Nasturtiums (Tropaeolum majus) grow quickly from seed in moist soil. They may be grown in any frost-free season. After sowing seeds, depending on temperature, flowering begins four to six weeks later, and the plant continues to grow and produce more flowers for about 10 weeks.
Garden nasturtiums originate from the cool tropical highlands in South America. They persist as herbaceous perennials since frosts don't occur and the temperatures don't get much hotter than 27 degrees C (80F). Modern nasturtium cultivars are usually grown as annuals that die out once the summer heat gets inhospitable or winter freezes kill plants. They grow as rambling vines or as compact bush types, depending on the cultivar.
When garden nasturtiums bloom depends on when you sow the seeds. Climate dictates the best time for sowing. In areas of the UK with colder winters, plant seeds in early to late spring. Blooming starts about a month later and continues for two-and-a-half months. To prolong the flowering season, sow seeds in successive increments every two to three weeks. In warmer regions, sow garden nasturtium seeds in autumn, winter and early spring.
Heat and sunlight
When air and soil temperatures are cool, in the 4 to 15C (40 to 60F) range, growth and initial production of flowers is slowed. Increased temperatures hasten growth and flowers, but too much heat -- over 29 degrees C (85F) -- can cause such a fast life cycle that plants wither before even two months old. In cool summer climates or in cool seasons, plant garden nasturtiums in full sun to hasten and extend flowering. In hotter conditions, it's better to plant the seeds in a bright location with indirect light to prevent soil and air from getting too warm.
Once you plant garden nasturtiums, they tend to grow, flower, set seed and germinate without any work on the part of the gardener. This ability to self-sow and spreading in numbers is called naturalising. Bees visit the flowers and pollinate them so seeds develop. While the original garden nasturtium may wither away, daughter plants that sprouted from the mother's seeds perpetuate the planting bed. Thus, it may seem a clump of original plants continually bloom, even though it may be second or third-generation plants.