Make certain friends with rock garden and wall plantings

“Certain plants, like certain friends, you enjoy having for a visit, but do not care to see remain forever and a day.” 

— Henry Sherman Adams, “Making a Rock Garden,” 1912

This kind of dry wit makes H.S. Adams’ gardening book a delightful and still relevant read for anyone interested in making a rock or wall garden. (Download a free digitized copy of the book at www.forgottenbooks.com.)

Adams was referring to plants that tend to “take all the room in sight if they are allowed to, and they must be watched closely, or else discarded altogether.”

I recently read Adams’ 50-page treatise after I used lava rocks from a dismantled raised bed to build a border around a pond in the backyard (see pictures on my blog). Two pages into the book, I realized I had already unwittingly disregarded Adams’ number one rule for rock gardens — they should look natural. He also cautions against building a rock garden close to the house.

Yet, I have a bunch of lava rocks piled around a pond next to the back door. How can I make my rock border look more natural? My intention is to take Adams’ advice this time and create a border garden with low-growing plants particularly suited to living among rocks, also called saxatile plants.

My lava rocks are porous, so they will absorb water and help keep the plant roots cool and moist. Other kinds of rocks recommended for rock gardens include limestone, sandstone and shale.

My pond receives a bit of morning sun and lots of afternoon sun, so I need plants that will grow well with about 6 hours of sunshine, plenty of summer heat and minimal water. Rock garden plants also require soils with excellent drainage and low to medium nutrient content. I’m using a purchased topsoil mixture slightly amended with compost, but gardeners can make their own rock garden soil by combining one-third coarse sand or fine gravel, one-third peat moss, and one-third loamy soil or compost.

The key to planting in rock crevices is to use seedlings or small plants purchased in cell packs; don’t try to force a larger plant into a space that can’t accommodate the root ball. Wrap the plant roots loosely in moistened peat moss and then gently push the plant into each crevice. Position the plant so the crown is about an inch above the soil. Pack the soil around the plant, making sure to gently press the base of the plant to remove air pockets. Also, it’s important to keep a lip of rock visible to help prevent the soil from washing away.

Gardeners can also sow seeds and bulbs in cakes of soil that are pushed into the rock cracks.

Rock garden plants usually don’t need to be fertilized, and they need only minimal watering during summer. Protect plants in winter by mulching with shredded leaves; in spring, remove the dead foliage and freshen up the soil around the plants. If the plants die off in the center, add fresh soil to encourage new growth.

I was amazed to learn of the array of plants that are suitable for my rock border. Many of the plants recommended by Adams more than 100 years ago are still suggested for rock gardens and walls today. Some perennials include:

Spring bloomers: rock cress (4-6 inches; pink or white flowers); candytuft (6 inches; white/light pink flowers); alyssum “Basket of Gold” (6-12 inches; yellow flowers); thrift (6-12 inches; pink or white); creeping phlox (4-6 inches; purple or pink flowers); blue star creeper (2-4 inches; pale blue flowers); dianthus pinks (6-12 inches; pink or white flowers).

Summer and fall bloomers: sedum (2-4 inches; pink or mauve flowers); sempervivum (6 inches; pink starry flowers); snow-in-summer (6-12 inches; white flowers); soapwort (6 inches; pink flowers); iceplant (3-6 inches; variety of colors); Dalmation bellflower (4-6 inches; light purple flowers).

Several of these plants will spread rapidly, but Adams cautioned gardeners against overexuberance. “Certain friends” must be curtailed to allow the rocks their well deserved moments in the sun.

— Rhonda Nowak is a Rogue Valley gardener, teacher and writer. Email her at rnowak39@gmail.com. For more about rock gardening, visit her blog at http://blogs.esouthernoregon.com/theliterarygardener/.

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