Although very little is “permanent” in gardens, we use this term to mean mulches that are intended to remain without replenishment for many years. Mulches that don’t decompose include all the mineral mulches—sand, gravel, and river rock—and coarse bark chunks.
Permanent mulches are used where plants don’t grow or around long-lived shrubs and trees. They are more difficult to haul and spread because of their weight, and may be more expensive than other mulches. To keep weeds from coming through the mulch, either place a weedblock fabric on the ground under it, or make the mulch deep enough to block light to the soil underneath.
Some types of patios may be thought of as mulches. Bricks, rocks, or pavers laid in sand have many of the same effects on the soil as mulches—they block weed growth and prevent evaporation from the soil, but allow water and air to penetrate to the soil, promoting root growth. You can plant in this type of patio by removing bricks or pavers to make “planting pockets.” Plants grow well here for the same reasons that they grow well under mulches. Patios that are impermeable to air and water, such as concrete or bricks laid in concrete, do not have this effect, and impede the growth of roots because the soil under them does not contain enough oxygen or moisture.
Although they are low-care, permanent mulches are not completely maintenance-free. All types need to be kept clean. If leaves and plant litter falls on them, they must be cleaned up. If allowed to remain, in a few years the litter will cover the mulch and support weed growth, effectively destroying both the appearance and weed control effects of the mulch. Coarse mulches made of heavy particles remain in place when blown, and a blower is the most effective way to clean them. Finer materials, such as sand or fine firbark, may be moved by a blower. Clean them with a rake, drawing it lightly across the mulch to avoid shifting it.
Be careful not to get soil on the mulch when digging nearby. Soil sifts into the mulch and plugs up its air spaces, again destroying it as a mulch. Enough soil on a rock mulch makes rocky soil, not mulched soil.
Sand makes an attractive mulch if the right sand is selected and it is maintained properly. Don’t use it under plants that drop leaves or a lot of litter. It’s too hard to keep clean, and if soil and organic matter gets mixed into it, it stops acting as a mulch and becomes sandy soil.
Select a coarse graded sand for a mulch. Graded sands have been sifted so all the particles are of the same size. Because there are no finer pieces to plug up the spaces between the sand particles, graded sands have the highest amount of air. This means they drain rapidly, dry out quickly, and don’t support weed growth. Many ungraded sands contain particles of different sizes, so the smaller particles plug up the air spaces between the larger particles, restricting air flow.
Some ungraded sands have been graded by nature. Beach sands and many river sands are composed of grains all the same size. If you gather your own beach sand, wash it thoroughly to remove salts.
Sand with “fines,” or dust, is really sandy soil, and will not act as a mulch, but as a layer of soil. Weeds will grow in it, and water evaporates readily from its surface.
Spread sand 2 to 3 inches deep.
“Gravel” is here used to include all the rock materials with grain sizes from the size of a pea to a golf ball. This includes all sorts of ornamental rock. Gravel, like sand, is usually dumped, then spread out with a rake or shovel instead of being placed one at a time like river rock.
Like sand, gravel must be kept clean, but it’s easier to clean than sand because you can blow it. You might have to blow pea gravel and other small types gently, but you can usually lift off leaves and debris with a blower without disturbing the mulch. However, the smaller sizes may be difficult to rake because they mix with the leaves and are picked up.
Fir bark and other ornamental chunks of bark are permanent in the sense that they last for years without decomposing. Treat them as gravel.
This largest category of mineral mulch includes pieces large enough that they are usually placed individually instead of being spread with a rake. It isn’t usually practical to make river rock deep enough to prevent light from reaching the soil. Spread a layer of weedblock fabric on the ground before laying the rock, or lay the rock over a layer of gravel.
River rock does not rake easily because the spaces between the rock are so deep. It must be blown to remove leaves and litter. Although leaves may not spoil its appearance if they sift between the stones, they will eventually plug up the spaces with humus that supports weed growth.