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.
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.
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.
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.”
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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