It may even be necessary to keep the plant itself over the winter. This course is followed in case of seedling water-lilies which, during the year, have not advanced sufficiently to produce either seed or tuber. Though tubers have been formed, they may be immature and likely to decay before the next spring and it then becomes necessary to resort to other methods to retain the variety.
The experienced grower readily foresees which particular plant is liable to be in this undesirable condition in the fall and lays his plans accordingly. As soon as he has decided that the plant is worth keeping something which usually can be foretold in late summer – he digs it up and repots in a shallow box, five to six inches deep and about two feet square or even larger. After boxing, the plant is returned to the pond, where it will continue to grow until fall, at which time it can be transported bodily to a tank in the greenhouse. By keeping the water in the tank at ordinary greenhouse temperature, that is to say, 60° to 65° F., growth will not be interrupted, though somewhat retarded, and the tubers have an opportunity to mature. Following this method, it is highly probable that in the spring we will not only have the parent plant but likewise a number of small tubers, which will bloom the same season.
The temperature of the water in these growing tanks should be about the same as that of the house in which they are placed, that is to say, about 65° F. A little more heat is desirable for Nymphaea Sturtevantii, and a greenhouse pipe may be passed through the tank to supply the requirement. In an emergency, heating the tank with a coal-oil stove placed under it may be resorted to, but cannot be recommended as a general rule.
Keep the water level so that the edges of the pots are about two inches below the surface. When the leaves appear, keep them just floating, using blocks underneath to regulate the heights of the pots.