Q: I have tried and tried to get tuberoses to bloom. All the bulbs grow, but only 1 or 2 out of 100 will bloom. What’s the problem? — Vicky Newell, Alexandria, VA
A: Tuberoses can be capricious, but in your case the most likely problem is that the bulbs you’re planting are too small to make flowers. For blooms the first summer you’ll need bulbs about the size of a golf ball. They’ll be more expensive, about $1.75 apiece, but are well worth it for the heady fragrance and big spikes of brilliant white they bring to the final month of the gardening season. Tuberoses are only marginally hardy in zone 7, where you live. If you had mulched your plants with a few inches of salt hay or pine needles, your flowerless tuberoses would have survived most years, and having grown larger, they’d be able to flower the following summer. Most gardeners in the northern half of the country treat them like annuals and buy strong, new bulbs each spring. This year, you should do the same.
Grow them like vegetables — in full sun with fertile, well-drained soil and a steady supply of moisture. Plant the bulbs only after all danger of frost has passed and the earth is warm — about when you’d plant tomatoes. Set the bulbs about 6 inches apart and deep enough to cover with 2 to 3 inches of soil. The foliage will take a couple of weeks to emerge, but then it will grow strongly if the weather is warm and the soil well-watered. Flowers will start to bloom about three months later and continue until frost. Where summers are short, gardeners start them in pots about five weeks before planting time.
To store the bulbs over winter, dig them up after the first frost kills the tops. Cut off the tops and pack the bulbs loosely in barely damp peat moss, sawdust, or sand, and keep them where the temperature is at least 45 degrees. Admittedly, this is a hassle, and Northern gardeners report uneven success with getting overwintered bulbs to bloom a second time, which is why so many of us just buy big new bulbs every summer. In the South, temperatures don’t get cold enough to harm the bulbs, but many gardeners there find that digging and overwintering them keeps the bulbs vigorous. You live in that wonderful hardiness region known as borderline — mild enough so that a thick mulch, in most years, is all your tuberoses need to survive and return to bloom gloriously.