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/Aquatic Gardens, Design, Construction & Maintenance

Rock Selection & Use

by Robert Fenner

Big Rocks, big weight?

Aquatic Gardens

Ponds, Streams, Waterfalls & Fountains:
Volume 1. Design & Construction
Volume 2. Maintenance, Stocking, Examples

V. 1 Print and eBook on Amazon
V. 2 Print and eBook on Amazon

by Robert (Bob) Fenner

A rock is a rock is a rock; or is it? The answer is no; many types of rock are unsuitable for waterscape use. Some are too soft and gradually dissolve away; many types are chemically toxic to pond life. All rocks are a danger to you, the aqua-scaper, whether they're sharp or not. Their massiveness and lack of handy handles poses a danger in lifting and placement. Happily there are specialized tools and folks who know how to use them for moving rocks of hundreds or thousands of pounds.

What would a water effect be without accompanying waterscaping rocks et al? Plain. The beauty in arrangement of rock and other decor items is a matter of personal taste; I'll give you some suggestions on how to improve yours.

Material Selection: Sources/Considerations

There are two ways to go about aquascaping your aquatic garden; either focus on 1) what materials are readily available, or 2) start with an absolute dream of the size, type, color, texture qualities of the decor you'd like and then set out to acquire or fabricate it.

Did you ever see the movie "The God's Must Be Crazy"? One of the most amazing revelations to me from that show was that there are areas where there are no rocks to be had.

Do you live someplace where the hardest object is a coke bottle? Well, fear not; there are always artificial rocks to be had or made.

Faux Rocks: come in several formats easily defined on their manner of construction and composition. The simplest for the home user/contractor are pre-made CRF's, concrete-reinforced fiberglass (photograph). These are light-weight, naturalistic and quite tough if not physically abused. See the references for water garden, your local rock and gravel (nursery) businesses for sources of phony rocks.

You can also make your own or have someone skilled do it for you. The most primitive technologies for these ersatz rockworks involve making a "cage" of tied (or welded) steel rebar, covered by wire, metal lathe (be careful) and/or canvas bags. Cementatious material is applied to this; mortar by hand for small jobs, gunite, shotcrete, mortar mix by pumper/sprayer for big ones, and worked with hand tools for cut-in and texture. A color finish and additional texturing are added with spray oxides, brushes and possibly the zenith of artificial rockworking.

Artificial rocks are made in several ways, of different materials... These "spray" foam and fiber productions are strong and lightweight... yet still need protection (by careful placement) from kicking and landscape gear damage. 

The Zenith of Artificial Rock-Working: What is it? I'm glad you asked. The most realistic rock look is achieved by copying real rocks. I'm not joshing; you go to a rock face you admire, clean off loose matter, apply a releaser and coat with several layers of latex rubber "paint" (see manufacturer's specific recommendations). This mask dries, is peeled off and used to apply the texture to your phony rocks. It sounds easier than done, but the results can be striking.

Artificial rock art and technology is applicable in all situations. Oftentimes faux rockwork is the most cost-effective or only possibility where there's no access or real rock readily available.

A very nice, large project of not one piece of real rock. These are all CRFs, Concrete Reinforced Fiberglass. 

Real Rock:

Foremost concern should be given to the appropriateness of the rock (wood, shells...) in mind. Is it going to release toxins into your water? All rock should be suspect, even one's collected in existing natural water settings.

The very best test for safe use is a simple bio-assay; a small piece of the rock is boiled, the rock and water allowed to cool and placed in an aquarium or jar with a few inexpensive "feeder" goldfish. If they survive for a good week, the rock is probably okay for your use.

A secondary, but still serious consideration is how sharp the rock is. Some volcanics (especially pumice and obsidian) will cut you and your livestock up pronto. If you use them, keep these rock types out of harms way.

Where can you get information on what's available and has been used in your neighborhood? Call landscape contractors, visit your local nurseries, and check around for a koi, garden, even tropical fish club in your area. Ask the folks there.

The real thing... heavy, hard to move around... possibly toxic... Notice that the placement around the perimeter... on a safety bench


This is the world of smashed fingers, broken toes and bad backs. Please be careful. I have witnessed many casualties from rock mis-handling. Big rocks moved via power tools (skip-loaders, back-hoes, cranes...) and chains, straps are the worst, but "trust no large" object should become your watchword. Keep your hands and person out of the way!

When collecting your own stone, wear thick gloves, use moving carpet or cardboard in your vehicle to stabilize the load and take your time. Even small features may require several (from the Middle-English meaning many) tons of rock. Where in doubt, buy the rock from a commercial concern and have them deliver it. You may never spend money better.

A rock around the size of your coffee table can weigh as much as an automobile; don't try to stop it from falling or rolling. Make preparation in advance of moving for where each rock is to be placed and provide them with a "pad" of carpet, mortar... "And he that rolleth a stone, it will return upon him. Proverbs 26:27.

Placement Beauty Hints:

There are numerous treatises, mainly in Japanese, on how to go about selecting and spatially orienting rocks in a garden. Some highlights: They generally should be arranged in small odd numbered groups (3,5,7), with any shape (e.g. pointiness) or textural (e.g. swirls of color) qualities arranged in a consistent, confluent manner. The rock should however not be of the same size, color, texture...

A pet peeve of mine is how much of the rockwork is apparently exposed. Unless, you have flat-bottomed material, or are making artificial rocks, please bury or place them so they look like they're at least 2/5 underground/water.

Want my best suggestion for becoming a rock-master? Take a hike! No, really, go for a few trips to the outback, preferably by foot, or through "landscape picture books". What do you see? Beautiful rock arrangements, naturally. What about them is attractive to you? That's what you want to approximate in your aquatic garden.

Small Boulders, Gravel...

Keep these things out of the water feature; loose material makes maintenance an impossible nightmare. If you must have the "look" of a "natural" stream bed using these rocks/aggregate, mortar them permanently in, leaving no room around for "gunk" to get in-between and rot.

As per the above beauty hints, bury at least half to 3/5's of the rocky material in a good stiff (colored) mortar bed, wet-sponge off and acid-wash excess mortar away. You'll thank me forever for this advice, or alternatively curse whoever ignores it when you have to manage a system with loose rock/gravel.

Bibliography/Further Reading:

Hall, Kevin. 1990. Rock business solid for landscapers; suppliers leaving no stone unturned. California Landscape Magazine 6,7/90.

Watson, Thomas T. 1979. Geology for the aquarist. Freshwater and Marine Aquarium 6/79.

Aquatic Gardens

Ponds, Streams, Waterfalls & Fountains:
Volume 1. Design & Construction
Volume 2. Maintenance, Stocking, Examples

V. 1 Print and eBook on Amazon
V. 2 Print and eBook on Amazon 

by Robert (Bob) Fenner
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