If you’re growing night-blooming moonflowers (Ipomoea alba), you have to wait patiently for their flowers to appear. Firstly, they like a long growing season and plenty of heat. I started mine early, indoors in pots, and our summers certainly provide enough heat. Secondly, and most interesting, moonflowers are short-day plants. Simply put, they wait for late summer’s short days and long nights before producing flower buds.
It seems a contradiction that a heat-loving plant native to South America waits for short days and long dark nights to produce flowers, but that’s a basic premise of photoperiodism. There are long-day plants, like clover and Carpathian bellflower (Campanula carpatica), wanting long days and short nights. But moonflowers, and also chrysanthemums, are short-day plants and require extended dark hours.
Moonflowers have a photo-receptive protein in their biology that measures the periods of uninterrupted darkness, triggering bud growth when enough consecutive dark hours are available in a 24-hour period. Shine a spotlight directly on them in the darkness and you won’t get flower buds. (Fortunately, lunar and starlight aren’t sufficient to interrupt the budding process.) My moon vines are six feet (1.8 m) from an outdoor security spotlight that’s on all night, but the light is aimed away from them.
Not all plants are sensitive to photoperiodism. Roses are day-neutral plants, not caring how long or short the nights are. Day-neutral plants flower in response to developmental stages in their growth, like sufficient new cane growth to initiate rose buds. Or in response to environmental changes like vernalization, a period of moderately low (but not frosty) temperatures that can induce flowering in plants like orchids and Christmas cactus.
But, back to the moonflowers. I saw the flower buds forming 10 days ago, and prepared to celebrate. It’s an occasion to watch their big flappy petals swirl open, and the event becomes an entertaining ritual. One evening, I sniffed sweet perfume drifting through a window and went out to investigate. There was the first flower, bold as brass and very white, already attracting a circus of moths. The large, dark buds are a bit prehistoric-looking, with a sort of spiky construction around them. The flowers stay open all night, and begin to fold as sun brightens the morning.
My moonflowers open at about 5:45 p.m., but that will change as the days become shorter and the darkness they relish comes earlier. My plants are shaded at that time; if they were in sun, they would probably open later.
Moonflowers are large plants, and they seem to require quite a bit of water. I give them a big drink each night to keep their wide leaves from wilting. Flowering continues for about six weeks, and by then autumn is firmly established.