GloucesterTimes.com, Gloucester, MA

May 3, 2013

For Memorial Day flowers, start begonia tubers now

North Shore Gardener
Barbara Barger

---- — 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!

Keep the planted tubers moist and at room temperature until the sprouts appear, which doesn’t take long. At all stages of growth, begonias are kept in strong light, but not directly in hot sun. When the first shoots appear, it’s time to begin feeding with a weak 5-10-5 or liquid manure fertilizer every week. Fertilize carefully — be careful not to burn the little plants by splashing fertilizer onto the leaves.

Where are you going to use your tuberose begonias? Use them wherever you want precious color in the shade or in tree-filtered light. These shade-loving plants can best be used in protected areas — they’re quite fragile. What’s a protected spot? Not where the hose gets pulled through the evergreens and pachysandra. Not where the kids play ball, and not where the paper person throws the newspaper. Not where a broken downspout pours a torrent of water from the roof during every rainstorm. And beware of unleashed dogs that might run through your begonia plantings! Consider using them in containers for a lush, luminous look.

For the most spectacular blooms, continue feeding begonias all summer. During the summer, remove dead flowers and foliage to prevent fungus and mildew. Good air circulation helps prevent mildew, which is common on most begonias.

Next year, you can propagate more of your favorite tuberous begonias by division. Just like a potato, the tuber can be cut into pieces containing at least one eye on each piece and planted. Dust the cut surface of the tuber with charcoal and start in peat, just like a whole tuber.

When buying new tubers, look for a firm tuber — think of the way you choose a potato: firm, not mushy with any soft spots. Pink or white sprouts, which are signs of growth, are acceptable, but avoid tubers with excessive sprouting. The sprouts will grow but won’t give you head starts — the pale, weak new shoots will probably break off or rot anyway — not grow into strong plants.

If tubers grow from seed, then why not grow tuberous begonias from seed? It’s far too late to grow tuberous begonias from seed this year. But maybe next year. For flowers this year, start now with a tuber. Seeds are as fine as dust and need very careful handling. They’re carefully dusted onto pots or flats of moist peat, and covered with a sheet of glass until they sprout. Begonias can be grown from seed in about a year.

Prices for tubers range from about $1 or $2 to about $3 or $4 each, depending on the variety and size. As with all tubers, choose the largest ones you can find for the best result the first year. The cost of tubers is not as expensive as it might seem. You can easily save them from year to year if you lift these tender tubers in the fall after the foliage turns yellow. Tubers are then gently dried and packed away in a frost-free place until another year. And well-kept tubers keep getting bigger and better every year. They’re worth saving.

North Shore Gardener by Barbara Barger is a periodic feature of Friday’s Living section. Reach Barbara at nsgardener@comcast.net or write c/o Gloucester Daily Times, 36 Whittemore St., Gloucester, MA 01930. Previous North Shore Gardener columns can be found at www.nsgardener.com.