The Gardener Within: Joe Lamp'l, a master gardener and author, shares how to create a hypertufa or concrete container for your garden.
The weathered surface of a stone planter adds Old World character to a garden. However, the high cost and heavy weight of these timeless containers can break your budget and your back. The good news is there is a handsome alternative to natural stone that you can make yourself.
Hypertufa is a mixture of Portland cement and other inexpensive ingredients that can be transformed into lightweight, custom jardinieres that look like they’ve been in the garden for ages.
There are a staggering number of recipes for hypertufa, and many techniques for molding and sculpting containers, from simple boxes to elaborate focal points complete with fountains, waterfalls and even whole miniature landscapes. For this project, we’ll concentrate on a basic hypertufa mixture and a simple box container. After you’ve gained confidence with the process, you’ll find there’s no limit to the simulated stone objects your imagination can come up with.
Hypertufa recipe: Equal parts milled sphagnum peat moss, vermiculite and Portland cement.
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Materials and Tools: Shoebox or other cardboard box; foam packing “peanuts” or Bubble Wrap; masking tape; plastic sheet or painter’s drop cloth; large mixing container; dust mask; rubber gloves; water; craft stick; three large drinking straws; wire brush (optional). And wear gloves and a dust mask and work in a well-ventilated area — this is a dusty project.
To make your own hypertufa container, use whatever size and form you’d like. If your form is soft, such as a cardboard box, pack it with foam packing peanuts or Bubble Wrap to keep it from collapsing under the moisture and weight of the hypertufa mix. If you’re using packing peanuts, avoid the biodegradable ones made from puffed corn. They’ll disintegrate when water soaks through the cardboard.
Mix it up: With rubber gloves on, add 10 cups each of dry Portland cement, vermiculite and milled sphagnum peat moss to the mixing container. Work the ingredients together in their dry state, and then add about 1 gallon of water a little bit at a time. Don’t let the mix get too wet. It’s ready when you can form a ball that sticks together and holds its shape but isn’t crumbly or gooey.
Pack it on: Place the foundation form lid side down on a flat work surface, covered with the plastic painter’s drop cloth. I use a 24- by 24-inch sheet of ¾-inch-thick plywood. Pack handfuls of the mixture on the bottom and sides of the box 1 ½ to 2 inches thick. If the mixture sags and slips off the sides of the box, the mixture is too wet. Add a cup of peat moss to thicken it. If it’s too dry, pat a little water on it to moisten it.
Check the thickness: Mark a craft stick at 1 ½ inches as a measuring tool, and check the thickness of the hypertufa mixture all over the form. Pay special attention to keep the material thickly applied at the corners and edges of the box. Insert the three drinking straws into the bottom to mold drainage holes (you’ll take them out when the container is dry). Cover the container with plastic so it doesn’t dry too fast and place it in a cool, dry place for 24 hours.
Unveil the container: After 24 hours the material should be dry and firm to the touch. Remove the straws from the drainage holes, then carefully and gently turn the container over and tear out the foundation box and peanuts. Use the wire brush to soften any hard edges and add texture to the surface. Let the container continue to dry and cure for about a month in a cool, dry place. Like any concrete project, the longer it sits the harder it becomes. With hypertufa, the lighter it becomes, too.
Remember that the Portland cement in this recipe is alkaline, and has to be leached out before using the container for plants. I leave mine out in the weather for one or two months and let the rain and elements remove the alkaline residue.
Once you have the basics down, try different foundation forms such as plastic containers, metal pots and pans — even terra-cotta pots. Be sure to use a release agent like cooking spray, petroleum jelly, vegetable or mineral oil or WD-40 on these materials, or the hypertufa will literally bond itself to them.
Joe Lamp’l, host of “Growing a Greener World” on PBS, is a master gardener and author. For more information, visit www.joegardener.com.