Rock Garden Soils   

By Lori Chips       

With rock gardening being a somewhat specialized activity, it is no surprise that the subject of rock garden soil(s) evokes insecurity on the part of new converts. Although I get numerous questions about this, I instantly know someone is serious when they pull out a pen and paper and want specifics. In my mind, the person is probably ready to dive headlong into the discipline that this sort of gardening actually is. This question though, can never have an absolute or a single response. Interestingly, too, my answer centers not around the plants, or even the site, but on the person asking the question. I need to know exactly what the gardener wants.

The first thing a gardener needs to do is honestly answer this: "Is it a bona fide rock garden I want, filled with its compliment of rare and interesting (and challenging) alpine plants?" If the answer is yes, this will entail preparation, commitment, and maintenance; not to mention embarking on a learning curve that, no matter how engrossing, may prove life (or at least garden) changing. One way to take this leap is keeping it small to start out. A crevice bed might be planned, or even something near a step or behind an existing wall could be tucked in. This is often a very good way to get your feet wet and to figure out if rock gardening is for you.

But most often the gardener who is inquiring has a larger area in mind, and simply because of extant rocks and stones and a sloped space it has occurred to them that this would be a prime spot to place an alpine garden. Let me inject a word of warning here that, setting aside vegetable gardening, rock gardening is probably the most high maintenance sort of gardening there is.

Sometimes what the person is really thinking of is a "rockery effect" in their space. And, this is doable and can be charming. It usually means organizing some picturesque rocks, some gravel mulch (though even this is negotiable for this solution) and opting for smallish scale plants and shrubs, leaning rather heavily towards the lowest maintenance items possible. Creeping junipers, low mounding conifers, the shorter stature perennials, and even a few of the least cranky rock garden plants. If this is truly the ultimate goal then it is best to own it, create it, and enjoy it. But true alpines, in almost all cases, will not thrive here.

Ready for a real rock garden? Here goes.

Alpine plants require full sun and sharp drainage. By full sun we mean a minimum of five hours a day straddling the noon hour. (Six o'clock in the morning until eleven o'clock is not enough.) Your palette of plant choices diminishes rapidly for partial and mostly shady sites. Alpines come from above the timberline; shade plants do not.

Soil preparation to achieve sharp drainage is probably the most crucial part of building your rock garden. There are at least as many recipes for soil as there are alpinists out there, but here is a formula that has proved effective for us.

First, remove all perennial weeds. Use whatever means you like, but don't just tear them out of the ground and think you are ready to plant. The roots of weeds have incredible regenerative powers. Don't underestimate them. If I can, I spend a full year eradicating weed problems in advance of building a rock garden.

Second, dig out at least 12 - 18 inches of the extant soil. Though the above ground portion of that adorable cushion plant will fit comfortably underneath a china teacup, its roots can and will grow for yards. This is simply the nature of plants that come from a harsh environment with stony, lean soil below them. They forage far and wide for nourishment, and just because you have brought them down to sea level and tried to feed them doesn't mean they will be happy. They will most likely die. Rich soil and our high humidity are a deadly combination for mountain plants.

So after you have dug out your soil, replace it with the following scree mixture:

• ¼ by volume, preferably organic purchased pathogen free soil.
• ¾ by volume, ¼ to 3/8 inch gravel. This is sometimes called river run gravel or pea stone and no doubt several other names.

Recognize that this ratio means your mix is mostly gravel. By the way, when purchasing gravel be sure it is not limestone unless you are specializing in a limestone bed for plants that prefer that PH. And, make sure the colors of your gravel (it should have more than one tone, one flat color looks, well, flat.) match up well with the larger rocks and stone you are using. Finally, purchase enough of it to spread a generous three-inch mulch over your entire rock garden after you are finished mixing and shoveling in the scree mix.

Judicious amounts of the following amendments can also be added:

• Fertilizer (anything from Osmocote to Sustane to clean compost.) Use about 1/8 the amount recommended for perennials or shrubs; these plants do not want to be over fed.
• Turface (a clay particle for increasing cation-exchange*)
• Pumice (hard to find & expensive but effective.)
• Small amounts of trace minerals (greensand, Spanish River Carbonatite, glacial flour.)

Mix everything together thoroughly on a tarp or in a wheel barrow and shovel it into place. Do not make the mistake I once made and attempt to mix everything together in situ. In other words, don't dump all the ingredients into the bed and mix it up there. Your shovel will bring up native soil into the mix thus thwarting all your hard work, and will re-establishing the seed bank of weeds you eradicated in the upper layers of your newly-prepared bed.

After adding your new soil to the bed, it is good to allow the bed to settle for a while before planting it. How long? Well, that depends on how patient you are. A couple of weeks and a rainstorm or two will help. Waiting until the following season is better. Given all the gravel you have used it is interesting to note that this is not considered a lean soil for alpines. Straight gravel or pure sand would be.

The next step is arranging attractive stones to your liking. A rule of thumb and common wisdom dictate using the largest stones you can get (and are able to move) and burying them so that two-thirds of the stone is below ground. Always try to place the stones naturally with their widest flattest side down. Use only single-source stone, don't mix up the colors or textures because it will shatter your continuity and will look extremely unnatural. For that reason, it's important to choose stone similar to what is found in your area. Brilliant reds in the midst of local granite in New England will look jarring no matter what your expertise. Obey the "rules of stratification" as much as possible. Look at each piece of stone and determine the predominant angle of strata, then place all your stones with a similar tilt. Without much trouble you will end up with a convincing replica of a natural outcrop. While placing stone consider the placement of any larger "anchor" plants, usually shrubs. Since both of these elements play a dramatic part in the final look, it is good to consider them together.

Next, mulch. That's right, mulch before placing and planting your alpines. If you mulch after planting these little gems two things happen. First you will probably bury the crowns too deeply. Second, you will probably end up with too thin of a layer of mulch. (You want two to three inches.) Finally, (sad experience talking here) slinging shovelfuls of gravel around baby alpines can seriously damage the plants. Incidentally, the gravel mulch is not put in place for simply aesthetic reasons. It is an important cultural element as well. It keeps the vulnerable crowns of the plants dry. It keeps the soil cool. It holds down weeds, or at least makes any that blow in easier to pull, and it helps prevent frost heaving. When all is said and done, it looks pretty sharp, too.

There are many specialty beds that rock gardener's build: pure sand beds, limestone, wall gardens, crevices, bogs and woodland. But the gravel scree is no doubt the classic bed and can be home to many numbers of tiny, choice, and exciting species. You have definitely gotten dirty past the elbows (and knees too no doubt) but you have created a home for some more challenging plants in the process.

* Cation-exchange capacity: in soil science, this is the degree to which a soil can absorb and exchange cations. A cation is a positively charged particle. Particles of soil and organic matter have a negative charge. Minerals can absorb to the negative surfaces and are not easily leached by water. They provide a reserve of nutrients to plant roots. Clay (like Turface) is important in the mix because of the high surface area to volume. Clay offers lots of "parking spaces" for plant food.

Lori Chips © 2012

Dryopteris erythrosora, A Fern for All Seasons   

By Mimi Dekker

By now, all but a handful of ferns in my gardens remain visible above ground. Long gone are the delicate dancing fronds of Adiantum pedatum and the bright green, giant wings of Matteuccia struthiopteris. Athyrium 'Ghost', A. 'Branford Beauty' and A. nipponicum 'Regal Red', each of which add a unique textural and colorful element to my gardens, have all been reduced to brown fluff. But one fern in particular still shines like a beacon on this dreary, winter morning - Dryopteris erythrosora, commonly known as autumn fern.

D. erythrosora 'Brilliance' on a dreary winter day.

Rewind to last spring when Dryopteris erythrosora distinguished itself with its young foliage, ranging in color from bronze to coppery pink, and orange. By early summer, the fronds turned a dark, glossy green, and from time to time new growth emerged for a revitalizing splash of color. By fall, these wonderfully arching and turning fronds were covered with bright red indusial which cover the young sori. As the fronds move and bend in the autumn breeze, so too, were the sori exposed.

Dryopteris erythrosora hails from the woodland hillsides and mountain slopes in Japan, China and Taiwan, where it spreads by short creeping rhizomes. Single plants form a vase-shaped clump and multiples create a colorful, tropical-like groundcover, a tapestry of copper-red, pink and green from spring to late fall. It also works well as a specimen plant or a backdrop for shade-loving, flowering perennials. Its genus name comes from the Greek words drys (tree) and pteris (fern), and from erythros (red) and sora (sori) in reference to its red sori. With no serious pest or disease problems, what's not to like?

D. erythrosora, red sori.

Autumn fern tolerates a drier soil, but is most successful in moist, humus-rich soil, with a pH range of 6.1 to 7.5. Provide morning or late afternoon sun, but avoid mid-day. It is hardy to zone 5 and stays evergreen in frost-free areas. Site them in locations sheltered from strong winds to protect the fronds.