Aquatic Plants: Going Below the Surface

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Oh, my aching muscles...

How to Repot and Divide Water Lilies and Marginals

By Mary C. Gutierrez

As spring progresses, water lilies and marginals send shoots to the water’s surface to begin their seasonal cycle anew. Have you looked below the water’s surface? While you may have been busy this spring potting decorative plants in patio containers, you might not have thought to lift your aquatic plants to see what’s going on inside their pots.

Like other perennial plants, aquatic plants increase in size each year. Many aquatic plants grow from rhizomes or tubers that would spread out to form a larger clump each year if they were growing in their native habitat. Since they are confined to pots, they can become crowded and stunted if not maintained.

There are a few supplies and techniques that differ from dividing and potting up terrestrial plants. First, the planting containers need to accommodate roots that grow horizontally rather than vertically, so shallow but wide containers are the best. Water garden experts recommend using either plastic pots without drainage holes or mesh bags designed specifically for growing aquatic plants.

Second, you need to use clay loam or special aquatic plant potting soil. Regular bagged potting soil usually contains peat, ground bark, perlite and possibly pumice—all of which will float to the top of your pond as soon as you submerge your aquatic plant. Since I have an abundance of clay soil in my garden, I collect a few shovels-full in the area where I’m working. I sift through the soil to remove plant roots and other debris.

In most cases, you will fill a container one-half to three-quarters full of clay loam, and will arrange the plant’s rhizome, bulb or roots in the pot before topping it off with more clay loam. Aquatic plants and marginals prefer to have their crowns at the same level, or just above, the soil’s surface.

Once you’ve positioned your plant in its new container and filled it with soil, top off the pot with an inch of pea gravel. The gravel will support the plant’s crown and keep soil from washing into your pond.

Before the season progresses any further, roll up your sleeves and divide your aquatic plants! Here are some suggestions for repotting different types of aquatic plants.

Water Lilies (Nymphaea)

Water lilies have vigorous rhizomes (fleshy roots) that can cause their pots to bulge after just one season. The second season after you bring home a water lily you may notice that it has two crowns sprouting leaves instead of the original one. (I poke a couple of fertilizer pellets down into the soil of my second-season plants.) If your water lilies have been in their original pots for more than two seasons, they will produce fewer and smaller leaves and flowers. It’s time to repot!

Remove your water lily from its pot and use a hose to wash the soil away from its roots. If you simply want to repot your water lily, select the strongest-looking crown, and cut away excess rhizome, roots and damaged foliage, leaving four or five inches of healthy rhizome attached—and discard the rest. If you want to propagate additional plants, leave sections of root attached to all the crowns that look healthy and pot each up separately. Use an old kitchen knife, a root saw, or pruners to cut the rhizome.  Fill your container three-quarters full of soil. Place the plant’s crown against the side of the pot and spread the roots toward the center of the pot. Then, backfill around the roots with more soil to just below the plant’s crown. Add your one-inch layer of pea gravel, fill the container with water, and place it in the pond.

Marginal Plants With Rhizomes or Bulbs

Use the same technique to wash soil away from the rhizome of plants like iris or cattail. Cut the rhizome into good-sized, four-inch sections, making sure each section has an “eye” or crown. Remove any soft, mushy portions and discard.

Fill a wide container with soil, leaving four inches or so between the top of the soil and the top of the pot. Lay the rhizome horizontally on top of the soil, with the crown near the edge of the pot. (Positioning the root at the edge of the pot allows the plant’s roots to grow across the surface of the soil.) Just cover the rhizome with soil and add an inch of gravel. It’s ready to return to the pond. This technique works for any marginal plant with a large horizontal rhizome.

Marginal Perennials

There are a number of plants that grow happily in flower beds as well as in the pond. These plants are divided in just the same way as they would be for terrestrial planting. The only difference is that the soil should be clay loam, and it needs to be topped off with pea gravel. Leave the crown of the plant just level with or slightly above the surface of the soil and the gravel will support it.

Unlike water lilies, marginal plants need to have the tops of their containers just a few inches below the water’s surface. They are ideal for placement on the shelves or shallow areas near  your pond’s edge. They won’t be happy where the top of the pot is a foot or more below the water’s surface.

Aside from the differences in containers and soil, repotting aquatic plants isn’t any more difficult than other garden plants. Your plants will thank you!


An AquaPlanter bag, photo courtesy TetraPond®

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