and Marginal Plants
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Overwintering Your Tropical Waterlilies
If you live north of USDA zone 9, you will have to overwinter your tropical lily out of the water. As your lily grows throughout the season, it may multiply and become two or more plants. Repot these lilies into different containers, or at least move them away from each other in the same pot. This will prevent the lilies from forming one large mass of tubers that will have a great tendency to rot.
When fall approaches, you should stop fertilizing your tropical lilies. This will help slow their growth and prepare them for overwintering as tubers. After your first frost or two, go digging into the pots (I know it will be cold!) or pull the pot out of the water. Feel around in the soil and find all of the hard nut-like tubers. You will find them just below the crown of the plant. They will normally range from the size of an acorn to the size of a golf ball, but may be as small as a pea or large as your fist. Take all of the tubers out of the soil and rinse them thoroughly with a strong jet of water. Trim off any remnants of roots or stems from the tuber. Many people store them in plastic bags, but rodents can eat through the bags. I suggest storing them in glass jars. Just be sure they don't get knocked off the shelf and break. They must be stored in damp (not wet) peat, sphagnum moss, or sand. The temperature must stay above 50 degrees Fahrenheit. Check on them once a month throughout the winter to be sure they don't dry out. Spray them with water if they have become dry.
In the spring, when the water temperature reaches 60 degrees F, replant the tubers in fresh, fertile soil. Make sure you put the tuber just at the top of the soil surface. The pointy side goes up, round side down. If possible, elevate the pot close to the surface so it will get more sunlight and warmer water. The tuber will set out leaves and eventually the plant will set out roots. When the plant is strong, separate it from the tuber. You can turn the tuber in the soil so that it will send out the next plant in a direction different than the first. Each tuber can put out several plants.
Overwintering your marginals
Many tropical bog or marginal plants can be overwintered outside in the pond. If your marginal will be spending the winter in the pond, you may trim the foliage back to a few inches above the water line, or wait until the spring and trim back then. I prefer the latter, but you must keep an eye out for insects that may use the foliage to overwinter. Semi-hardy plants for your zone may be moved to the deepest part of the pond where it will not freeze. Do not trim plants that will spend the winter under the water until they come back out in the spring.
Tropical marginal plants can usually be wintered inside as houseplants. Bring them in well before the first frost and keep them in a saucer of water in the highest light area you have. You may want to transplant them at this time into fresh soil to be sure you don't accidentally take any unwanted creatures into the house. Be sure to thoroughly rinse the foliage - top and bottom of the leaves, since many creatures lay their eggs under the leaves. Supplemental lighting is recommended so they get 10-12 hours of light a day. Do not fertilize these during the winter because they need a rest period.
Submerged plants can overwinter below the ice. You may use a net to be sure they stay deep, or trim them back so that there is no foliage that will freeze - which would contribute to the algae bloom in the spring. Parrot feather has been known to overwinter in mud, but I would bring it inside for the winter.
Floating plants such as water hyacinth and water lettuce can be overwintered indoors or in the greenhouse, but please be sure you can provide the following:
Parrot feather, Neptunia aquatica, Bacopa, water mint and water poppy can be planted in hanging baskets with no drainage and overwintered in a sunny window, greenhouse or sunroom.
Be sure to take your plants in early enough so that they don't begin to 'shut down' and go dormant on you because of decreased light. If you have fish, you can coordinate your plant overwintering procedures with your decreased feeding schedule.
If you have lots of plants, it may be a good idea to overwinter bulbous plants in paper bags or in peat or sphagnum moss. Crinum americanum and taros will produce hard tubers and will overwinter that way. Just dig up the bulbs, rinse all the dirt off and be sure they dry thoroughly. Then just overwinter them as you would your annual flower garden bulbs.