IF YOU think of a dishpan made of stone you will see that it would make an attractive container for growing a collection of houseleeks, sedums, thymes or, for that matter, alpine plants that require flawless drainage and freedom from the rambunctious weeds of the open garden.
In some places, in the past, it was possible to buy old sinks made of stone for a dollar or so and these were wonderful, I am told - and do not doubt it.
But today we shall consider particular mixes of concrete that serve well for making one's own "stone" sink, or trough. I have two recipes here, one from Michael Jefferson-Brown's "Small Garden Design," (John Gifford, Ltd., London) and the other from the Delaware Valley Chapter of the American Rock Garden Society.
Jefferson-Brown considers a thing too rarely considered: how to convert old glazed sinks or hand basins into stone. Structurally such sinks are fine as they are, but not many gardeners wish to have an assortment of white glazed sinks sitting about the terrace. He recommends a good bonding agent (suppliers of concrete sell them, the same firms that offer special concrete finishes and additives) to cover the glazed sink.
While it is tacky he applies a coating of "hypertufa," patted on with the hands, as much as will adhere without falling off.
He recommends, for the hypertufa mix, one part portland cement, one part sharp sand and two parts sifted sphagnum peat. (Which is not the same as peat moss, by the way.)
The stuff is mixed dry, and mixed by hand very thoroughly, then enough water is added so the mixture will stick to the sink without running off.
"The final effect," he says, "is a very approximation to one of the old stone troughs hewn out of solid rock; indeed, they can easily be taken for the original stone product."
Now the second, or Delaware Valley approach, seems to me even better, since it does not contemplate a supply of old sinks or tubs, but instead you start from scratch.
As I saw this method demonstrated at the Philadelphia Flower Show, one begins with a 24-inch square plywood board (simply as a work surface).
On it you set two or three cans, such as tomatoes come in. It is not critical what size the cans are, but something squattier than large grapefruit juice cans. You want the stone trough to be perhaps six to eight inches deep, and the cna are simply part of the form over which you mould your concrete.
With the can sitting on the board, the cans touching, and with the open end of the can on the board, you pack damp sand around the cans.
Think of three squatty smokestacks covered with alva from a nearby volcano. The cans are completely covered by the sand, so you have a small rounded hill of sand sitting on your plywood board. The only reason you use cans in the first place is to give strength to the mound of damp sand.
This little hill of sand, then, slopes down from the cans, and covers the tops of the cans no more than half an inch.
Over this little hill of damp sand you lay a plastic bag such as cleaners use over suits and dresses. I assume any light plastic would do, since its only purpose is to keep the concrete (which you are going to apply over your little hill) from sticking to the sand.
Now over this plastic (which follows the contours of your sand hill) you use a layer of chicken wire, either inch or inch-and-a-half mesh. You cut a flat sheet of the mesh roughly the size to cover the hill, and gently shape it to fit flat against the plastic which covers the sand.
Then remove the mesh, keeping it ready for use later.
Now that your sand mold is resting on the board, with the cans inside for structural strength, and the plastic all over the mound to keep your concrete from sticking to the sand hill, and with your wire mesh reinforcing sheet ready for the moment you need it - with all things ready, you now mix your concrete.
Mix one-and-a-half parts of sphagnum peat moss (peat moss, notice, it does not have to be shredded sphagnum, but just ordinary sphagnum peat moss such as you buy in bales or bags at garden shops, hardware or grocery stores) with one-and-a-half parts horticultural perlite.
Perlite is the granular light-weight stuff you have often noticed in potted plants from greenhouses. Its purpose in potting soil is to assure good texture and drainage, but its purpose in your concrete is to take the place of crushed stone, which you would use if you were making ordinary concrete.
The peat moss should be neither bone dry nor especially wet. If it is saturated with water then squeezed out by hand and left a day or two to dry, it should be about right. Or if you have some outdoors where the weather has got to it (many gardeners keep a supply outdoors the year through) you will probably find it moist but not really wet, and that will be fine.
You mix your dry but not bone-dry peat moss with your dry perlite, so that the mixture is uniform, with the perlite equally distributed through the peat moss. You then mix dry portland cement (remember one scoop of portland cement to one-and-a-half scoops meat moss and one-and-a-half scoops perlite is the formula, and do not alter the proportions) with the dry peat-perlite mixture.
Again, mix it all thoroughly.
Add water and mix until it is the consistency of "slush." Most people add too much water to any concrete they make. I myself always add too little water. It is a question of temperament, probably. If you add the water gradually, you will avoid getting your mix too sloppy and runny. You want it firmer than gumbo, but not so firm as bread dough.
You want it so it will not all run off when you apply it to your sand hill covered with plastic, but you do not want it so dry that it crumbles or falls off in lumps.
When you think it is right, apply it one-and-a-half inches think all over your plastic-covered sand hill. Then set the wire-mesh form that you made earlier right on the wet cement mixture.
Then the little concrete shell you have made will have walls two inches thick, with the layer of wire mesh half an inch in from the outside.
You will be pleased to see that your plastic-covered sand hill does not start to collapse or change its shape, when you apply the wet cement because the tin cans inside the sand hill give it strength.
Your let the thing set for 48 yours, after which you will be delighted to see that by tugging on the plastic sheet, it comes off nicely from your concrete. At this point you have a sort of large bowl with concrete walls two inches thick. This is the time (you have let it set 48 hours) to make gently three or four drainage holes in the bottom, using a screwdriver or a star drill. You gently tap these holes, tapping the screwdriver with a hammer. Needless to say, do not see how quickly or fiercely you can bang through.
If you wish, you can go over the surface with a wire brush if you think your new stone sink looks too smooth.
Let it sit, empty, outside for a month. The Rock Garden Society says six months, so that the free lime of the cement has time to leach out.
But I would not wast all that time, and would fill the vessel with water, let it sit five days, empty it, let it sit another five days full of water, empty it, scrub it with full-strength vinegar (the sort used in making salad dressing) and rinse it out. Then I would go ahead and plant things in it.
You will be surprised, possibility, at the splendid appearance of your work. Creeping thymes (there are many sorts, some with yellow leaves, some with gray, some with green) look fine in such a container, and so do any number of small sedums or hens and chickens. Or Corsican mint or alpince pinks or whateve you like, except it should be rather small and of course you should use the appropriate soil. Ordinary garden dirt made rather gritty by the addition of plain sand will serve well for thymes and sedums.
In the winter the earth will freeze solid. I would be cautious, therefore, of entrusting any rare little bulbs to the stone sink unless it were protected during the winter with a deep mulch or something.
Suppose you have a wide shallow metal bowl that you use for fruit - you could turn it upside down, cover it with the sheet of plastic, and use it in place of the sand hill. Ingenious gardeners will think of other things. Not t oo ingenious, I hope.