DIY Natural Pond

Promote Microorganism Growth

Insects, frogs, fish and other living creatures add organic matter to a pond, as do pond plants and nearby shrubs and trees. This organic material would build up and overwhelm the water were it not for the pond’s secret weapon: microorganisms. These organisms are everywhere — in soil and on rocks and plants — and they feed on organic matter. The more surfaces there are to support microbes, the cleaner the water will be.

To use microbes to your advantage in your DIY pond, plan for half of its surface area to consist of a shallow ledge, about 8 inches deep, around the pool’s perimeter. Place a ring of large stones along the inside edge of the ledge, closest to the deeper water. Then, position pebbles (about a half-inch in diameter) on the rest of the ledge. The larger stones will prevent the smaller ones from rolling to the bottom of the pond. The surfaces of the small stones will be the perfect places for microorganisms to grow and become your pond’s filter. In no time at all, the stones will become slimy, demonstrating that microorganisms are prospering and cleaning the water.

Control Nutrient Levels with Water Garden Plants

If nutrient levels become too high, algae can quickly take over a pond and choke out everything else. The secret to maintaining an algae-free pond without using a pump, filter and chemicals is to control nutrient levels — and the best way to do so is to have lots of plants growing in the water to out-compete the algae.

To create a balanced environment in your DIY pond, plant the ledge with “marginal” water plants that will thrive in the bog-like conditions at the edge, or margin, of a water garden. You want the marginal plants to take in nutrients from pond water, so don’t plant them in soil or pots. Simply set them among the small stones on the ledge, and secure the roots with a larger stone. The plants’ new roots will soon anchor them to the ledge. Good plants for shallow water include arrow arum, pickerel rush, water iris and cattail. Don’t introduce the highly invasive yellow flag iris or any other plant that’s a problem in your area.

Reduce Algae Growth by Limiting Light

An additional way to inhibit algae is to keep the water’s internal light levels low by covering about 50 percent of its surface area with plants. Water lilies are perfect for this purpose because their leaves provide a lot of shade, and their roots take up excess nutrients that algae would otherwise thrive on. For small ponds, select water lilies with smaller leaves, because big varieties can quickly overpower an area.

Different types of water lilies prefer different water depths, but most will be happy in the deepest part of your pond, either sitting directly on the bottom or raised up in pots. These plants don’t need soil, so plant water lily roots in pots filled only with stones. Simply lower the pots into the pond, and nature will do the rest.

Tips for Backyard Ponds and Garden Water Features

The size of your natural pond won’t be critical to its success, but you’ll probably wish you’d made it bigger. A depth of at least 3 feet will help fish overwinter in cold climates (Zones 4 to 6), while 2 feet will be sufficient in warmer areas.

Consider including a shallow beach-like area with a sand or pebble bottom along the pond’s edge. This will make it easier for wildlife to take a drink and provide a way for an animal to escape if it falls into the water. Plus, a beach will make the pond even more visually appealing.

Not all ponds require a liner. How can you learn whether you’ll need one? Dig a trial hole and fill it with water. If the water stays at an acceptable level in the hole for several days, you don’t need a liner. The design of a natural garden pond with or without a liner will be exactly the same.

If you do need to use a pond liner, usually made of plastic or rubber, you should always add an underlayment to protect against punctures. I prefer synthetic carpet squares or scraps because they’re thicker than flimsy commercial underlayments and will last just as long — and you can get them free. Consider an overlayment, too. Virtually no experts recommend using an overlayment, even though you could easily damage the liner by stepping on one of the stones that ring your pond. I recommend adding an overlay of carpet wherever you plan to place large stones — on the planting ledge, for example. This will protect the liner from both above and below.

Frogs usually appear soon after the pond is filled, and will take care of mosquitoes. If you live in an area without frogs, consider adding fish to control the mosquito population. Koi are beautiful fish, but they dig up plants and produce a lot of waste. Goldfish are a better option for a natural backyard pond. I add a dozen goldfish to my pond each spring if the previous tenants didn’t make it through winter. As a rule of thumb, add one goldfish for every 10 square feet of water surface area. Don’t feed the fish, because you want them to eat algae and other pond life.

A pond’s sides can collapse if the soil around the edge and under the liner gets too wet. What will happen if a heavy rain over-fills the pond, or if you forget to turn off the garden hose while filling it? Design the pond with a specific low spot to allow for overflow, and add some extra liner in that area to direct the water where you want it to go.

Online sources and pond books are full of advice on maintaining correct water chemistry. Ignore them! Don’t adjust pH or nitrate levels, and don’t add chemicals for algae control. Natural ponds don’t need any of this.

A Natural Pond Theory That Holds Water

Seven years ago, I tested my theories by building a pond measuring 20 by 30 feet. Today, water plants thrive on a ledge that covers about half the pond, and water lilies cover most of the remaining surface area. There’s no pump or filter. Wind and jumping frogs are the only things that move the surface of the water. I’ve never cleaned out the bottom of the pond, and I’ve never changed the water (although I do top it off once a year to compensate for a lack of rain).

In the early years, as my plants were getting established, I did have string algae, but it declined as the water plants filled in. For the past two years, my pond has been virtually algae-free. In fact, the water is so clear that I can see 4 feet deep, to the bottom of the deepest section.

Frogs breed like crazy in my backyard pond. Deer, raccoons and an opossum drop by for regular drinks. Dragonflies and all kinds of insects love the pond. Two types of native bulrushes have seeded themselves on the planting ledge. Plants and microorganisms provide all the maintenance and filtering. My natural pond is living proof that you can build a low-maintenance, less expensive, wildlife-friendly water feature on your property.

Secrets to a Successful Natural Pond

1. Keep nutrient levels low in your natural backyard pond by placing lots of small stones and pebbles on the ledge to provide surfaces to support microbes, which will break down organic matter.
2. Include plenty of water plants on the ledge to feed on decaying organic matter. Grow water lilies to shade the surface and inhibit algae.
3. Keep the fish population low and don’t add fish food.
4. Don’t clean the rocks or the liner.
5. Sit back and relax while Mother Nature does the work.

Best Water Garden Plants for Natural Ponds

• Arrow arum (Peltandra virginica)
• Broadleaf arrowhead (Sagittaria latifolia)
• Bulrush (Scirpus)
• Cattail (Typha angustifolia and T. latifolia)

• Corkscrew rush (Juncus effuses spiralis)
• Hardy water canna (Thalia dealbata)
• Pickerel rush (Pontederia cordata)
• Umbrella plant (Darmera peltata)

• Water iris (Iris laevigata and I. hexagonae)

• Water lily (Nymphaea)

Resources

The Water Garden covers the basics of pond planning and construction.

Robert Pavlis is a plantaholic who’s been gardening in southern Ontario for more than 30 years. A background in biochemistry has helped him sleuth out gardening facts, which he promotes on his website, Garden Myths.