If you’re overwintering herbs in containers outdoors, you want your hardy plants to be in hardy pots. For those of us who live in areas where winters are harsh, options narrow for plants that will survive outside; so, too, do choices for containers. Terra-cotta often crumbles by spring, and ceramic containers may crack during freezes. Plastic is generally winter hardy, but it may not suit your sense of garden aesthetics. Whiskey barrels work well, but they may be too big and too heavy to move around. If you’re like me, you make do with what you’ve got, but you’re probably on the lookout for alternative ideas for inexpensive outdoor containers.
While visiting Ithaca, New York, last summer, I saw some stalwart plant containers that intrigued me. They looked like stone, rugged and sturdy. They were fashioned in varying sizes and sometimes unusual shapes, from troughs to flaring bowls. They had an odd and rustic charm, and they fit easily and naturally into garden settings. I was told these containers were home-made, constructed from hypertufa—a mixture of portland cement, peat moss, and vermiculite.
When I got home, I had to try my hand at it, and I soon realized that these simple containers have many virtues to recommend them beyond their homemade charm. They’re easy, practical, inexpensive to make in quantity, and because they’re porous, they drain well. You can make them in sizes and shapes to suit your needs, and they are lighter in weight than they look. They’re fun to make, children enjoy helping, and it can be a good project for winter months if you have an area such as a basement where you don’t mind a little mess. Sometimes the finished product is less than elegant, but the style suits me: I’m more interested in tough and enduring containers than in gracious statements. They will last for many years.
The stony texture of hypertufa is pleasing and appropriate for herb plantings and rock gardens. Creeping thymes, for example, will scramble happily over the sides. Soft green or gray foliage and gaily colored flowers are offset beautifully by the rough texture.
The containers age well and take on a weathered look. Should they break or crack (not likely if you reinforce them when you make them), you can patch them with more hypertufa so effectively that the damage is barely noticeable.
Barbara and Robert Cotts have made many of these pots, and they regularly conduct classes on the technique at Cornell Plantations, the arboretum and botanical gardens at Cornell University. They first saw these handcrafted hypertufa containers in 1986 in England, where they are very popular, Barbara says. Traditionally, the hypertufa is shaped into rectangular troughs 2 to 4 feet long. When the Cottses were last in England in 1993, they noticed them in gardens everywhere.
There are many recipes for hypertufa. Varying the ratio of ingredients results in varying textures, as does substituting fine-textured masons’ sand for the vermiculite. Some people add strengtheners or colorants, and there are any number of techniques for shaping the pots. I asked Barbara to coach me through my first attempt at making a container, as she has tried many variations.
Where winters can be severe, as in Colorado or New York, or for containers that may be moved around or handled, the hypertufa needs to be tough. The Cottses tried embedding a layer of chicken wire in the hypertufa to reinforce it, but found that unwieldy and time-consuming. Eventually, they began adding a concrete strengthener called Fibermesh to the mix. Smaller tabletop hypertufa containers don’t need it, but the larger garden troughs benefit from the added strength.
The Cottses’ recipe follows. Wear old clothes and rubber gloves because this procedure is messy and hard on your hands. The cement throws up a lot of dust, so wearing a painter’s mask is advised. Make the containers in a place where they can sit undisturbed for a couple of weeks; a spot outdoors or in a garage that you can hose down when you’re done is convenient, but in the winter try the basement, as the containers must not freeze while they cure.
• Rubber gloves
• Dust mask
• Wheelbarrow or other large container to mix hypertufa in
• Mold(s): For a trough, use a rectangular box made of styrofoam, wood, or heavy cardboard sturdy enough to support the weight of the hypertufa mixture. For other shapes, use any containers you like, so long as the circumference of the bottom isn’t larger than the top. Recycled plastic pots and rubber dishpans work well.
• Container to hold water
• Lightweight plastic sheets, dry-cleaning bags, or garbage bags, cut open
• Small lengths of wooden dowel 1/4 inch in diameter (or an electric drill with 1/4- to 3/8-inch bit to make drain holes in container)
• Wire brush
• 2 parts portland cement
• 3 parts vermiculite (or perlite)
• 3 parts peat moss, sifted to remove twigs
• Fibermesh, available at many ready-mix cement companies. For the name of a dealer near you, call (615) 892-7243.
1. Measure out the dry ingredients and mix them together in a wheelbarrow or other container. To make a small garden trough, you can use a styrofoam ice chest measuring 9 by 18 by 7 inches as a mold. For a mold that size, you’ll need about 10 quarts of portland cement, 15 quarts each of vermiculite and peat moss, and a big fluffy handful, or about a cup, of loosely packed Fibermesh. (An empty 13-ounce coffee can hold 1 quart.) Mix with your hands, and add enough water—about 3 gallons for the mold mentioned above—to make the mixture wet enough to work but not drippy. It should have the texture of cottage cheese. Place the mold upside down on a sheet of plastic, and cover it with another piece of plastic, tucking the plastic under smoothly at the corners.
2. Starting at the bottom and working up, apply the mix on all sides, 11/2 to 2 inches thick, covering the top of the mold last. Press the hypertufa firmly onto the mold, eliminating any air pockets. Pat it to a smooth surface. Draw a board across the top so that when the planter is turned over it will have a flat bottom. If desired, insert dowels into the mix at the top (which will become the bottom) of the container to provide drainage holes. Rotate or jiggle them every day or two until the pot is partly dried so that they’ll pull out easily.
3. After about four hours, or anytime during the first day or so, wire brush the surface for the desired texture and round the corners with the brush if you want. Cover it loosely with plastic or other material, and for the first several days, keep the surface of the container wet by misting it occasionally. This will ensure that it cures evenly. The more slowly it cures, the tougher it will be. Remove the plastic in about a week (depending on the weather). Let the planter sit uncovered where you made it for four or five days longer, then remove it carefully from the mold.
4. When the pot is thoroughly dry, two to three weeks after you made it, spray it with a hose to remove any extra lime that may have come to the surface. If you did not use dowels, drill several holes in the bottom of the finished container with a masonry bit.
5. After you have wire-brushed the surface, you’ll notice short polypropylene fibers of the Fibermesh sticking out like whiskers all over the outside of the pot. You can burn them off by passing a propane torch or lighter flame rapidly over the surface, or just let the sun and weather take care of them.
• Do you have wet hypertufa mix left over after you’ve made a container? Pat it out flat on a piece of plastic into a pleasing shape at least an inch thick. When dry, set it flush with the ground and it becomes a convenient stepping stone for the garden.
• A good way to make a trough wider or longer than about 2 feet is to construct a two-part mold out of wood or sturdy cardboard. It consists of two bottomless rectangular molds the size of the inner and outer dimensions of the container; your work surface, covered by a plastic sheet, becomes the bottom, and the inner mold supports the sides until the hypertufa hardens. All mold surfaces should be covered with plastic so that they don’t adhere to the cement mix.
• If you want a colored hypertufa container, try adding a small amount of concrete colorant—sold in powder form in red, black, earth tones, green, even yellow—as you mix the dry ingredients together. A little goes a long way, so start with 1/4 cup per batch and adjust the amount to your liking. It will lighten as the container dries.
• Potassium permanganate crystals, obtainable from chemical companies and pharmacies, can be used to speed up the curing process. Mix the crystals with water, then brush the wine-colored solution over the surface of the container. Wait several hours before washing it off, and dispose of the used solution away from plants and animals. Use with care, and follow directions on the package.
• For a cool, shady spot in the garden, you might want a planter that looks like old stone covered with moss, and you can speed the process along. In Gardener’s Day Book, Richardson Wright tells how to hasten the antiquing by washing the hypertufa with a thin solution of molasses or a mixture of milk and stale bread. That will provide a culture medium for algae, which will form a surface that looks mossy.
• Perhaps you have an old porcelain or soapstone sink you’d like to recycle as a planter. Rosemary Verey, in Classic Garden Design, describes her technique for covering a sink with hypertufa. First, she scrubs it to rid it of any grease, and when it is dry she applies a thin coat of bonding adhesive such as Euco Weld. When the coating is thoroughly dry, she applies hypertufa to the surface. A thick layer is not necessary; 1/2 inch will do. She covers the outside of the sink and enough of the inside so that no white will show when the trough is filled with soil. As it weathers, it takes on the look of real stone.
• To repair a cracked or broken trough, brush out all the soil from the area and patch it next time you have a batch of hypertufa mixed. Let it dry before reusing.
• You can mix large quantities of dry ingredients in the proper proportions and store in sturdy trash bags in a dry place until you’re ready.
• The larger the container, generally the better the chance your plants will overwinter successfully outdoors. To prepare planted containers for winter, remove any annuals and fill in the resulting depressions with soil mix. Cut back plants you want to leave in place so that they don’t touch the rim or sides of the planter. When cold weather arrives, cover the plants with Christmas-tree or other evergreen branches for protection. Some winters you may lose a few plants, but you’ll learn what you can rely on to come through the seasons in your climate.
Did you Know? The first commercial application of portland cement was in the making of flowerpots.
As associate editor of The Herb Companion, Kathleen Halloran finds many opportunities to get her hands dirty.