Spring is never out of sight when you have a window garden. Today, for instance, during a snow flurry, and with temps in the low 20s, my Thunbergia grandiflora decided to bloom. This is one of the loveliest vines I possess, for it captures the blue of the Cote d’Azur in each of its large, lustrous flowers.
A successful adventure with Thunbergia begins with full, direct sun, moderate humidity (50 percent), and cool temperatures (60-65 degrees F.). Other essentials are a clay pot and a trellis of some sort upon which the vine can climb. My plant thrives on the broad sill in my library, where it receives direct sun from late morning through early afternoon. The pot rests on an oversized saucer filled with pebbles and a quantity of water, which, with evaporation, produces beneficial humidity around the plant. Two lengths of wire, bent to form arches and inserted into the pot, provide a decorative trellis for the vine.
Next to insufficient humidity, overwatering is the biggest threat to this Mediterranean tourist, but a clay pot greatly reduces the chance of death by drowning. Wait until the top inch of soil feels dry to the touch, and then water thoroughly. I fertilize thunbergia with every watering, just as I do most of my other houseplants, at the rate of a 1/4 teaspoon of high-phosphorous food to a gallon of room temperature water.
Like the poinsettia and the kalanchoe, thunbergia sets its flower buds in the fall, as the days begin to shorten. If your plant resides in a room that receives lamp-light in the evening, move it to a closet from 8pm until 8am for three weeks. There, in complete darkness, flower buds will begin to form.
When in May flowering ceases, cut the plant back to two inches. After all danger of frost has past, bring it to a semi-shaded position on the porch, where it is sheltered from strong winds. And be sure to check the plant regularly for water. Outdoors in a porous clay pot, moisture might be required daily. As the vine grows — and it will do so with surprising speed — twist the vining lengths around the wire arches. Further pruning may be necessary in July, but withold all further cutting by the first of August.
Labor Day is the time to bring the plant indoors. But before it enters your house, give both pot and plant a thorough insecticidal spraying. Also, scrape out as much top soil as you can, and replace with a fresh mixture. So treated, this Blue Sky Flower plant will never require a larger pot.
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