Houseplant Help 5 Tips to Keep Finicky Begonia Rex Alive


The American Begonia Society calls Begonia rex “the showboat of the begonia world.” That may be, but the king begonia, or Begonia Rex Cultorum Group as they are more formally known, may just be my Achilles’ heel: a plant that simply will not thrive for me, however much care and attention I lavish on it. I have killed every one I have owned—either a swift execution, or a lingering, painful death over several seasons.

But oh, how much I want to grow these beautiful plants, with their foliage shaped like lopsided hearts (or in botanical parlance, obliquely ovate leaves) splashed, striped, and swirled with silver, pink, plum, purple, chocolate, and scarlet.

Moist Air

There are hundreds of cultivars of B. rex to choose among, usually classified by leaf size—small (three inches or less), medium (three to six inches), or large (more than six inches). Their main quibble with modern homes is dry air, Booman tells me, causing brown edges on the foliage followed by leaf drop. “The plants are native to areas with high humidity, so they don’t pump water to the leaf edges very efficiently, as compared to weeds in our yards, which seem to survive no matter how dry we let them get,” Booman explains.
But don’t despair if your once-glorious rex became a shriveled mess over winter. Leaf drop—provided it occurs outside the growing season—doesn’t necessarily signal death. That’s because many cultivars have profound winter dormancy. “Leaves naturally begin to yellow and fall no matter how much fertilizer you apply. Dormant leafless plants take on the appearance of a pile of snakes coiled on the soil, as the twisted old rhizomes wait for a signal from the sun that longer days have returned, making it safe to push out new leaf growth again.”
I don’t know whether to laugh or cry about the fact that I’ve probably composted a few dormant begonias in my time, but it’s also worth noting that key to these survival strategies is the moist air that the plants find in their native climes: The Begonia rex species from which all the plants were bred comes from Assam, northern India.
And exactly how much humidity? Fifty-five percent is the Begonia rex sweet spot, but higher will work too: “I have seen plants happily growing in wet seeps at the edges of caves in Panama where humidity hovers near 100 percent day and night,” Booman notes.

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Distilled Water

The other key to success with rex begonias in Booman’s book is tap-water quality. That’s because “hard” water with high mineral salt content causes leaf edge burn in rex begonias because they have no mechanism to deal with salt loads. “They evolved in areas of high seasonal rainfall, and no salts in the water. When given salty water, they pump it to the leaf edges, where it evaporates, leaving these same salts at the pores. The accumulation burns the leaf edges brown and dead.” If you live in an area with hard tap water, switching to distilled water or rainwater will fix this.

Aggressive Drainage

Booman has another trick: Water thoroughly so that at least 20 percent of the water flows out of the pot, and don’t let the plant sit in the water for a second, so that the salts cannot be taken up by the plant.

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Don’t Overwater

But don’t overwater, either, as this is the number-one mistake with so many houseplants, rex begonias included.

Booman agrees: “People see the tropical appearance of the leaves and assume they must water it like a pothos or philodendron. Wrong. These plants are the ‘cacti of the humid jungles’. The soil must be allowed to get light in weight, almost dry, before they are watered again. The roots are very fine and delicate. Too much water rots the roots off the plants, especially at cool temperatures, e.g. below 55 degrees Fahrenheit. So water heavily when you do water, but do not water often.”

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Monthly Meals

And finally, feeding is vital to get the foliage looking at its best. Booman advises feeding with a soluble houseplant feed once a month, April through August, at a concentration of 180 parts per million nitrogen. Nitrogen is all-important for leaf production, and although rex begonias do flower, you can pinch out or snip off the flowers when they come in winter to chivvy the plant into putting all its energy into leaf production.

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