Alpines in Troughs and Sinks, by John Husbands
In the last issue of The Alpine Gardener I wrote about the various ways of acquiring troughs and sinks: buying the genuine article, buying a reproduction or making your own. What I propose to do in this second part is to proceed to the next logical step, preparing your trough ready for planting and choosing the plants.
THE TROUGH OR SINK
The first thing to consider is where to position your trough. Once it is filled with compost and planted up it will become quite a heavy item and wont be so easy to move, especially if large. The majority of alpine plants do best in an open sunny position away from overhanging tree branches where the drip from rain and melting snow may cause problems. The ideal place would be a south-facing paved or gravelled patio or courtyard. They can make a very attractive feature in such a setting, especially when three or four various shapes and sizes are grouped together. Placing them under the kitchen window or a bay window just a few centimeters away from the brickwork can bring an otherwise stark and bare wall to life when the plants come into flower. It is always advisable to raise your trough from the ground on a plinth. This has the advantage of bringing the plants up to a more convenient height for viewing and maintenance. It keeps the drainage hole clear and they just look better raised up. Walling stone, bricks, breezeblocks or tiles can be used. If you have a hypertufa trough - either cast or a covered glazed sink you can cover the breezeblocks with hypertufa or even cast a number of plinths to match your sink! Matching plinths made of lightweight polymer resin and compressed limestone are available for the replica troughs and can be purchased from many garden centres nowadays.
Your trough will need at least one drainage hole, with a diameter of not less than 3cm. Place a few pieces of broken clay flowerpot or perforated zinc over each drainage hole. Next, cover the bottom of the trough with about 3 cm of coarse drainage material: gravel, broken pots, stone chips or something similar. (If you have a very shallow trough you can perhaps dispense with the drainage material if you have open well-drained compost.) Over the coarse drainage material, place about 3 cm of coarse bark chips, coarse peat siftings or turf with the grass side down. This is to prevent too much of the finer compost being washed down towards the coarse drainage. The next job is to fill the trough to within 3 cm of the top with a suitable compost. What then is a suitable compost for troughs? I have usually gone for a fairly basic mixture as suggested by the late Joe Elliott, in Alpines in Sinks and Troughs.
All quantities are by bulk:
Three parts good quality loam
Two parts moistened sphagnum peat
One part coarse sand
If you use unsterilised garden loam you may unwittingly introduce worms into your trough. A few bags of John Innes Potting Compost No 2 or 3 could be used instead. Nowadays, there are strong environmental arguments not to use peat and, whilst there are now a number of alternatives available to the gardener, such as bark, wood fibre, wood waste, cocoa shell, coconut fibre (coir), refuse derived humus, sewage sludge and recycled landfill, their availability depends on where you live. I now use a fine-grade of composted bark, available in 80 litre bags, but I have to travel quite a distance for it as none of my local garden centres stock it. Whatever you use as a peat alternative, the main criteria should always be that the product should be crumbly, not smelly and with no or few recognisable raw materials. As for sand, I use coarse concreting sand. Make sure that the sand is washed; any reputable builders merchant should be able to supply this. Mix all of your compost ingredients thoroughly, adding a 3-5 mm coarse grit if you want a more open, faster draining compost, or more peat if you intend to grow peat loving plants. When filling the trough with your compost mix, do a little at a time, firming down each 10 cm layer (not forgetting the sides and corners) at a time. Do this layer firming down until you are within 3 cm of the top enough for a good topdressing of stone chippings, slate or gravel. However, do not put the topdressing on until the rocks have been positioned and the plants put in.
A few well-placed rocks not only give a pleasing finish to a trough; they also have a practical purpose. Many alpine plants benefit from having their roots in the cooler conditions under the rocks. Planting close to the north-facing side of a rock will partially shade from the hot summer sun those plants that do not like direct sunlight. Tufa is a wonderful material to use in troughs and a great many alpines will grow and flourish in this medium much better than in any other. It is very soft and porous and holes can be bored in which can be planted a large variety of saxatile plants. These will retain their natural characteristics and find a contentment that would be hard to beat. If
Gentiana verna (Angulosa of hort.)
PHOTO: JOHN HUSBANDS
find that tufa is difficult to obtain, you can easily cast your own rocks of
hypertufa at very little cost and effort. There are of course types of rock
other than tufa that are suitable for troughs: sandstone, Westmorland stone,
slate and indeed many other materials can be used in landscaping. Carefully
selected tree-stumps could replace rocks to give the effect of plants growing
amongst woodland ground litter; this would suit some ericaceous plants and small
woodland bulbs. A recent trend gaining in popularity amongst a number of alpine
growers is crevice gardening, and this is quite adaptable to troughs. This is
done by placing slabs of rock on their ends and then planting the crevices created.
John Pages informative article in The Alpine Gardener (71:257) describes
this in greater detail. Whatever type, shape and size of rock is used it is
important that it is well bedded-in the compost with no air pockets beneath
where ants or woodlice might set up a colony. Use a piece of wood or a trowel
handle to firm the compost all around the under-surface so that there is no
movement. It is much better to use two or three large rocks than several smaller
ones. I believe that around 45% of the surface area of a trough can be taken
up with rock. This may appear to be a lot, but once you have tried a few dummy
runs by moving the rock around and viewing your creative achievement from all
sides you will see how pleasing it looks. The largest rock will look better
if it is placed off centre towards one corner of the
trough and may take up to 30% or more of the surface area. The smaller rock(s) may take up about 15% or 20%. Try to visualise what plants you will be planting in the valley between the rocks, which plant will be suited to the north-facing side of that larger rock and which will look most effective tucked into that far corner to tumble over the side. I always derive immense pleasure and fulfilment from this stage, for I know that I am close to placing my first plant. So let us now move on to choosing the plants!
There is an enormous choice of superb plants suitable for growing in troughs and sinks. Indeed, very many plants grow much better in these than anywhere else in the garden. The more vigorous and fastergrowing alpines should not be allowed a place, as these would soon smother the small, delicate and slowergrowing ones. The following is purely a personal choice of some of my all time favourites that have proved successful and given much pleasure over the years. For me, the most beautiful of all trough dwellers is the Spring Gentian, Gentiana verna, and its many forms. It has the most brilliant blue flowers of any plant. It is not longlived and should be perpetuated by
PHOTO: JOHN HUSBANDS
young plants grown from seed; it is a lime-lover and does best in gritty, humus-rich soil. A complete contrast is the New Zealand G. saxosa,
cup-shaped white flowers have grey-purple veining on the petals. This, too,
is short-lived and should also be kept going from seed; on occasions it will
self-sow. Do try Centaurium scilloides, which has tight tufts of small, rounded,
shiny green leaves and bright rose-pink flowers. It will happily seed itself
about and is not in the least invasive. Morisia monanthos represents a monotypic
genus from Corsica and Sardinia, inhabiting coastal sands. The attractive, dark,
glossy green, fern-like leaves are the perfect foil for the bright golden crucifer
flowers that appear from March to July. Asperula arcadiensis is an exquisite
species from Greece with pink tubular flowers that erupt from the delicate,
woolly, grey foliage. This has been with me since I bought a plant in the early
1970s. I grow it in several troughs as well as in a raised bed.
The smaller-rosetted sempervivums are ideal trough plants. Long ago I planted a few rosettes of Sempervivum arachnoideum in a 3 cm hole in a piece of hypertufa. I
snipped off any flower stalks that the plant attempted to put out and, in 25 years, it had formed a perfect iron-hard dome of just over 16 cm. Whilst the true species
may be small in number the countless hybrids, forms and clones will give the alpine gardener a vast choice of distinct and worthy houseleeks. I feel that it is always best
to go to a nursery and see them in the flesh, so to speak, as some catalogue descriptions do little to describe the beauty and variety of these plants. A few others of distinction are: S. nevadense (from the Sierra Nevada of Spain) with attractive apple-green rosettes, which are tinged red at the tips. S. calcareum Greenii has jade-green
rosettes, each leaf tipped a rich maroon, while the tiny S. minus bears a distinctive purple colour at the base of the leaves. The houseleeks not only revel in holes drilled
in tufa but are equally at home when sandwiched in between rock crevices. Another genus that often thrives in holes and crevices is of course Saxifraga, in
particular the Kabschia and Engleria kinds (two subsections of the section Porphyrion). The many forms of S. oppositifolia will usually put on a great show by completely covering their dark-green foliage with blooms from February to April. One of the finest is Ruth Draper with large rich red flowers. S. x boydii Aretiastrum has robus
sulphur-yellow flowers, with a slight greenish tinge. Both S. wendleboi and S. burseriana Gloria have lovely white flowers. S. x edithae Bridget another very attractive
Saxifraga oppositifolia Ruth Draper
PHOTO: JOHN HUSBANDS
produces tight domes of silvery-blue rosettes and elegant arching heads of rosy-red
flowers, each with a ring of yellow stamens in the centre. Do include some of
cultivars of S. x megasaeflora in your choice, for they are indispensable to
any trough. I have mentioned but a handful of the many saxifrages available
from the specialist nurseries advertising in The Alpine Gardener. Choose campanulas
with caution, as some are too rampant to be let loose in a trough, but there
are a few
notable exceptions. C. lasiocarpa from the Rocky Mountains and Japan is a gem worth trying. I have C. betulifolia growing in the corner of a large trough where it has been for many years but has never attempted to encroach on its neighbours. C. zoysii is a treasure from the eastern and south-eastern Alps and if you can protect it from slugs it is well worth trying it in a hole in tufa. Another outstanding plant for the same location is Physoplexis comosa. Planted several years ago in a trough it has never yet failed to produce its clusters of unusual bottle-shaped flowers. This too is a martyr to slugs and snails which, given the chance, would devour the whole plant in just one sitting!
Dianthus should be chosen with discretion, but some of the smaller ones, such as D. haematocalyx, D. freynii, D. alpinus and D. microlepis are excellent subjects.
Personally, I think that the majority of bulbs should not be planted in troughs with other alpines, with perhaps the exception of really diminutive ones like Narcissus rupicola, N. assoanus and N. rupicola subsp. watieri: none grow more than 15 cm high. The choicer Oxalis are fine, so too Rhodohypoxis. The vast majority of dwarf conifers offered in garden centres will become too big for a trough in as little as three years. If you want the really dwarfest, slowest growing conifers it is always better to seek them out
PHOTO: JOHN HUSBANDS
a nursery which specialises in these plants. They will often be smaller and
more expensive than those offered by a garden centre. Because of their slow
takes many years to produce a good-sized saleable specimen. Some writers state
that no trough is complete without a dwarf conifer, but I do not believe this;
of the fifteen that I have, only three have one. Having said this, the most
popular one for a trough is Juniperus communis Compressa which forms
a dense, very slowgrowing
flame-shaped bushlet and will remain in character for many years. I have Picea Tom Thumb, which after 12 years has a diameter of about 16 cm, while Chamaecyparis obtusa Mariesii has formed a dome of 12 cm in just over eight years. A small cutting of Chamaecyparis lawsoniana Pygmaea Argentea was placed in the corner of one of
the troughs but in eight years grew the size of a football and was carefully moved into one of the raised beds. A few other dwarf shrubs that are suitable trough subjects: Salix x boydii, a choice miniature willow, which in five years will grow approximately 30 cm tall; smaller hebes such as H. tetrasticha, H. Tiny Tot and H. buchanani Minor are slowgrowing enough to remain in a trough for many years; Cotoneaster congestus Nanus will hug the side of a trough with its tiny spoon-shaped evergreen leaves on gnarled stems. Prune it back from where you dont want it to encroach on other plants and it will be an attractive feature for many years.
When the rocks have been positioned and the planting completed we come to the final stage of covering the surface of the compost with stone chippings, (often
referred to as topdressing). Not only do chippings give a pleasing and natural finish to the whole trough but they prevent a certain amount of evaporation from the soil
on hot days. They also prevent soil being splashed onto the foliage of plants during a heavy downpour of rain and discourage the germination of weed seeds. A covering
about 3 cm deep is sufficient. Carefully tuck the chippings under each plant so that none of the foliage is in contact with the soil.
I hope that this article will give some guidance and inspiration to growing alpine plants in troughs and sinks. There are no hard and fast rules. They come in many
shapes and sizes; pump troughs (with a rounded end), octagonal and corner troughs are just some of the shapes. They can be deep or shallow, huge or small. One of the
most attractive I have seen was a shallow trough filled with Sempervivum at Wisley many years ago. The biggest was Joe Elliotts Saxon stone coffin, weighing in at
nearly a ton! A trough can be planted up with just one genus or even one species, Kabschia saxifrages or Cyclamen for example or, as already mentioned, Rhodohypoxis.
You could plant one up with alpines from one continent or region (America, New Zealand, Mediterranean etc.) or of one family (Compositae, Scrophulariaceae,
Leguminosae etc.) I have mentioned only a few of the many plants that are suitable. You will find many others that will grow and flourish much better in a trough than anywhere else in the garden. You will probably not stop at making just one trough; become hooked and before you know it you will have a dozen or more. Your planted
troughs will intrinsically appeal to friends and relatives who will covet your creations and may even persuade you to make one (or more) for them. I am sure you will
derive great pleasure and a justifiable sense of achievement when you stand back and see how appealing your collection can be.
The following five web sites offer a wide range of genuine stone troughs: