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Tuberous Begonias Care At A Glance

Tuberous Begonias are frost-tender plants that thrive where they receive bright light but little or no direct sun. Given an early start, they put on a glorious display all summer long. We grow our Begonias in pots and hanging baskets, but they can also be planted in the ground (once the danger of frost has passed). Either way, unless you live in a frost-free climate, you must lift and store the tuberous roots in fall to carry the plants through winter.

Starting Tubers in Flats or Pots

To get a jump on a short growing season, start tubers in flats (shallow trays with drainage holes) or utilize 4-5 inch pots.
The potting mix should have a light texture and be well drained.

NOTE: Typically, a peat-based mix, blended 3 to 1 with play sand or additional perlite is ideal.

Thoroughly pre-moisten the starting mix.
Fill trays or pots with the pre-moistened mix, leaving about 1.5-2 inches.
Handling the tubers with care (especially if they have begun to produce new growth, which is very fragile), place them, hollow side up, on top of the potting mix. Space the tubers 2-4 inches apart in a flat (one tuber per 4-5 inch pot) and cover them with ½ inch of potting mix.
Then water sparingly and place the container in a window that provides bright but indirect light.
Keep the potting mix moist, Not Soggy.
To hasten growth, set the container on a heating mat or a radiator (place 1-2 dish towels between the tray and radiator to prevent overheating). Tubers that have not already begun to sprout when you receive them will generally show signs of growth within 2-6 weeks from planting depending on conditions.

Transplanting

After the first 2 leaves have emerged, transplant tubers started in flats into 4-5 inch pots.
If all danger of frost has passed, tubers can be transplanted into a lightly shaded outdoor bed that has already been amended with organic matter such as compost or peat moss.
Lift and move the tubers carefully to avoid damaging the roots and replant them with the top of each tuber not more than 1-2 inches below the soil surface.
When planted, Begonias should be positioned with the new leaf tips facing forward, because the blooms will ultimately face the same direction.
Plants in 4-5 inch pots should be moved into 7-10 inch pots when the roots fill the pots. If time and energy permit, a final move to 12-inch pots will yield especially spectacular plants.
After the threat of frosts, set pots outdoors in a bright but shady location. Too much sun will cause leaf scorch, however, deep shade prevents optimal growth and inhibits flowering.

Staking

When upright varieties are 4-6″ tall, push heavy, 18-20″ long bamboo or aluminum stakes into the potting mix or garden soil on the side of the plant opposite the points of the leaves. Place the stake a few inches away from the main stem to avoid injuring the tuber.
Fasten the plant to the stake with garden twine or soft plastic tape looped in a figure-8 around the stem and stake. As the plant grows, you may need more ties to provide additional support.

Summer care

Tuberous Begonias thrive in organically rich soil that is evenly moist but well drained. Soggy soils can lead to cultural issues such as the stems that snap off at the base.
Fertilize plants once a month with a balanced (6-6-6), water-soluble fertilizer like ALGOPlus mixed as directed.
Keep plants tidy by removing spent flowers; cut the flowers off close to the stem using sharp clippers.
The only disease that can bother your plants is powdery mildew, a fungus that appears as white powder on the leaves.

NOTE: Powdery mildew is easier to prevent than it is to cure, so use proper prevention measures.

Place your plants in a location with excellent air circulation.
For reoccurring powdery mildew issues, apply a recommended fungicide like Revitalize® as a preventative spray. Use this solution once every 7-10 days during hot, humid weather. Other good fungicide options include mineral-based Sulfur or Copper, as well as synthetic types such as Captan® or Fung-onil® by Bonide®. Read and follow all label instructions and safety notes for any pesticide products used.

NOTE: Neem oil is considered weak and ineffective as a fungicide.

Overwintering

Allow plants to grow through November (or until frost) to store energy for the next season.
Force container-grown plants into dormancy by gradually withholding water.
Dig plants grown in the ground with a ball of soil and let them dry out in a shed or in the garage.
When the stems break free from the tubers, shake off excess soil and allow the tubers to cure in a well-ventilated, dry location for 4-5 days.
Then store them in dry peat moss or dry sand in open flats, crates, or buns in a cool (45°-50°F), dry place.
Replant the tubers as suggested above in late winter.

Growing Hanging Basket Begonias

Hanging Basket Begonias--varieties with trailing stems--require much the same care as upright Begonias, except that they look their best in a shallow container that can be suspended from the eaves of a house or from an arbor.
One Hanging Basket of Begonia tuber in a 12″ container makes for a spectacular and long-lasting display.

NOTE: If the stems of a Hanging Basket Begonia grow upright and refuse to trail over the edge of the container, this simply means that they are not receiving enough light.