When planning on adding water plants to your garden, be prepared to do a lot of “winging it.” No matter how much pre-planning you do, in the end, you’re going to go out, buy whatever water plants you can find, and then figure out where to put them. Unless you’re lucky enough to live near a well-stocked pond supply store or nursery, you often have to settle for whatever you can find.
Once you begin setting your water plants, try to mix a variety of colors, forms, textures, and bloom times. Take into consideration the neighboring shrubs, trees, and ornamental plants that act as a backdrop to your water plants to heighten and smooth out the overall dramatic effect of your landscape.
Keep in mind that garden design principles apply equally to water plants. You can use a single large plant as a specimen, but smaller plants in groups of three or more will have the strongest effect and look more natural. Don't overdo things, however. Remember that the focus of your pond is the water. Use plants to add to the overall effect, not to divert attention from it.
Controlling Invasive Species
Before you buy any plant, find out how well behaved it is. Water-garden plants have a tendency to be invasive in ideal climates. Some states ban certain water garden species because they have been known to clog natural waterways and even disrupt sewer and storm-drain systems. Invasive plants also can create a maintenance headache; they'll battle you for control, and you'll spend hours pulling them out of the pond.
If you do want to use invasive plants, always put them in pots and be certain that the roots can’t escape out of the drainage holes. We usually place landscaping fabric (the stuff used to keep weeds from coming up in the flowerbeds) in the bottom of the pot before planting.
Take into account, too, how your plants and fish are likely to interact. Duckweed, while invasive, is a favorite food for goldfish and koi. The fish like to munch on it between meals, and that helps keep it under control. As for other plants such as water lilies and hyacinth, koi are extremely rambunctious and will often uproot and shred the plants. They do it either for food or for fun, or maybe just because they’re koi.
Water movement is another element to keep in mind when choosing your water plants. Most floaters and some deep-water aquatics, including water lilies, prefer fairly still water. Other plants, such as watercress and bull rush, thrive in moving water. Others still do equally well in either setting.
Although fountains are beautiful, the splashing spray can push floating plants away or keep a plant’s leaves constantly wet, which could ultimately kill the plant.
The hardiness of a perennial plant (its ability to survive winter cold) can often be extended by simply lowering the plant’s root system below the anticipated depth of the winter ice. If you live where winter temperatures reach -20° F, for example, plants hardy to -10° F may thrive if you set their pots deeper than normally found in the pond. As spring approaches and the cold snaps are history, move the plants back to their original growing depth.
While plants in natural settings root in the mud on the bottom of the pond, those in garden ponds (except for floaters) thrive best in pots. The potting medium you use for water plants versus garden plants, however, is different.
Since garden soil has a lot of fertilizer, humus, and other organic matter that can decay and foul the water, it’s best to use a special water plant potting medium, which is not soil at all but rather small pebbles. The potting medium won’t break down and end up contaminating your pond.
Water plants draw their nutrients directly from the water; so, this type of potting medium or even aquarium or pea gravel makes an excellent alternative to soil.