however, they spread by rhizomes or stolons, which are the shoots that spread under or over the soil surface. The first year or two the plant may stay in a single clump, but all it is doing is growing a root system. Then look out - the underground rhizomes shoot out from only a few inches to several feet and the clump suddenly becomes a monster that takes over the garden.
As you pull the extra sprouts spreading into the garden, look to see if they are connected to other plants by a stem. If not, it is extending by seed. If the grass is spreading by rhizomes or stolons, then you will have to decide how big you want the clump to be. Dig a trench around the plant - some grass species will be held back by occasionally yanking out all the rhizomes trying to cross the trench. Others will require a solid plastic, steel or wooden barrier buried in the trench.
Among the worst spreaders, or best if you need to cover a large area, are blue lymegrass, cordgrass and ribbongrass, which has a white-striped variegated form. These can be planted in such places as parking lot islands or areas between buildings and sidewalks where they are held in place while forming low-maintenance green beds.
E-mail questions to Jeff Rugg, Kendall County unit educator, University of Illinois Extension at email@example.com.
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