Answer: Dieffenbachia is a tropical plant and it will die if left outside in cold weather, so it is usually grown as a container plant.
As with most plants, this one is not best started in water. Although it may form roots, the roots will not adapt to soil when the time comes to plant it, and water does not provide sufficient nutrients for it to survive and be healthy enough to grow.
For most people it is easiest to purchase a small plant and grow it on in a pot, moving it to larger pots as it grows.
Otherwise, to propagate it yourself, take a tip cutting from a vigorous shoot. Remove the foliage from the bottom half. Trim the remaining foliage by about half. Place the cut end into barely damp soilless potting mix (usually composed of milled spagnum peat moss and vermiculite and perlite), firm the soil, water lightly once, and place in a clear plastic bag or under a clear dome made by cutting the bottom off of a clear plastic soda bottle. This will help keep the atmosphere humid while the plant is rooting. Place the pot in a bright location but out of direct sun so it won't overheat. Open the bag from time to time to allow for air exchange and make sure the soil is still damp but not soggy. Check for roots and when there are some, gradually remove the cover to accustom the plant to normal humidity levels and move it to its normal growing location. When the roots fill the pot, repot to the next larger size.
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