Surprise! The flowers come in pinks and lavenders too, as well as the usual oranges and yellowsl.
When it comes to nasturtiums, we gardeners are most familiar with the common majus cultivars. Those can be showy enough, as proved by the annual April display of them that cascades down courtyard walls at the Isabella Stewart Gardner art museum.
There are, however, 80 or so other species of Tropaeolum native to South America, many with even more exotic blooms. Several of those lesser-known nasturtiums stray from the typical orange or yellow hues and from the usual growing season.
A few sprout in autumn, climb in winter, bloom in spring, and drop into dormancy over the summer. That group includes blue nasturtium (Tropaeolum azureum), purple nasturtium (Tropaeolum austropurpureum), and Bolivian or Chilean nasturtium (Tropaeolum tricolor)—whose three hues are red, yellow, and violet. Summer-growing types, such as the scarlet Scottish flame flower (Tropaeolum speciosum) and the green-tipped coral ladies’ legs (Tropaeolum pentaphyllum) follow a more typical pattern, blooming in summer and “napping” in winter.
Like clematis vines, these nasturtiums like having their feet in the shade and their heads in the sun, preferably in well-drained, neutral or mildly acidic soil with a trellis for support. Most can be perennial in USDA zones 8 through 10, though azureum and hookerianum are listed as only hardy to zone 9.
If you must grow them in pots, make those pots at least 7 inches deep, so the tubers can be buried 6 inches down. Once the plants begin dying back, you can cease watering them altogether until they emerge from their dormancy. Provided that they ever do!
The most provoking thing about tuberous nasturtiums is that they don’t always wake up when they are supposed to. Like Sleeping Beauty, some of them may decide to rest for longer than one season, and nobody knows what magic kiss it takes to rouse them again.
I did succeed in germinating a single Tropaeolum azureum plant from seed once, but--since that was over ten years ago--I can’t now remember what happened to it. I may have assumed that its dying down meant it was dead rather than dormant, or it may have been one of those tubers that refused to wake up again. In any event, it is now long lost.
I probably followed Norman Deno’s instructions for germination (from Seed Germination Theory and Practice), so I’ll relay those to you here. He recommended that you keep the seeds at 70 degrees Fahrenheit in damp paper towels for one month before moving them to 40 degrees for germination.
If you prefer not to use paper towels, you probably should sow them in a soil mix rather than seed starting mix, since they shouldn’t be transplanted until after they go dormant for the first time. Dave’s Garden member Ursula suggests a combination of compost, garden soil, and sand, though you probably can substitute potting soil for garden soil. You then should cover the seeds with about 1/2 to 3/4 inches of sand.
My records indicate that a single one of my seeds began to show roots just a couple weeks after being moved into the more chilly conditions. Deno asserted that the germinating seeds must be left at 40 degrees until stems emerge, so I probably kept my sole success in the refrigerator until I saw a stem before potting it up.
According to the Pacific Bulb Society, the seeds of winter growing Tropaeolums should be sown in the fall, and I see that I started mine in late September. However, you can plant summer blooming types (sometimes called alpine species) in the spring.
Despite the old advice that one should be nasty to nasturtiums to get them to bloom well, I would treat these tropaeolums with more care. After all, if they don't like you, they can just play possum for as long as they want!
Photos: The Tropaeolum azureum image is by Ursula, the speciosum image by KMAC and the hookerianum image by melangemerchant, all from the Dave's Garden PlantFiles. The pentaphyllum image is by jeffdelonge, courtesy of Wikimedia Commons and this license.