Planting in Landscape Rock
Rock is not Mulch
I don’t really want to lecture on the evils of using rock in the landscape, I use them too. But I will lay out a few pros and cons and focus on how to successfully grow plants in a rock bed.
But first a note about the rock. When selecting a landscaping rock we are naturally focused on the decorative aspect. But I have learned the hard way to focus first on function. Rock is hard to keep free of leaves and debris, which will decompose and help seeds to take root. Using a leaf blower to clear debris from landscape rock can be tricky but with experimentation does work well. Lightweight lava rock can easily blow out of the bed as will small rocks. Large heavy rocks stay put better but also trap the debris well, preventing easy clearing. Small rock compacts and does not trap debris easily so it does allow pretty easy removal with a low setting on the blower.
What I find most important is using a rock that won’t crumble and disintegrate. That ruins the whole idea of using permanent rock rather than mulch that decomposes and needs consistent refreshing. The crumbled rock also quickly adds to the organic matter that aides in weed proliferation. A good hard rock will last a lifetime. River rock is smooth and hard and available in different sizes. Lava rock is rough and fire hardened with interesting color. Trap rock is also fire hardened. Granite and quartzite of course are also very hard. “Softer” stones susceptible to crumbling can usually be identified by simply scratching at the surface to see if you can dislodge particles. These are generally limestone, dolomite or sandstone.
Since rock does nothing to benefit your plants or the soil and does not present ideal conditions for growing, a pretty big benefit needs to be there. For me, that benefit is cost and labor. I spent 20 years buying hundreds and hundreds of dollars of mulch every single year. I did get a lot of mulch for my money since I purchased by the truckload and picked it up myself. But that also meant dragging enormous amounts of mulch off a drop side truck and hauling it in wheelbarrows up and down hills. Great workout. But I’m not so young anymore. Much of my mulched area was a never ending project area of encouraging ground cover to fill in between what now are enormous evergreens. But, around the foundation of my home I prefer rock. regardless of drawbacks.
Yes, there are some benefits to using landscape rock, especially for foundation beds. First and foremost, it is a one time purchase if you select the right rock. That’s a pretty big benefit in my book. You can also select a rock style and color that enhances your home and landscape. That goes a long way for curb appeal. Two big wins so far for me. As for foundation plantings, typically we at least try to select plants that will survive and fit the space for at bare minimum 10 years, hopefully much longer. So removing rock to replace plants should be a very rare event if you are doing it right and are very lucky.
But foundation landscaping has changed a lot over recent years and we do love to incorporate some shorter lived flowering shrubs and perennials, even annuals and edibles. Now we have some problems with that rock. For starters, you really do need a porous landscape fabric under the rock to prevent the rock from embedding and burying into the soil. (Please don’t use plastic) The fabric will help prevent weeds from sprouting from existing seeds and roots in the soil but still allow water to penetrate. Every hole that is cut for a plant now allows and escape for weeds. Weed seeds will blow into and be dropped into the rock, sprouting all kinds of thing to be removed. But of course weeds are a never ending battle for us no matter what, so nothing new there.
Plants in general do not like to grow surrounded by fabric and rock, it can constrict the natural growth pattern. Perennials especially have difficulty growing in rock. The rock and fabric can also become a problem when it becomes necessary to dig up and divide the perennials. Rock also gets very hot in the day sun and retains that heat well. This can heat up plants and their roots which is can be damaging especially to sensitive plants. Some may not be able to survive with the added heat. These issues can be overcome for the post part with proper planting , care, and plant selection.
Keep in mind for plant selection:
Most shrubs, evergreens and ornamental trees will do fairly well in rock with landscape fabric.
Heat sensitive plants such as coral bells may struggle if not fail
Select native or sturdy perennials that will thrive without pampering. Consider specific rock garden plants.
Select perennials that do not require frequent division.
Tips for planting in rock with landscape fabric:
Plan placement of plants carefully to accommodate mature size to prevent moving or replacing plants. Shrubs and trees generally mature at about 10 years but they do still keep growing for decades.
Cut holes in the landscape fabric large enough to accommodate the full root ball of perennials. Remember that you will eventually have to dig it up to divide it.
Cut holes in the landscape fabric large enough to accommodate the full grown trunk of shrubs and trees. If the fabric interferes the trunk may grow right around the fabric or the trunk may become girdled.
Keep the rock away from the base of plants, allowing at least 4 to 6 inches of bare soil. The soil can be covered with an organic mulch or compost but should not come within a few inches of shrub or tree trunks.
If you intend to plant annuals, select specific areas to section off with edging. You can use organic mulch with no fabric to make planting easy every year.
Monitor young plants until well establish to ensure they are not overheating and that the soil is not dried or over watered.
One last note about foundation plantings especially if you install landscape fabric an decorative rock - Soil underneath may not receive as much moisture as exposed soil. If you leave a large enough opening around each plant that will help water to contact the soil and reach the roots. Do not use more than one layer of landscape fabric as that will further inhibit water absorption. And note foundation beds under overhangs may receive very little water even if the bed is mulched. Be sure plants closer to the foundation receive adequate moisture.