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Mendocino Coast Botanical Gardens, 47 acres of botanical bliss fronting the Pacific Ocean

Growing and caring for Tuberous Begonias

Growing Begonias
Choosing the proper environment for begonias is essential. This is particularly true in regard to light conditions. Begonias do best in partial shade or filtered sunlight. Often this is the key to success. Exposure to excessive sunlight results in burned foliage and flowers. At the other extreme, too much shade results in lush foliage and few, if any flowers. The more sunlight a begonia receives without burning, the larger and more abundant the flowers will be. Some protection from wind is essential. Begonias are quite brittle and are sometimes damaged by strong winds. Light breezes are beneficial. Since begonias are tender, they should be started indoors and set out after all danger of frost is past in the springtime. The first light frosts of fall and winter will not harm the tubers. However, in cold climates, such as the eastern United States, it is advisable to lift the plants with some soil and move them to a frost free place before a freeze sets in. In this way, loss of the tuber by freezing will be avoided.

Inspect the stored tubers around February 1, discarding any that have become soft. Mold on the surface is a sure sign that rot is present. Check the tubers again about April 1. By then, most should be showing pink or white buds and some may have sprouts. Move those without buds to a warmer location (65 to 75 degrees). Handle the sprouted tubers with care, as the sprouts, which are the future stems, are easily broken off.

When a greenhouse or similar facility is available, tubers may be started as early as March. If no greenhouse is available, wait until mid-April. The starting mix is the same as the growing mix, but without fertilizer. In a plastic bag, combine the mix with water until uniformly damp. Use flats or pots filled to a depth of four inches. In flats, space the tubers four to six inches apart. Plant tubers concave side up, covered with half an inch of damp mix. Tubers showing no buds or sprouts may also be planted. Maintain the temperature between 65 and 75 degrees.

Keep the mix damp but not soggy. Provide good light when sprouts are an inch high. When the stems have grown four to five inches, replant in the growing mix with fertilizer (described above). Harden before moving the plants outside permanently. For most areas, this is best done around the first of June. Further instruction is the same as for the seedling plants.

To grow in a pot, place a piece of broken crockery over the opening in the bottom of the pot to prevent the soil from plugging the hole, thereby insuring perfect drainage. Fill the pot one-third to one-half full with potting mixture. Carefully remove plants from the starting flat. Place in pot and fill in with pot mix around the roots being sure to add enough soil to cove the root mass lightly.

The most important factor in planting begonias in outdoor beds or large planter boxes is perfect drainage. The level of the bed should be even with or slightly higher than the surrounding areas. The most desirable soil is fairly loose yet has sufficient body and nutrients to allow normal development of the plants. Moisten the soil thoroughly several days before planting. When setting plants out in beds, each plant should be places so that the points of the leaves face the front of the beds as most of the flowers will face this direction. Use care to see that the soil is not left mounded around the stalk of the plant.

The most important factor to be considered in preparing any soil for tuberous begonias is perfect drainage. Begonias will grow in any good soil that drains well. For potting, use a mixture of approximately four part well-decayed leaf mold, one part garden loam, and one part coarse sand. If decayed leaf mold is not available, any good potting mixture well suffice. The mixture should contain humus, and it should be well drained. For outdoor beds, if you have a heavy clay-type soil or a soil which drains poorly, add leaf molds, sand, or both. If you have a very light or sandy soil, add leaf molds, peat moss, or some other organic materials. If manure is used, it should be mixed into the soil well in advance of planting time. Fresh manure is not advisable.

For potted plants, water thoroughly, but carefully. Then do not water again until the surface of the soil shows dryness. For plants in the ground, water the entire planting bed thoroughly but carefully. Use care not to let the soil become soggy. The next watering should not be until the surface of the soil starts to dry out. As the plants develop, the watering will become more frequent. An often repeated rule is water only when the surface of the soil shows dryness. In hot climates, mist the foliage at least once each afternoon.

Feed with half-strength water-soluble fertilizer with trace elements every ten to fourteen days.

Pest and Diseases
Tuberous begonias are relatively free of injurious pests and diseases. Slugs and snails can become a problem but are easily managed by baiting regularly. Most slug and snail granules will also control earwigs, which sometimes eat small, round holes in the leaves of begonias.

Stem rot appears as soft brown spots on stems, often starting at a cut, bruise, or damage caused by snails or slugs. Small spots may be cut away with a sharp, sterile knife or wiped away with a clean, dry, rough cloth. With large, deep areas of rot, cut off the stem far enough below the rot to expose clean tissue, then dust all cut surfaces with a rose dust, preferably one containing sulfur.

Powdery mildew is the most serious disease affecting tuberous begonias. However, its prevalence is decreasing every year with the development of more resistant hybrids and the use of improved control materials. As with most plant diseases, prevention is the best cure. Spray the plants with a mild fungicide every month, starting early in the season. If the mildew shows up in between spraying, re-spray the infected plants.

Pruning and Maintenance
Extraordinary flower size is seen in begonias grown in greenhouses, protected from the elements. The flowers will face the direction in which the leaves point. If there are several stems, flowers may face in several directions. Pinch the first few flower buds when they are dime-sized, being very careful not to break the growing tips. Each flower bud has a center (male) and two side buds (usually female). To get the largest flowers, remove the two side buds. Do not remove them on the hanging basket types. When the lower leaves begin to brown, remove them to allow better air circulation, which is most important. When removing leaves or picking buds or flowers, leave a short stub attached to the stem. It will heal and fall off in a week or two.

When the plants are about one foot high, support them with stakes. The stems are quite brittle and are easily broken off. Handle them with care. Tie your plants to the stakes using flat plastic ribbon rather than with wire, cloth, or cord.

After the first light frost, or before the first hard frost, cut growth back to six inches high. Lift the tuber together with a ball of soil and store, without watering, in a frost-free location with some daylight. The stems will separate from the tubers in a few weeks. After the stem is removed, gently wash the tubers without damaging the roots. Dry them in a sunny spot for a few days until the surfaces appear dry. The tubers may be stored in trays in a cool, dark location. If the humidity is low, cover them with slightly damp peat moss. Leave the trays or containers open for air circulation.

For more information on begonias, see the website for the American Begonia Society at