Q: I’ve got about a dozen tuberous begonia bulbs that I dug and stored away last fall and want to use them this year — when can I plant them in the garden?
A: If you want begonias this summer, you need to start them indoors, right now!
With our cold spring and short summer season, we need to start them indoors. If you want to see blooms before August, then give tuberous begonias a six-week head start before planting them in the garden. If we put them into the cold ground too early in the spring, they might rot — or they might survive and eventually sprout around July 4 and bloom a month later. That’s too late!
Start tubers now (it’s already a little late), and they’ll be ready around Memorial Day, the same time that it’s safe to put your houseplants outdoors; nighttime temperatures are reliably in the 50s.
Transplant tuberose begonias outdoors in a shady, moist place. Starting tuberous begonias is no big deal — it’s much easier than starting seeds. Start the tubers in individual pots or in flats of moist peat moss or potting soil. Individual pots are easier for just a few tubers, but if you’re growing more than a few tubers, a seed flat will make a dozen or two tubers simpler to handle. To start tubers in a flat or a pot, plant the tuber in damp peat moss. Place each tuber in the moistened peat and press gently into the soil surface — press; do not bury the tuber with peat! Water thoroughly, and do not permit the pots/flats to dry out.
Now, which side of the tuber is the up side? And which side is the down side? We get dozens of calls every year about which way is the correct way to plant these cup-shaped tubers. It’s simple to remember: Plant the cup-shaped tuber up! If you happen to plant them upside-down, the sprouts will eventually grow to the surface — they will survive — but the sprouts will have to grow down and around the tuber to get to the soil’s surface, and that’s a waste of time. Plant the cup-shaped tuber up, and you’ll be on the way — the right way!