Tuberous begonia sprouting from a tuber. Photo: Michèle Bénard
Question: Last year, I let my tuberous begonia (Begonia x tuberhybrida) grow without intervening and the stem became so tall and heavy that it finally broke. This year, I wanted to find out if pinching it would help, but I found nothing on the Internet. It’s growing vigorously and the stem seems strong, but it doesn’t look as if it’s going to produce any branches. What should I do?
Answer: Yes, you can certainly pinch it (that is, remove the growing point). You could also prune it back, shortening it even more. Both will reduce the height of the plant somewhat and also increase the number of stems, as both pinching and pruning stimulate branching. This will ensure more flowers during the summer.
That said, there is still a limit to how effective of this kind of pruning can be. Some tuberous begonia cultivars—especially large-flowered varieties—are not given to producing numerous side branches and their stems are naturally quite brittle. It may therefore still wise to plant these varieties in a spot protected from strong winds or to attach them to a stake at planting time. That way you can make extra sure there’ll be no damage during their summer outdoors.
By the way, if you do cut your begonia back, you can use the cut stem as a cutting. Just insert it in a pot of moist soil and make sure you cover at least one node (point where a leaf is attached to the stem) with potting mix, as new roots sprout from the node. You’ll discover that begonia stem cuttings readily flower even the first summer and will produce, by early fall, a new tuber that you can bring in and store for the winter.