This section is from the book "The Florists' Manual", by William Scott. Also available from Amazon: The Florist's Manual.
This useful genus is very familiar to all. The tall nasturtium is one of the best plants for covering fences or walls when given some strings or brush to climb on. The dwarf nasturtium is used as a bedding plant and in mixed borders its round, compact clumps have a fine appearance. They are always treated as annuals and although growing most freely in our summers they will not endure the slightest frost.
The dwarf varieties of the nasturtium are not always a success as bedding plants, because they are planted in too rich a soil and the leaves hide the flowers. Plant in rather poor soil in the full sun, and you will have better results. The double forms of the nasturtium were formerly used as bedding plants, but we have many better plants for the purpose. A dark scarlet variety of the tall form is often grown in an 8-inch or 10-inch pot and trained up a rafter where it will give many flowers in any greenhouse where the temperature is not under 50 degrees.
Both the tall and dwarf forms are too easily raised from seed for us to think of them as bedding plants, but as plants for our veranda-boxes they are of consequence, their fine, bright green leaves and showy flowers being always liked by our patrons and soon filling up and making a fine show. We never remember having any left when trade was over by the middle of June.
Obtain the best strain you can of the tall growing kinds, and sow the middle of March. As soon as they have formed their seed leaves pot into 2 1/2-inch and keep them in a temperature of 50 degrees. They will grow fast and in any soil. If grown on the bench they will quickly become unmanageable, so we put them on a hanging shelf where they will get air and light and be occasionally short of water, which does not hurt them at all, but induces them to flower. Their roots, when put into the basket or vase, have not much room to spread, and they flower freely.
There are many named varieties of the tall growing nasturtiums, but we find a good mixture answers every purpose. In the dwarf or Tom Thumb strain for bedding you can buy any variety true to color and some of the foliage of these is very distinct.
Varieties of T. Lobbianum grow as finely as the common nasturtium. The foliage is somewhat smaller, but they flower in great profusion.