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How to make a simple hypertufa trough
By Brien Darby, Greenhouse Production Assistant on Dec 2, 2011
From time to time, the greenhouse team at Denver Botanic Gardens will build hypertufa troughs. These troughs are a great addition to a garden, especially for showcasing some of the rock garden plants, native wildflowers, and cacti that might otherwise be lost in a larger landscape. We sell our planted troughs at the Spring and Fall Plant Sales and occasionally throughout the season at the Shop at the Gardens. However, if you are interested in making your own hypertufa troughs, I would like to share with you our process and recipe for making a simple hypertufa trough.
For this project, you will need the following supplies:
- Portland cement (either white or gray--if you are using a dye, the color of the cement will effect the dye)
- Sphagnum peat moss
- Concrete dye
- Synthetic concrete reinforcement fibers
- A plastic mold, such as a large bowl, a cat litter tray or a dish pan
- 1mm or thicker plastic sheeting (this can be a thick trash bag, a painting drop cloth, etc.)
Where to get supplies
In the Denver area, most of these supplies are readily available. The peat moss and vermiculite can be found at most garden supply centers or at a large retail horticulture products supplier. The Portland cement, concrete dye, and synthetic reinforcement fibers can be found at any specialty concrete supply store.
The task that will take you the longest in your trough making endeavor is the materials prep. The cement and vermiculite can be used as is. It is advised that the peat moss be sieved to remove large particulates and to provide a smaller grain material. However, if your goal is a coarser appearance, the sieving is not necessary.
The materials that take the most prep time are the synthetic fibers. When they are purchased, the fibers have the appearance of clumps of white strings. The fibers are added for stability and need to be consistent throughout the batch. To obtain this consistency, the fibers need to be ‘fluffed’ before they can be added to the mixture. This can be done by rubbing the fibers between your fingers until they take on the appearance of a pile of cat hair. It can be quite time consuming, but definitely worth the effort.
- Dust mask
- Rubber gloves
- Wire brush
- Propane torch
- A bucket or container for measuring your dry ingredients
- Wheelbarrow or suitable container for mixing the hypertufa
Mixing the hypertufa
Once you have obtained and prepped your materials, it is time to mix the dry ingredients. It is advisable to always wear a dust mask and rubber gloves when working with Portland cement. We have experimented with several recipes; the following recipe is our favorite for strength and appearance:
- 2 parts Portland cement
- 3 parts vermiculite
- 3 parts peat moss
- 1-2 cups of dye (depending on the color you are hoping to achieve)
- 3-4 cups of ‘fluffed’ synthetic fibers (this will translate to about a 1/4 cup unfluffed fibers). These fibers will not be added to the dry mixture; rather, they will be mixed in as you are adding water.
The size of the trough (or troughs) you are hoping to obtain will determine the amount of hypertufa you mix. If your goal is to make one small trough, the bucket you are using to measure out your ‘parts’ should reflect this size.
Once you have mixed up your dry ingredients in the wheelbarrow/mixing container, it is time to add water. It is important to only add a little bit of water at a time; if your mixture becomes too wet, the end result will not resemble hypertufa.
If you have a partner to aid in this process, the extra set of hands will be very helpful. While one person is using a shovel to turn the mixture, the other person can be incrementally adding water and synthetic fibers.
This is also a good time to make any adjustments to the color of the mixture by adding more dye, if necessary. The desired result is a mixture that when squeezed in your hand both holds its shape and releases just a few drops of water. If you squeeze the mixture and it feels squishy or you can visibly see a lot of water forcing out, you have added too much water.
Filling the forms
Now that your mixture is ready to go, it is time to start making the trough by adding the mixture to the form (the plastic container) that you have chosen. As you will have to cover the finished project later with the plastic sheeting, it is a good idea to lay the sheeting down before you begin the molding process. As this is also a bit of a messy project, the sheeting will protect whatever surface you are working on.
In general, regardless of the size of your trough, you want the walls and bottom to be between 1 ½-2 inches thick. Begin by adding shovelfuls of mixture to your form. This mixture needs to be compacted (to form the base) either by pushing with your hands or using a block of wood to push it down. If you do not compact the hypertufa, as it dries it will form holes in the trough and will generally lose stability.
As you are forming the base, start working your way of the side walls of the form. If the walls of your form are somewhat steep, it may be difficult to compact the mixture against the form. If this is the case, you can try compacting the mixture in your hands and then apply it to the inside of the form.
As you continue to build the walls of the trough, remember to keep the thickness consistent. It is very important that you make a drain hole in the bottom of your trough. If the trough is larger, you may want to make two or three holes. This is the best time to make the hole; if you forget, it will be necessary to drill it out once the trough has dried.
Once you have molded the hypertufa in to the form and you are happy with the way it looks, it is time to begin the curing process. This is a two stage process; the first part lasts between 24-48 hours and the longer curing should take about four weeks.
Immediately after finishing the molding process, you will cover the trough with the plastic sheeting. Between 24 and 48 hours you will remove the trough from the form and leave it under the plastic sheeting. When to remove the trough is determined by the hardness of the mixture. If you can scratch it with your fingernail, it probably needs about 12-20 more hours. If you need a screwdriver to scratch the surface, this is about the right hardness. In general, smaller troughs take longer to cure than larger ones.
To get the desired appearance for your trough, this is the time when you would use the wire brush to rough up the outer surfaces of your trough. Most likely the plastic form will have left the trough looking shiny and smooth. Typically, hypertufa troughs have a rough and more weathered appearance. The wire brush will help you alter the texture to your liking.
To make the trough as strong as possible, the longer curing method is recommended. Once you have removed the trough from its form and altered the texture, place the trough back under the plastic and keep it at room temperature for four weeks. You will want to occasionally check the trough to make sure it is not drying out. If it feels dry, moisten it with water.
After the curing process, if all goes well, you are ready to plant your trough! Remember to use well-draining soils and chose plants that are suitable for troughs.