Choosing and Installing Substrates

Last time I discussed how I chose a location, planned, and installed the container for the pond I recently built at the Biosphere II.  My pond is designed to be low maintenance and attract aquatic insects, so the things that I chose to put into the pond were important – different insects look for different things when choosing a home.  Today, I’m going to go over the things I chose and why I chose them.

There were two main things that I wanted to balance in my pond: diversity of insects and the quality of the water.  Many insects depend on a particular water quality to survive, but the pond is also an educational display.  Being able to see into the water is a good thing if you’re trying to convince people to look for the bugs!  I also wanted to provide many different objects in the water to attract as many different types of insects as possible.  Choosing the right plants and ornamental/landscaping features are important if these are your goals.

I started by adding a tick layer of pea gravel, small cobble, and large cobbles to the bottom of the pond (large cobbles not pictured):

Many insects like to sit on the bottom of ponds and want things to hold onto.  There are several species of insects that I find only on rocks, so I wanted to make sure there were plenty available.  By including three different sizes of rocks, I created 3 different habitat types that might attract three or more different types of insects.  These rocks were also very dusty and I knew I would end up with a layer of fine sand or silt at the bottom of the pond once it was filled, creating yet another space in which some insects will live.

Next, I added the cinder blocks you see in the image.  These were installed to provide a platform onto which I could later put my plants.  I also needed to create a space that would be relatively silt-free in which I could put the pump that would circulate the water.  I’ll talk about the pump in my next post.

Many people install a layer of garden soil on the bottom of their ponds so that they can plant plants directly into the pond.  While this most accurately imitates a real pond, I didn’t want to do this for one important reason: adding a lot of garden soils to the water introduces nutrients into the water.  While these are good for promoting plant growth, something you might desire if you are building a pond for growing plants rather than attracting insects, putting a lot of soil in ponds drive nutrient levels up.  This in turn promotes algae growth and can cause massive algae blooms (rapid reproduction and growth of algae) and causes problems in the pond.  Algae blooms tend to make the water very opaque (see image at right), which decreases the educational value of my pond.  The algae can also block the light from reaching any submerged plants near the bottom of the pond, potentially killing them.  Further, when the algae absorb most of the nutrients from the water, they die off in hoards, fall to the bottom of the pond, and rot.  The rotting process consumes oxygen, so the oxygen levels in the pond drop.  Sometimes this can kill the insects that live in the pond.

You can control algae with algaecides, but that is a maintenance heavy task and my pond needed to be low maintenance.  Instead, I decided not to put much garden soil into the pond to keep the nutrient load as low as possible.  Many aquatic plants can survive in rather low nutrient environments, so I planted the rooted plants in pots, then submerged the pots in the pond.  This had the added benefit of allowing me to keep the plants contained so that they wouldn’t spread and fill the entire pond.

I bought some of my plants (including the horsetail at left) and those all came pre-planted in soil.  For the plants that were donated and/or harvested, I largelyfollowed the recommendations of the Tucson Water Gardeners and used unscented kitty litter in place of soil and planted my plants in water plant baskets that I got from Home Depot.  I added a thick layer of kitty litter, a very thin layer of garden soil, and finished with a thick layer of kitty litter on top.  This way, the soil is held in place by the litter (soil can float to the surface if it’s not weighed down) and there is very little soil added to the pond, but enough nutrients for the plants to thrive.  I’ll give an update on how my plants are faring next month when I know how well it worked!

I wanted to include a variety of plants in my pond.  This makes the pond look nice, but the right balance of plants also contributes important habitat to a variety of aquatic insects and helps keep the water looking its best.  For the insects, I wanted to use several different types of plants: rooted marginal plants (those that have their roots in the water and the vegetation extends well above the surface), floating plants (rooted or unrooted plants that float on the surface), and submerged plants (plants that are entirely underwater).  The rooted marginal plants provide vertical habitat for insects that like to climb, such as damselfly nymphs.  Floating plants are great for obscuring insects that swim in the water column like beetles and backswimmers.  The submerged plants are great habitat for the insects that stay underwater all of the time and like to hide, such as mayflies.  Lots of different types of plants mean lots of different types of insects, so including a variety of plants was a priority for me.

The plants also contribute to the quality of the water in the pond in several important ways.  Submerged and floating plants contribute a lot of oxygen to the water as they photosynthesize, something the insects, especially those that live on the bottom of the pond and don’t go to the surface to breathe, need to survive.  (For more information, see my posts on aquatic insect respiration and improving aquatic respiratory efficiency.)  All of the plant species absorb nutrients from the water, keeping the nutrient levels down.  The plants also block the light from about half of the pond.  Low nutrients and low light are good things if you want to keep the algae population under control, so hopefully my plants will prevent huge algae blooms from occurring in my pond.

As far as the specific plants I used go, I used 9 different species.  For marginals, I used horsetails (Equisetum hyemale – an interesting plant), cattails (Typha latifolia), arrowhead (Saggitaria sp.), an unidentified rush species, and a yellow water iris (likely Iris pseudocrorus).  My floating plants include an unidentified floating plant I got from the University of Arizona’s Environmental Research Lab and parrots feather (Myriophyllum aquaticum – an invasive aquatic plant commonly sold as a pond plant that I only planted because it is FAR away from natural water sources).  My submerged plants are a type of hornwort (Ceratophyllum sp.) and a waterweed (Elodea sp.).  All of these are supposed to thrive in Tucson’s climate, so we’ll see how well they do at a slightly higher elevation.  (Thank you to the Tucson Water Gardeners for donation of the hornwort, waterweed, parrots feather, and arrowhead, and to the UAERL for the irises and unidentified rush and floating plant!)

I filled up the pond with a hose and put the plants in their pots into the pond.  The water was muddy when it was first filled, but the silt settled out pretty quickly – the water was clear a few days later when I went back to B2 to work on it again.  Good thing, because it didn’t look so great at first:

If you read my last post, you know that I had to make my pond safe for kids to be around, which involved installing a barrier.  The mesh you see just under the water is what I call the anti-drowning shield for my pond.  It’s not pretty and it was a beast to install (the worst part my the experience!), but it keeps kids out of the pond too.  It’s a good thing and necessary.

Next time I’ll talk about my pump, installing the water and the electrical components for the pond, how I’m combating mosquitoes, and the educational components of my display.  Later this month I’ll give an update on how my pond’s doing!  Hope you’ll check back!

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Posts in this series:

• Part 1: Introduction
• Part 2: Choosing a Pond Location and Installing the Container
• Part 3: Choosing and Installing Substrates
• Part 4: Water, Electricity, and Avoiding Skeeters
• Part 5: The Educational Value of an Insect Pond
• Part 6: Update

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