Over wintered tuberous begonias questions - Ask Extension
Hello,
I have a number of tuberous begonia bulbs that I have over-wintered from last year. Some were in pots, others in the garden. They've all bee...
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Over wintered tuberous begonias questions #925317
Asked March 02, 2026, 6:50 PM EST
Hello,
I have a number of tuberous begonia bulbs that I have over-wintered from last year. Some were in pots, others in the garden. They've all been in a dark, relatively cool place in a bed of sawdust over the winter. I did this last year also, and had about a 90% success rate with starting them. But they were quite mixed in when and how vigorously they grew. I have a couple of questions:
About when should I pull them out and get them started.
I've read conflicting instructions on starting them: 1) to place them in soil with the bulb just barely covered by soil, and 2) placing them on top of the soil and then plant after they get going. What is your recommendation?
I've also read that one needs to be careful not to over-water them early in their cycle. Any further guidance on that?
Last year, I put them in an unheated greenhouse that gets limited sun with a plant heater mat underneath the pots. I suspect that the plant heater didn't provide enough heat to encourage growth as it took quite a long time for many of the bulbs to start (the pots were mostly 4-5" tall, so I just don't think the heat from the mat made it up to the bulb). I was thinking this year that I would keep them in the house in a fairly well lit area that doesn't get direct sunshine. Any thoughts on the best approach?
For reference, I live in near-in Portland at about 600' altitude and on the North side of a hill with quite a lot of mature mostly maple trees around the house, so sunshine is somewhat limited.
Any help or guidance appreciated.
Roger Demuth.
Multnomah County Oregon
Expert Response
Hi Roger,
Thank you for using Ask Extension.
I'm going to answer some of your questions out of order.
Overwatering tuberous begonias: Because the tuber has a depression on the top of the bulb, that 'cup' can hold water. If you over-water, the chances of having water collect and start to rot the tuber is high.
As far as putting the tubers out in the garden, night-time temperatures need to be above 50F which is probably another month towards mid-April in our area, but the fact that you have a shady yard can make the warming up of the soil a little slower.
Starting the tubers inside the house will give you a jump start.
As far as starting them inside, it might be worthwhile to start them in four-inch pots on top of 2 inches of moist potting soil, leaving them uncovered until you see the leaves sprout from the top. At that point you can add more potting soil over the tuber. One reference said to use grow lights but it's doubtful that a sunny window will be too much light this time of year.
Thank you for using Ask Extension.
I'm going to answer some of your questions out of order.
Overwatering tuberous begonias: Because the tuber has a depression on the top of the bulb, that 'cup' can hold water. If you over-water, the chances of having water collect and start to rot the tuber is high.
As far as putting the tubers out in the garden, night-time temperatures need to be above 50F which is probably another month towards mid-April in our area, but the fact that you have a shady yard can make the warming up of the soil a little slower.
Starting the tubers inside the house will give you a jump start.
As far as starting them inside, it might be worthwhile to start them in four-inch pots on top of 2 inches of moist potting soil, leaving them uncovered until you see the leaves sprout from the top. At that point you can add more potting soil over the tuber. One reference said to use grow lights but it's doubtful that a sunny window will be too much light this time of year.
