Tender or tropical plants that grow from tubers, bulbs, rhizomes or corms—like dahlias, tuberous begonias and canna—bring color and flair to gardens and containers throughout the year’s warmest months. These plants are typically added to the garden at the start of the growing season, when frost is no threat. At that time, they can be planted as dormant bulbs or as pre-sprouted plants purchased from the garden center.
There is a third option for starting these summer beauties, of course: sprouting them yourself indoors, starting in late winter or earliest spring. Similar to annual seed, dormant warm-season bulbs entice us with low cost, more choices of species and cultivars and, especially in cold regions, a head start on the growing season, leading to bigger plants and earlier flowers. Even where winters are short and mild, gardeners may prefer to start warm-loving tubers indoors as a safeguard against the rot that can be brought on by wet weather.
All of the plants addressed in this article can be transplanted outside after the threat of frost has passed and the soil has warmed to approximately 65 degrees (F). Typically this will coincide with the local planting-out date for tomatoes and other warm-season crops.
As with annual seedlings that have been started indoors, tuberous plants should be hardened off before taking their final planting position. Do this by placing them first in a shaded, sheltered location for longer hours each day and gradually moving them into stronger sun, before finally leaving them outside overnight. If temperatures threaten to dip into the 40s, keep the plants under cover.
1. Dahlia
Dahlias are invaluable for the bright, lasting color that they add to the garden from midsummer to fall. Cultivars run the gamut of flower size, shape and hue, while plant size is equally varied. Starting dahlias from tubers lets you take advantage of all these options, especially if you seek them out from specialists.
In cold-winter regions, dahlia tubers can be started in pots several weeks before the expected last frost date. This results in bushy young plants ready to gently transplant into the garden once the weather has warmed. Use a pot just big enough to fit the tubers, setting them so the eyes, or growing tips, sit about two inches below the soil line. (If you're planning to take stem cuttings, detailed below, you can set the tuber a bit high in the pot, so that the crown is just above the soil line. This will make it easier to see the stems that you’ll be cutting.)
Set the pot in a bright space and keep the growing medium just barely moist. After the shoots appear, water more thoroughly. Once the shoots have formed several sets of leaves, pinch them back by simply snipping or pinching the stem off just above a pair of leaves. This will encourage the plant to make more branches and therefore a bushy shape. If more than five shoots sprout from the tuber, remove them so that only five continue to grow. It may seem counterintuitive, but in the long run this will result in more flowers than if you had allowed additional stems to continue growing. It will also improve air circulation through the mature plant.
You can treat the excess shoots as stem cuttings to root new plants, if desired. Snip off the lowest set of leaves. Stick the cutting into a pot of light potting soil with good drainage, placing it at the edge of the pot so that the roots will grow touching the pot's sides. This will encourage budding at the roots. Keep the pot moist and warm. To conserve moisture, you can seal it in a plastic bag. Within a few weeks the cutting should take root; you will know it has when it resists a tug on its stem. Pinch the stem, then, to encourage a bushy habit.
2. Canna
Shade-tolerant and moisture-loving, cannas grow well in heat and humidity. Their large, upright leaves bring coarse texture to mixed beds or containers. Some cultivars offer variegated foliage. Large, bright flowers appear in late summer. Cannas are often thought of as big plants, but compact cultivars are available, growing to just 24 inches tall.
Cannas despise cold soil and their rhizomes can be slow to sprout. Starting them indoors four to six weeks before the anticipated last frost ensures you’ll have bulky cannas by the Fourth of July. Lay the rhizome on its side in the pot, buried two or three inches deep. Just moisten the potting mix to start. When the shoots appear (after several weeks), they will develop quickly and relish more water. Keep growing the plant indoors until it’s time to plant out warm-season crops like tomatoes.
3. Calla lily
Calla lilies make excellent cut flowers, and their long bloom period means that new blooms will quickly replace those taken for the vase. Many cultivars also possess freckled foliage that can carry the scene in lieu of blossoms. Like canna, calla lilies can be a little slow to start from rhizomes, but worth the wait. Time and treat them similar to canna. Make sure that the rhizome is placed so that its eyes, or growing points, face up.
4. Elephant ears
The colloquial name “elephant ears” can be a bit confusing (as common names are indeed wont to be), because it applies to several similar-looking plants. The key difference between them is size and stance.
The smallest so-called elephant ears are caladium, growing just knee-high. Caladium rival the familiar coleus, a tender perennial, in perking up a shaded space. Shield-shaped, white-, pink- or red-splashed foliage is caladium’s hallmark, along with an affinity for warm weather and low light. Caladium tubers can be started about three weeks before the expected last frost. It can be difficult to tell which way is up on a caladium tuber. When in doubt, set the tuber on its side; the roots and shoots will head the right way. Cover with two inches of potting mix.
Short-statured caladium make beautiful small accents or edging. The other elephant ears, Alocasia and Colocasia, require a spot at the middle or back of the border, or a prime position in an oversized pot. Both alocasia and Colocasia boast large, broad foliage. Alocasia stands more upright, while Colocasia takes on a rounded silhouette. The former offers more colorful cultivars, while the latter provides the very biggest “ears.” Both plants grow from large bulbs that can be started four to six weeks before the last frost, with their pointed tops set about an inch below the soil surface.
5. Tuberous begonia
Tuberous begonias bring luscious color to the garden, whether they’re grown in hanging baskets, window boxes, mixed containers or garden beds. Their large, often ruffly flowers are available in a wide range of colors, and their appealing foliage provides the perfect backdrop.
To get your begonias growing and ready to bloom in time for spring, start the tubers indoors about 10 weeks before your typical last frost date. There’s no need to plant them at first; simply set the bare, dry tubers in a warm, bright spot. Within about two weeks, reddish buds will sprout from the tops of the tubers, followed by the beginnings of stems. (The top is the side that’s cupped.)
At this point you can transfer the tubers to pots. Plant them about an inch deep with the sprouts facing up. Use small pots—four inches at the most. This helps prevent rot. Use evenly moist potting mix when planting the tubers. To increase humidity, you can cover the pot with a plastic bag or other clear cover. While the tubers are rooting, water only if the soil dries out. Once leaves emerge, remove the cover and start to water more regularly, but avoid creating soggy soil. Keep the plants in bright light.
When two or three leaves have developed, transplant the begonia to a pot a couple of inches larger and continue to grow the plant on toward spring.
