If your begonia tuber is getting too big for its pot, you can fix that . . . by dividing the tuber!
By Larry Hodgson
Question: Should I ever split a begonia tuber or just keep putting it in bigger pots each year? I have a few of those dinner plate-sized ones.
Answer: That’s a question that certainly takes me down memory lane! I used to grow a lot of tuberous begonias (Begonia × tuberhybrida, B. boliviensis, etc.), many more than today. And those tubers do get bigger year after year. Some really do become nearly as large as a dinner plate! Yet, the aboveground part of the plant—stems, leaves, flowers—don’t necessarily increase in size correspondingly. Some small begonias produce big tubers and some big ones have tubers that remain fairly small.
True enough, certain tuberous begonias will get bigger annually for a few years, but they then seem to stop. From there on, the “green part” remains much the same size from year to year.
Big Tuber, Small Pot
You eventually reach the point where tuber is ridiculously big for the size of the pot. I mean, how can you even water it correctly when the tuber is so wide it entirely covers the potting mix? What can you do?
Well, you can divide it, but . . . it’s a risky business. That leaves a large open wound subject to rot, one that never really heals over. With time, the outer tissue does dry, forming a sort of scab. However, no new tissue forms to cover the wound.
And now you have half a big tuber instead of a whole one. That doesn’t change much when you pot it up. If it was 8 inches (20 cm) wide before and looked somewhat like a deflated basketball, it will still be 8 inches (20 cm) wide and look like a deflated basketball ball, but a deflated basketball ball that has been cut in half. Not so much better, as you’ll still need a big pot for a fairly small plant.
Divide and Conquer!
If you decide to divide yours, do so in late winter or early spring, as it wakes up. Locate two different sprouts, ones that are well separated on the tuber. (The further apart they are, the less there is a risk of rot reaching them.) Just use a clean, sharp knife and neatly cut the tuber in two between the sprouts, revealing white flesh. You can’t help but notice the resemblance with a potato!
Dust the wound with sulfur or some other fungicide, then let it dry. I notice some websites say to let it dry for a few hours. I used to let the wound dry for 2 weeks or more, until the cut surface was a crusty brown.
Then you simply pot the tuber up, as discussed in the article Time to Wake Up Your Tuberous Begonias. You now have two plants to grow over the summer.
An Alternative Treatment
Of course, this surgical procedure doesn’t really solve your problem. You’ll still have that ridiculously big tuber. So, downsize: take stem cuttings instead.
When you’ve potted up your begonia tuber and it’s growing nicely (again see Time to Wake Up Your Tuberous Begonias), it’s time to react. By then, it should be about 3 inches (8 cm) tall. Just snap a stem off at its base or cut it free. Then root the cutting in regular potting soil that you will keep slightly moist. Just like any cutting, really. You simply insert the bottom 1/3 of the stem into the growing mix. No rooting hormone is necessary.
Cover the cutting with a clear plastic dome or put it inside a clear plastic bag. That will maintain high humidity and will moderate temperature fluctuations. Set the covered pot in moderate light at regular home temperatures. Within a few weeks, it will be well rooted. Then you can remove the dome or bag and grow it as a summer bulb.
Yes, your freshly rooted begonia will bloom the same summer, usually abundantly, and will have formed an as-yet small yet fully viable tuber by mid fall. You can bring it in and store it for the winter.
And you might want to simply drop the overgrown tuber into the compost where there are all sorts of microbes just waiting to digest it!