Knowledgebase
Adding plants to a rock garden #868390
Asked May 14, 2024, 12:56 PM EDT
Three years ago I bought a house in Longmont. The front yard is largely grass with a border of rocks about three to five feet wide in places. They are smaller rocks and in some places there are shrubs and some random plots of tulips and daffodils early in the summer. My initial goal was to remove the rock and replace it with dirt to plant some waterwise shrubs, flowers and grasses but that involves a lot of labor so in the meantime I’ve been growing some of the plants I want in the future in large pots on top of the rock areas. Over the last two years several of those plants have self seeded and are starting to come up through the rocks. This is much easier than removing the rocks and I’m wondering if it’s feasible to fill in the entire area of rock without removing any of it? And if you have any suggestions for what plants might grow best through the rocks that are native to Colorado and have low water requirements?
Boulder County Colorado
Expert Response
Our apologies for the late response. Some of our emails inadvertently ended up in our junk file and were just discovered.
Thank you for the interesting question regarding growing plants in rock mulch. Depending on the size of your rocks, there are quite a few upsides to what you're considering.
I've cut/pasted a quote below from this fact sheet on mulch:
https://cmg.extension.colostate.edu/Gardennotes/245.pdf
From Rock Mulch
Rock and gravel mulch are often maligned as not beneficial to plant growth. Because they are sometimes grouped together as a single product, conclusions about them from research can be hard to draw. For example, many studies do not specify rock size when discussing “rock mulch” in comparison to wood mulches. In studies that have differentiated among rock sizes, though, mulch
with gravel less than one centimeter in diameter has been shown to suppress weed growth and reduce water loss from evaporation similarly to wood chip mulch. Additionally, surface temperatures in planting beds mulched with gravel remain cooler than paved surfaces or surfaces mulched with wood chips.
Landscapes in dry climates are most likely to benefit from gravel mulches. Pea gravel has been shown to improve water infiltration into the soil, especially in short, intense precipitation events like thunderstorms; larger rocks increase runoff. Even a thin layer of gravel mulch has been shown to 245-4 double the amount of precipitation that infiltrates the soil; a three-inch layer can increase water infiltration manyfold. In order to function as mulch, though, stones must be small enough – no more
than around a half-inch in diameter. Larger stones do not confer the benefits of mulch.
Gravel mulch can improve the soil too – by increasing the available soil moisture and warming the soil temperature beneath it, plant roots and microbes get a boost. The increased biologic activity can result in improved organic matter content and subsequent tilth, even though the mulch itself does not
decompose.
There are so many plants that would thrive in your rock mulch. Here's a low water native plants guide to help you with deciding on what plants will do well:
https://extension.colostate.edu/docs/pubs/native/FrontRange.pdf
Hopefully this is helpful, but if you have additional questions, please don't hesitate to reach out again.
CSU Extension Master Gardener
