Georgia Gardener Newsletter

Georgia Gardener Newsletter Design Tip: September 4, 2008

Rock Gardens

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From rain gardens to handle runoff in the September 21 newsletter to rock gardens for dry conditions, we are covering the full spectrum of specialty gardens that can handle just about anything that Mother Nature throws at us. Even with a return to normal or abundant rainfall, a carefully designed and built rock garden will have no trouble in handling the weather.

There are a few criteria that need to be established for a successful and attractive rock garden. The first is a slope or changes in grade. I've never seen a rock garden reach its full potential on flat ground. The amount of slope or grade change needed is minimal and can be accomplished by using medium to large stones to create mini terraces. If the rock garden is to be somewhat small, the height needed may only be a foot or two.

The second of course is the rock. Try to use rocks of varying sizes from boulders that may require some equipment to place to manageable rocks that you can set yourself. Larger rocks should be dug slightly into the soil or slope so it looks as though they've been there since the dawn of time and didn't just drop from the sky. Rocks should also be grouped to appear as natural as possible and not contrived. When selecting rock, look for those that have character such as mosses or lichens, surface cracks and variations in color. Choosing rocks for a rock garden is as personal as choosing a Christmas tree. Even if you plan to hire someone to do the installation, I highly recommend you choose the larger specimen rocks yourself. The rocks that are smaller (20-50 lb) are often purchased in bulk by weight.

As you're placing the rocks, work on replacing the soil. Compacted clay will not work as the major soil in a rock garden. It's important to add organic matter (compost or soil conditioner) and coarse sand or crushed granite to the clay since most of the plants used will require good drainage. I would recommend that your mixture be about 1/3 of each clay, organic matter and sand/crushed granite.


Succulents & Evergreens	Mixed Conifers

Plants for the Rock Garden

The majority of this article will focus on rock gardens that are in full sun. I'll have some suggestions at the bottom for those in shade. It's important that when choosing plants, you select those that have similar sun and soil requirements. Most of these plants are going to be at the top of most drought-tolerant lists and will consist mainly of conifers, evergreen groundcovers, succulents, perennials, grasses and herbs.

Placement of the plants should be to soften but not hide the carefully chosen and placed rocks. Therefore, I like to keep the majority of the plants low and to work with those that have a flowing or spilling habit. Taller upright plants should be spaced as specimens and should never take your eyes away from some of your larger boulders.

Conifers

Despite a lingering drought, our climate still receives significantly more rainfall than other parts of the country. Couple this with our heat and humidity and we have to make our conifer choices carefully. Some of the best conifers for our climate include the junipers (Juniperus spp.), false cypresses (Chamaecyparis spp.), cedars (Cedrus spp.), pines ( Pinus spp.), yews (Cephalotaxus spp.) and arborvitaes (Thuja spp.). Many of the plants have dwarf, weeping or even creeping cultivars.


Chamaecyparis obtusa 'Crippsii'	Cedrus atlantica 'Glauca'

Groundcovers

One of the most basic rock garden styles is to simply have an attractive arrangement of mixed sized boulders with groundcovers winding their way through the open spaces. Therefore, I usually like to recommend evergreen groundcovers for this type of garden. We have a wide variety of choices in hardy evergreen groundcovers. These include iceplant (Delosperma cooperii), pinks (Dianthus spp.), thrift (Phlox subulata), creeping sedum (Sedum spp.), creeping thyme (Thymus praecox) and creeping rosemary (Rosmarinus officinalis 'Prostratus').


Iceplant	Creeping Sedum

Succulents

Succulents in rock gardens bring back images of the southwestern U.S. However, there are many succulents that do well in our climate and don't necessarily resemble cacti. One of the exciting new items of recent years has been the development of winter hardy agave, some that can handle temperatures down into single digits. Other succulents to try in rock gardens would be sedums (Sedum spp.), prickly pear (Opuntia spp.), and hens and chicks (Sempervivum tectorum).


Blue Agave	Autumn Joy Sedum

Perennials

Perennials for rock gardens need to pretty tough in order to survive the radiant heat that will be generated from the nearby rocks as well as tolerate the fast draining soil. Some good candidates include perennial sages (Salvia spp.), wormwood (Artemisia spp.), Texas skullcap (Scutellaria suffrutescens), yuccas (Yucca spp.), rattlesnake master (Eryngium yuccifolium), blanket flower (Gaillardia spp.), blazing star (Liatris spicata), ornamental garlics (Allium spp.) and lily-of-the-Nile (Agapanthus spp.).


Hot Lips Salvia	Silver Mound Artemisia

Grasses

I recommend caution when choosing grasses for your rock garden. As a group, grasses are colonizers and can be aggressive if not invasive in nature. Some of the worst non-native invasive plants in this country are grasses. To play it safe, it may be wise to choose grasses native to your area or to simply skip this group of plants altogether. For the southeastern U.S. muhly grass (Muhlenbergia capillaris), panic grass (Panicum virgatum), purple lovegrass (Eragrostis spectabilis), little bluestem (Schizachyrium scoparium) and prairie dropseed (Sporobolus heterolepis) would be good choices.


Muhly Grass	Panic Grass

Herbs

Herbs whether grown for ornamental purposes or for cooking are an excellent choice for the rock garden. Their tolerance of sun, heat and dry soil is legendary. If you plan to use culinary herbs, make sure you plant them in an accessible location. You don't want to have to walk through the rock garden to harvest them. Some excellent choices are rosemary (Rosmarinus spp.) lavender (Lavandula spp.), thyme (Thymus spp.), hardy bronze fennel (Foeniculum vulgare) and chives (Allium schoenoprasum) to name a few.


Lavender	Rosemary

Rock Gardens in Shade

If you want to try your hand at a rock garden in shade, the installation and preparation for the rocks and soil will be the same. Your plant choices will be different and should focus on plants that are known to tolerate dry shade: prostrate yew (Cephalotaxus harringtonia 'Prostrata'), hellebores (Helleborus spp.), autumn fern (Dryopteris erythrosora), certain mosses and lichens, green n gold (Chrysogonum virginianum 'Eco-Lacquered Spider'), etc.