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How To: Build a Hypertufa Trough
by Gerald Klingaman

Of the various garden specialties, probably none is as diverse and free-form as rock gardening. Rockeries are an attempt to display plants in a naturalistic setting in the kind of habitat they might occupy in the wild. The plants can consist of the almost impossible-to-grow plants of the high Himalayas or dwarf conifers from the American West or succulents from Mexico. Whatever excites you can be grown in a rock garden if a little taste and common sense is employed.

Rock gardens began to emerge during the middle years of the 19th century in Europe as the expanding middle class became affluent enough to do a little sight seeing. One of the favorite tourist destinations was the Alps where visitors fell in love with the flowers of the alpine regions. Just as travelers do today, they brought these plants home and, after a suitable period of plant-sacrifice, learned how to keep these gems of the high country alive in their new lowland homes. One of the tricks learned early on was to ensure plants had really good drainage. Most of these alpine plants hated having their roots immersed in wet, poorly aerated soil.

England and Scotland were pivotal in much of the early rock gardening movement. Many of these new middle class gardeners were the first generation off of the farm, so naturally they had access to things back on the farm. One of the things they began to move to their city homes were the old stone bunkers used to feed livestock. These feed troughs were carved from soft tufa limestone which occurs sporadically in the British Isles. It turned out that these porous limestone containers provided just the kind of superb drainage that finicky alpine plants needed to thrive in a lowland environment.

Natural tufa formations occur across the southern U.S., but large formations are relatively rare. For this reason, natural tufa troughs are not readily available to American gardeners. We have to use our ingenuity to build our own. Being soft, lightweight and easily worked, handmade hypertufa troughs provide excellent places to grow plants that are susceptible to poor drainage conditions or small plants that tend to get lost in the landscape.

Sun Container Photo
Sun Container Photo
STEP ONE - Choose the shape

Hypertufa begins life as a wet cement mix so it can be formed into any shape desired. Troughs can be made in almost any fashion, but for beginners a rectangular cardboard box 12"x16"x7" makes a good starting point. Because the cardboard will absorb moisture from the wet concrete and sag, it must be covered with plastic before use. The covered box should be taped in place so it lays flat. I prefer to form my troughs over the outside of the box because the resultant trough will have a more organic, stone-like appearance. It's actually a little easier to form the trough inside the box, but the trough will be more boxy looking. Elaborate wooden forms can be used for deeper troughs.

STEP TWO - The basic ingredients

A good starting mix for hypertufa troughs is one composed of equal parts of Portland concrete, sphagnum peatmoss and perlite. Portland cement is a finely powdered material and it is used as the raw ingredient in all cement mixes. It is available at all home stores and most hardware stores in 70 or 94 pound bags. The bagged concrete ready-mixes in the familiar yellow bags will not substitute for the Portland cement.

Any good grade of sphagnum peatmoss can be used for troughs. Most peatmoss has a fair amount of roots and debris, which can be removed by running the peatmoss through a course screen. Because peatmoss can soak up so much water from the concrete mix, it is a good idea to premoisten the peat a day or two before mixing.

For the aggregate in our artificial tufa mix, I prefer perlite. Perlite is an expanded siliceous volcanic rock that is used as filler in many artificial potting mixes. It is available from better farm stores and some garden centers. It weighs only 6 pounds per cubic foot and will help keep the containers lightweight. With experimentation, you can substitute other ingredients for the perlite such as vermiculite, volcanic pumice or even sand

Fibermesh, fiberglass fragments used to strengthen concrete, can be added to the mix if your area is subject to severe freezing conditions or if you plan to make the sides of your containers very thin. It is usually added at the rate of 2 to 3 ounces for an average size trough.

Sun Container Photo
STEP THREE - Preparing the mix

Portland cement is a strong desiccant and does bad things to your hands, so you should wear rubber gloves while preparing the mix. For the size of trough mentioned above, mix 2 gallons each of Portland cement, perlite and peatmoss. This will be enough mud if you keep the walls one 1 thick. It is always better to have extra mud, so an extra half gallon of each should be sufficient to ensure you can complete the job.

Blend the ingredients together with a hoe in a wheelbarrow or any suitably sized container. When dry ingredients are thoroughly blended, begin adding the water. It requires approximately as much water as Portland cement, but the actual amount needed will vary depending on the moisture content of the peatmoss. Add no more than one half of the water and then mix the ingredients. Then slowly add a bit more water and remix. Continue this gradual mixing process until the mud has reached the consistency of moist cottage cheese. Allow the mix to stand for five minutes. If it seems to have dried out significantly, add a little additional water to regain the cottage cheese consistency.

STEP FOUR - Forming the concrete

Sun Container Photo Sun Container Photo Sun Container Photo Sun Container Photo

Once the water is added the chemical reaction that leads to the formation of concrete begins. You can safely work this mud for 30 minutes or more if you periodically stir the mix, but don't let the mix sit in the wheelbarrow too long or the finished trough will loose some strength. With the plastic-wrapped form upside down on a solid surface, begin patting the mud onto the form. Continue this process until the box is covered. Watch the corners of the box because these are often thin spots which may become weak places if not enough mud is applied. Forming the mud on the sidewalls higher than 7 inches is a problem. If you want a deeper trough, a wooden form may be the best solution. Balls of mud can be used to make feet as shown. Punch four to six holes for drainage in the bottom of the container.

When the mud is applied, trowel off the sides of the container and cover with a sheet of plastic. The curing process should proceed as slowly as possible. In the summertime work in the shade and spray the curing trough with water after 12 hours. During the winter, make sure curing is done where mud cannot freeze.

Sun Container Photo
Sun Container Photo
STEP FIVE - The curing process

If you've never worked with concrete before, it goes through a series of stages during curing. The first stage is when the mix gives up water, usually within an hour or two after the final time the trough is worked. Free water will appear on the surface of the mix. This is normal and the water will be reabsorbed as curing proceeds. Next the concrete enters the "green" stage when the mix is decidedly green and firm to the touch but still fragile as an eggshell. The green stage will last for 24 to 48 hours, with green gradually giving way to dark gray as the mix becomes hard and firm. As the curing process continues, dark gray gives way to lighter shades of gray. Concrete continues to cure for several months after mixing, but most of the strength will occur within a week of mixing.

STEP SIX - Texturizing the trough

Before the trough becomes too hard, the outer surface must be worked to create a distressed, stone-like appearance. A wire brush is the best tool for this operation. Judging when to texturize is an important decision. Too soon and the green trough may break apart; too late and the concrete will have firmed up too much. I usually texturize after 24 hours, but it may take additional time in the winter. Brush the green trough thoroughly with the strokes running horizontally. When the sides are done, carefully turn the trough over and do the upper surface of the trough. The inside of the trough does not need texturizing. In fact, I usually leave the cardboard box in place for at least a week after building the trough to ensure it is sufficiently hard to withstand the stress and strain of extracting the form. Use a screwdriver to create creases and gouges in the surface of the trough. If you used Fibermesh in the mix for reinforcing, strands of the fiberglass will be visible on the surface of the mix. You can remove them by passing a propane torch over the surface of the trough, just like singeing pin feathers on a chicken.

When the trough is texturized, place it in an area that does not freeze for at least another week to ensure curing is complete. Then, move the trough outside and allow rainfall and exposure to the elements to finish the curing process. After a month or two outside, the box is ready for planting.

Sun Container Photo Sun Container Photo
A version of this article appeared in print in Arkansas Gardener Volume III Issue I.
Posted April 2015


Gerald Klingaman, Ph.D., is an emeritus professor of horticulture at the University of Arkansas. He is now working full time as Operations Director at the Botanical Garden of the Ozarks. He gardens in Fayetteville.



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