Trough Gardening

A hypertufa trough with an elegant arrangement of plants and rock. Drawing by Rosemary Burnham from Davidsonia, Spring ‘74

A hypertufa trough with an elegant arrangement of plants and rock. Drawing by Rosemary Burnham from Davidsonia, Spring ‘74

Tufa consists largely of vegetable matter through which water containing carbonate of lime has seeped over a period of years, resulting in a light fossilized rock containing small holes and fissures. In this many of the small high alpine plants will grow and flower happily, retaining the dwarf characteristics of the plant in nature. The substitute, Hypertufa, has something in common; suitable plants can be grown successfully in this “rock,” pitted with holes where the peat has decomposed.

F.H. Fisher, in Portraits of Alpine Plants

Miniature plants can be enjoyed more fully, displayed more artistically, and maintained more easily in a miniature garden. And the most appropriate miniature garden for small, choice alpines is one made in a stone trough or sink.

The use of troughs for miniature rock gardens is a recent phenomenon. The idea origi­nated in England during the 1930s with such notable alpine plant growers as Clarence Elliott, proprietor of Six Hills Nursery. He inspired interest in them through journalism, especially his many articles in The London Illustrated News, and through his exhibits at the Chelsea Show of the Royal Horticultural Society. He used old stone feed and water troughs discarded by farmers with the advent of modern plumbing. Stone troughs could be obtained cheaply then, but now such things are rarely seen outside the gardens of alpine plant enthusiasts, where they are assuming the status of heirlooms.

Apart from the fascination of miniature aesthetic creations in general, there are practical reasons for growing plants in troughs. Trough gardening is an ideal way of managing collections of plants having specialized cultural requirements, because suitable exposures for troughs can be chosen, and the soil can be tailored to suit the needs of the group of plants each contains. What is more, troughs are usually raised a foot or two above the ground and rodents are less able to reach the plants. Routine maintenance becomes easier, too, so that even the elderly and infirm can participate in, and derive therapeutic benefit from, the care of these exquisite miniature creations.

Hypertufa trough at the University of British Columbia, Vancouver, displaying plants of the Wenatchee Mountains, especially Lewisia tweedyi. Author’s photograph

Hypertufa trough at the University of British Columbia, Vancouver, displaying plants of the Wenatchee Mountains, especially Lewisia tweedyi. Author’s photograph

Nowadays the inspiration remains intact, but the supply of genuine stone troughs has virtually dried up, particularly in North America, where, in any case, they were never plentiful. But there are alternatives to be found in many nursery sales yards. A container for alpines can be of any manageable length and depth, but should not be too shallow; an internal depth of six to ten inches is about right. Terra cotta containers come in all shapes, and, if glazed, all colors, too. In general though, they are too small and fragile for our purpose. An exception is the so-called strawberry pot, which, if of the large and sturdy kind, makes an excellent home for alpines. Cushion plants in particular — such things as kabschia saxifrages and cushion phloxes — drape such pots to perfection, growing from the many holes around them.

Concrete containers are found in many shapes and sizes. Most useful are those three or four feet in diameter that taper to a smaller base. Of these, the shallower ones are more in scale with alpine plants. Also quite common are long rectangular window boxes in concrete, although these are often too garishly ornamented to provide alpine plants with a comfortable setting.

Containers of wood and plastic, although perfectly serviceable from a horticultural standpoint, are frowned upon by alpine purists with unassailable devotion to real stone. I suggest, however, that if you find a suitable wood or plastic container, you go ahead and use it anyway. (I vividly recall the sight in an Oregon garden of the most magnificent collection of rare and difficult alpine plants that l have ever seen. They were growing and flowering exuberantly in large wooden containers made of railroad ties. Probably these should be described as raised beds rather than containers, though, as they certainly could never be moved.)

There is yet another possibility for growing alpine plants, one that l recommend highly: make your own container from hypertufa. Hypertufa is simply concrete with peat moss added during mixing. The result is a material that is more porous and rock-like, and therefore kindlier to plant roots than ordinary concrete. Hypertufa also lacks much of the weight, and of course the strength, of con­crete.

Sectional diagram of a planted trough showing: A, rocks and plants; B, layer of rock chips; C, prepared soil mix; D, layer of peat roughage; E, layer of fine gravel; F, drainage holes covered with wire mesh; G, hardware cloth reinforcing basket. After a diagram in Davidsonia, Spring ‘74

Sectional diagram of a planted trough showing: A, rocks and plants; B, layer of rock chips; C, prepared soil mix; D, layer of peat roughage; E, layer of fine gravel; F, drainage holes covered with wire mesh; G, hardware cloth reinforcing basket. After a diagram in Davidsonia, Spring ‘74

Making Troughs

The easiest method of making troughs uses cardboard cartons as molds. Two cartons are needed, one each for the outer and inner molds. The smaller, inner carton should be of a size that leaves a space of about two inches all around between the molds. A space less than two inches risks a finished trough too weak to support its load. A basket of half-inch hardware cloth is needed to reinforce the hypertufa. It should be made from a single sheet cut and folded up from the bottom to form four sides and secured with wire at the corners. (Metalworker’s snips, or pliers with a cutting edge, are needed for this.) When finished the wire basket must be two inches wider and longer than the smaller carton so that it lies at the center of the space between the inner and outer cartons. The height of the basket should be two inches less than the intended overall height of the finished trough.

During construction a one-inch layer of hypertufa is first spread over the bottom of the larger carton and the wire basket placed centrally on it. Another inch of mix is spread over the bottom of the basket and the smaller carton is then placed upright and centrally within the basket. Fill the space between the cartons with mix, tamping it down and into the corners as you go. When the full height of the trough is reached the top of the basket should be hidden by about an inch of mix.

Hypertufa

Proportions by volume for hypertufa are one part cement, one of coarse sand, and two of shredded peat moss. Mix these together in a dry state, then gradually add water until all is the consistency of creamed cottage cheese. It should be thin enough to allow tamping it easily into the comers of the mold without leaving air spaces, yet not so wet that it runs all over the place.

This mixture produces a perfectly good stone-like trough, but refinements are possible. One of these is the introduction of lines to resemble stratification. For this thin layers of another, harder, mix are introduced during mold filling. Lines should be even, but may slope a little from end to end. The harder mix is made, without peat, from one part cement and two of sand. Water is added until the mix flows slowly from a can. A little of this is poured over material already in the mold and filling with hypertufa continued. Unless a large trough is being made, one, or at most two, such lines will be sufficient.

Hypertufa trough made using cardboard cartons for molds by the method described here. Saxifraga oppositifolia is the prominent plant; there are also sempervivums and other saxifrages. Author’s photograph

Hypertufa trough made using cardboard cartons for molds by the method described here. Saxifraga oppositifolia is the prominent plant; there are also sempervivums and other saxifrages. Author’s photograph

Cardboard molds will spread outwards as they are filled. To avoid excessive sagging have a few heavy blocks ready to shore up the sides, but don’t worry about the rounding that a sagging mold produces; old troughs were seldom made with precision, Drainage holes are needed, but can be made later with a drill.

Concrete is best allowed to dry slowly, so cover the mold after filling with anything that will keep the sun off it and lessen evaporation. In hot weather spray it with water to slow drying. Let the newly cast trough cure for a day or two, then, while the hypertufa is still green — firm, but not fully hardened — carefully strip away the mold. The cardboard will be damp and come away easily. With a stiff wire brush, gently at first, go over the entire outer surface of the trough, working along the length to give a natural laminate texture that will augment, or serve in place of, the introduced strata lines.

Let the trough cure for another week at least, then fill it with water to which about half a teaspoonful of potassium permanganate crystals has been added. This neutralizes phytotoxic substances in the cement. Let this stand for two or three hours before rinsing the trough with fresh water. After two or three more days of curing a few drainage holes evenly spaced along the bottom of the trough can be made with a half-inch drill.

Cardboard molds, while quick and easy, have drawbacks; they can be used for only one trough, and not a large one. Cartons are not rigid enough for troughs larger than about twenty-four inches on a side. Larger troughs are best made with wooden molds. Wooden molds can be simple or elaborate, depending on the number of times they are expected to be used, but thought must be given in advance to the need for removing the mold while the hypertufa is green and fragile without damage to the trough. Hinges and removable metal corner brackets are useful solutions to the problem. Wood is best coated with oil — old engine oil is fine — to prevent sticking of cement to mold.

To avoid making molds ingenious gardeners have used all kinds of ready-made containers found at yard sales and in hardware stores: plastic buckets, wash-bowls, kitty-litter trays, and almost anything that can be paired to form the necessary space between them. The results are often surprisingly elegant and useful.

Old glazed earthenware sinks have been converted to attractive troughs with a covering of hypertufa, although I have no personal experience with this method. The sink must be coated with adhesive to form a bond with the cement. White glue, such as Wilhold, is brushed over the glazed surface and allowed to become tacky. At this stage a hypertufa mix rather dryer than for molding is pressed onto the surface by hand. (Wear protective waterproof gloves while handling cement mixes.)

Collection of stone troughs at Rodmarton Manor, Gloucestershire. Photograph by George Waters

Collection of stone troughs at Rodmarton Manor, Gloucestershire. Photograph by George Waters

Placing and Planting

Small troughs can be moved about — to give plants winter protection, perhaps, or to ring the changes in a prime garden location. Large troughs may be too heavy to move and must be placed before filling and planting. The placing will depend on the climate and the exposure needed for the kinds of plants the trough will hold. Most alpines thrive with all the sun available in a coastal climate, but in hotter, drier regions will prefer some mid-day shade. Whatever the site, there must be sky above uninterrupted by trees and eaves, and with no possibility of drip from either.

There may be ten to twelve drainage holes in a large trough — drainage is a most important consideration for the health of plants in troughs — and each hole should have a piece of wire screen over it to discourage insects from taking up residence in the trough. With the screens in place, cover the bottom of the trough with an inch of coarse gravel and over that a layer of rough material such as coarse peat or partially decomposed compost. The rough stuff is a kind of filter that prevents the soil mix from moving down and clogging the drainage holes. Shallow troughs may not have room for a rough layer and in them it may be replaced with a layer of fine gravel. The fine gravel may find its way down through the coarse eventually, but adequate drainage will probably be maintained.

Display of planted troughs at Harlow Car. Photograph by George Waters

Display of planted troughs at Harlow Car. Photograph by George Waters

Soil mix for alpines is the subject of endless debate. All manner of exotic substances are advocated, from crushed oyster shells to pulverized yak dung. (Here the alpine purist insists upon the real stuff from Upper Dehra Dun, not the inferior Nepalese product.) Oyster shell is considered a sure fire corrective for the notoriously non-flowering Aquilegia jonesii from the high dolomite screes of Montana, while yak dung is reputed to alleviate debilitating attacks of the vapors in choice Himalayan primulas. Nevertheless, the general run of alpine plants will be at home in a mix of one part each of loam, leaf mold, coarse sand, and granite grit (sold as chicken grit). Good garden soil may be substituted for loam, and if it can be partially sterilized by heating to 180° F for fifteen minutes, so much the better. Leaf mold is hard to find, too, and sphagnum peat moss may replace it, although offering less in the way of nutrients. Add a generous tablespoon of bonemeal to each cubic foot of the mix.

It’s hard to imagine any sort of alpine garden without one or two rock outcroppings. Two or three small pieces of suitably weathered, lichen-encrusted rock collected on mountain hikes can be combined to give the impression of one monolithic outcropping in the trough. While arranging the rocks, consider the disposition of plants and how they will associate with them in natural ways. Asymmetric balance in the scene, with perhaps rocks to one side of the trough and a dwarf conifer or other small shrub opposite, may give a harmonious effect. There is scope for much experiment in this.

Massive natural stone trough at Harlow Car Gardens, Yorkshire, with prostrate and columnar conifers, Linaria alpina, and other plants. Photograph by George Waters

Massive natural stone trough at Harlow Car Gardens, Yorkshire, with prostrate and columnar conifers, Linaria alpina, and other plants. Photograph by George Waters

Plants

In the choice of plants from the hundreds available, remember that not all alpines are suitable for troughs; some would too quickly outgrow their space. Discipline in this matter, and a great deal of added interest, can be found in adopting a planting theme. Themes are especially attractive if several troughs are planted. One trough might contain plants all from one area; another display souvenirs of friends or vacations. Collections of cushion and mat-forming plants are fascinating, such as groups of saxifrages, sedums, drabas, and sempervivums.

A visit to Craters of the Moon National Monument suggested a theme to one enterprising gardener; that lunar landscape in Idaho has large outcroppings of black lava rock and plants with strikingly silvery leaves. The result was a black and silver trough containing plants such as Veronica bombycina, Raoulia australis, Eriogonum ovalifolium, and some of the smaller celmisias. Silver foliage contrasting with black rock is the main attraction in this trough, with flowers in pale blue, yellow, and white as a springtime bonus.

But the best theme for a first trough is probably one that gives year-round interest, particularly if it is placed near a window from which it can be seen from indoors. It would need first a dramatic focal point, possibly a gnarled dwarf pine or juniper to set the scale. The choice of flowering plants makes demands upon the gardener because most alpines flower early in the year, and plants to supply the off season

need to be found. A number of bulbous plants fill the bill, especially autumn and winter flowering crocuses. Among these are Crocus medius, C. ochroleucus, C. Iaevigatus, and C. angustifolius. These have flowers of purple, white, lilac, and yellow, respectively, that appear before the leaves. And, of course, cyclamens are indispensable, especially Cyclamen cilicium, with dainty pink flowers in September and October, and C. coum, C. atkinsii, and their varieties, which begin in November and continue until February. Their flowers are deeper pink and are held above intriguingly marbled leaves. Among the many and varied irises there are those that rise early to greet the spring. Iris danfordiae brightens a mild February with yellow flowers, and I. histrioides with blue and purple. Of the snowdrops we could choose almost any, beginning with the November-flowering Galanthus reginae-olgae and continuing from January into April with G. nivalis and G. elwesii.

Stone troughs in Joe Elliott’s garden in Gloucestershire. Behind the white-flowered Dianthus squarrosus in the foremost trough is the gray-leaved Salix x boydii. Photograph by George Waters

Stone troughs in Joe Elliott’s garden in Gloucestershire. Behind the white-flowered Dianthus squarrosus in the foremost trough is the gray-leaved Salix x boydii. Photograph by George Waters

The choicest herbaceous plants for winter and early spring are, without doubt, the Kabschia saxifrages. These free-flowering cushion plants are the epitome of alpine vegetation. Their flowers of soft pink, yellow, and white rise above dense mounds of leaves on stems scarcely an inch high, and are often so plentiful as to completely hide the foliage. Many cultivars have been produced through hybridization and have names such as Saxifraga ‘Faldonside,’ S. ‘Jenkinsae,’ S. ‘Irvingii; and S. ‘Suendermanii Major,’ all of which are excellent. They are not difficult to grow, but do have one quirk: they absolutely insist on shade from the hottest sun, otherwise they will scorch, turn brown, and die astonishingly quickly.

As planting proceeds, adjust the soil level so that you have space for a one-half to one-inch layer of rock chips on the surface and a final level that remains just below the rim of the trough. Ideally the layer of chips will be of the same kind of rock as the outcrop; they can sometimes be collected at the same site. The rock chips aren’t merely ornamental — giving a natural look to the miniature scene — but help also in retaining moisture in the soil and discouraging insects and weed growth.

Maintenance of the trough garden — weeding, trimming, watering, feeding — differs only in degree from what is needed in any garden. Greater intimacy with the plants is possible, however, and consequently greater awareness of their needs. Less frequent feeding of more dilute fertilizer is generally advisable — say, balanced liquid feed (20-20-20) at half the recommended strength every other week during the growing season.

Trough Plants

This is a representative but by no means exhaustive list of flowering plants chosen to provide a long blooming season. Plants can be selected from this list to provide something in bloom almost year round. Kind of plant: Sh – shrub, Cu – cushion plant, EH – evergreen herb, HP – herbaceous perennial, B – bulbous plant. Height x spread indicated in inches. Position: S – sun, Sh – shade, HSh – half shade.

Aethionema armenum [Sh / 4 x 8 / S / June]
Alyssum serpyllifolium [Sh / 1 x 5 / S / May]
Andromeda polifolia ‘Minima’ [Sh / 2 x 6 / HSh / April]
Androsace carnea [Cu / 3 x 4 / S / April]
A. imbricata [Cu / 1 x 3 / S / April]
A. sempervivoides [EH / 2 x 6 / S / April]
Anemone vernalis [EH / 4 x 6 / S / April]
Aquilegia flabellata [HP / 4 x 6 / S / May]
A. jonesii [HP / 3 x 4 / S / March]
A. laramiensis [HP / 2 x 3 / S / April]
A. scopulorum [HP / 4 x 5 / S / June]
Arabis bryoides var. olympica [Cu / 1 x 3 / S / May]
Arcterica nana [Sh / 2 x 5 / HSh / April]
Armeria caespitosa [Cu / 2 x 6 / S / April]
Astilbe glaberrima [HP / 3 x 6 / HSh / July]
Bellium minutum [HP / 2 x 4 / S / June]
Calceolaria tenella [HP / 2 x 4 / HSh / June]
Campanula morettiana [HP / 2 x 6 / S / July]
Cassiope lycopodioides [Sh / 2 x 6 / S / April]
C. stelleriana [Sh / 3 x 6 / S / May]
Ceanothus prostratus [Sh / 2 x 8 / S / May]
Celmisia sessiliflora [EH / 3 x 6 / S / June]
Chionodoxa luciliae [B / 4 x 2 / S / March]
Crocus angustifolius [B / 3 x 3 / S / January]
C. imperati [B / 3 x 3 / S / February]
C. medius [B / 2 x 3 / S / November]
C. ochroleucus [B / 3 x 3 / S / November]
Cyananthus microphyllus [HP / 3 x 6 / HSh / August]
Cyclamen cilicium [B / 3 x 4 / HSh / September]
C. coum [B / 3 x 4 / HSh / February]
C. orbiculatum var. hiemale [B / 4 x 4 / HSh / January]
Cytisus hirsutus subsp. demissus [Sh / 3 x 6 / S / April]
Daphne cneorum var. pygmaea [Sh / 3 x 6 / S / May]
Dianthus alpinus [Cu / 2 x 6 / S / June]
Douglasia laevigata [Cu / 2 x 6 / S / March]
D. vitaliana [Cu / 1 x 6 / S / April]
Draba rigida [Cu / 2 x 6 / S / April]
Draba dedeana [Cu / 1 x 4 / S / March]
Dryas octopetala ‘Minor’ [Sh / 2 x 6 / S / May]
Edraianthus pumilio [HP / 2 x 6 / S / June]
Eriogonum ovalifolium [EH / 5 x 6 / S / June]
Fritillaria pudica [B / 6 x 2 / S / March]
Gaultheria humifusa [Sh / 2 x 8 / Sh / June]
Galanthus nivalis subsp. reginae-olgae [B / 4 x 2 / S / November]
Genista villarsii [Sh / 2 x 6 / S / June]
Gentiana saxosa [EH / 2 x 6 / HSh / July]
G. verna [EH / 3 x 4 / S / May]
Globularia repens [EH / 1 x 4 / S / June]
Hebe buchananii ‘Nana’ [Sh / 1 x 3 / S / June]
Hypericum reptans [HP / 1 x 8 / S / July]
Iberis saxatilis [EH / 3 x 6 / S / April]
Ilex crenata ‘Mariesii’ [Sh / 8 x 4 / Sh / May]
Iris danfordiae [B / 3 x 3 / S / February]
Lapeirousia laxa [B / 4 x 3 / S / August]
Leiophyllum buxifolium var. hugeri [Sh / 5 x 6 / Sh / May]
Leucojum autumnale [B / 2 x 3 / S / September]
Lewisia columbiana var. rupicola [EH / 4 x 3 / S / May]
L. rediviva [HP / 2 x 4 / S / June]
Muscari azureum [B / 3 x 2 / S / March]
Narcissus romieuxii [B / 3 x 3 / S / January]
N. watieri [B / 4 x 3 / S / March]
Oxalis lobata [B / 3 x 6 / S / September]
Penstemon davidsonii [Sh / 3 x 8 / S / June]
P. rupicola [Sh / 4 x 8 / S / June]
Petrocallis pyrenaica [Cu / 2 x 5 / S / May]
Petrophytum caespitosum [Cu / 2 x 6 / S / July]
Phlox douglasii [Cu / 2 x 8 / S / May]
Phyllodoce nipponica [Sh / 5 x 7 / HSh / May]
Polygonum tenuicaule [HP / 2 x 5 / HSh / April]
Potentilla nitida [HP / 2 x 6 / S / June]
P. farinosa [HP / 2 x 3 / Sh / April]
P. minima [EH / 2 x 4 / S / May]
P. clarkei [EH / 2 x 4 / Sh / April]
Primula x steinii [EH / 1 x 3 / S / May]
Raoulia australis [Cu / 0.5 x 6 / S / June]
Rhododendron impeditum [Sh / 4 x 6 / Sh / April]
R. radicans [Sh / 2 x 6 / Sh / May]
Rhodohypoxis baurii [B / 2 x 5 / S / Summer]
Salix reticulata [Sh / 3 x 6 / HSh / May]
Saponaria ocymoides ‘Rubra’ [EH / 1 x 6 / S / May]
Saxifraga burseriana [Cu / 2 x 6 / HSh / March]
Saxifraga ‘Irvingii‘ [Cu / 1 x 4 / HSh / March]
Saxifraga ‘Myra’ [Cu / 1 x 4 / HSh / March]
Scilla autumnalis [B / 4 x 2 / HSh / September]
Sedum spathulifolium [EH / 3 x 6 / S / June]
Sempervivum arachnoideum [EH / 3 x 6 / S / August]
Silene acaulis [Cu / 2 x 6 / S / May]
Soldanella minima [EH / 2 x 4 / HSh / March]
Talinum okanoganense [HP / 1 x 3 / S / May]
Thalictrum kiusianum [HP / 4 x 6 / HSh / June]
Trillium rivale [B / 2 x 3 / HSh / March]
Veronica bombycina [EH / 3 x 6 / S / May]
Viola verecunda var. yakusimana [HP / 0.5 x 3 / Sh / May]