Begonias: Brilliance in the Shade

Tuberous begonias are my favorite plants for shade. They are a fast-growing group of plants that maintain a brilliant display all summer and fall. Their beautiful flowers in luscious colors and distinct leaf color patterns make them prized plants for the gardener. For gardening purposes, they can be divided into two groups: tuberous-rooted and fibrous-rooted (common bedding varieties). We’re going to talk about the beautiful tuberous begonias.

Begonias adapt easily to outdoor culture in our climate, but need to be planted each spring. Tuberous begonias generally begin to flower in June or July and continue until frost. Though tender, the tubers can be lifted after frost kills the top growth and then stored indoors over winter.

Planting: Plant in spring when soil is moist, but not wet; remove the top few inches of topsoil and cultivate the remaining soil. If the soil is heavy, work in organic materials like compost, sand or peat moss. The goal is to have a spongy, loose, humus-rich soil. In beds, plant the tubers level with the soil surface and with the hollow side up. Water lightly at first until plants grow strongly, then keep the bed moist. Mulch with about two inches of hulls, moss, etc.

Potting Medium: Begonias are not especially fussy about potting soils, but they like a light, porous soil which should be pressed gently but firmly about the roots, if planting in pots. When potting, use the azalea pots, those that are shorter than they are wide, as most begonias do not have a deep root system. They detest being over-potted, and they do not grow in pots without drainage holes in the bottom.

Light: Proper light will encourage rich leaf color and profuse blooms. In summer, begonias need protection from midday sun. The rhizomatous and hairy leafed types need plenty of filtered light but no direct sun. (Leaves become pale with overexposure). Rex (variegated types) and other tender varieties will only grow outdoors in morning and late afternoon sunlight, however, the Rex begonias require fairly bright light to maintain the beautiful color patterns in their leaves. This is a general guideline: begonias with lighter leaf colors need protection from midday sun, whereas those with darker leaf colors can tolerate more sunlight.

Humidity: Begonias prefer a relative humidity of 40-60 %. Sounds like a lot? To increase humidity in outdoor patio pots, place plants on a drip tray filled with small, clean pebbles that have been washed in hot, soapy water and rinsed thoroughly. Fill the drip plate with water so that the water does not cover the pebbles; set the begonia on top of the pebbles where evaporation from the pebble tray will considerably increase the relative humidity. Pots can also be set in larger containers, packing peat or spaghnum moss between the two. Do not use misters on your begonias. In shady beds which are watered regularly to keep the soil just damp, sufficient humidity is usually not a problem.

Watering: Ideal watering time is in the morning, but do not wet the leaves. Many begonias have minute hairs where water droplets cling, causing a situation that could develop into a leaf spot. In general, water only when the soil surface is dry to the touch. Adjust your watering to changing growing conditions and to the needs of your plants. For example, during cloudy weather you will not need to water your plants as often as you would during sunny times and flower bud drop.

Fertilizing: A regular fertilization schedule will help produce beautiful begonias that bloom well. Start fertilizing about 3-4 weeks after you have potted your plants. However, do not fertilize a plant that is dormant or resting. A properly fertilized plant will be vigorous and more resistant to disease.

Pests and Diseases: Begonias are relatively free of pests and diseases. Overwatering, or poor drainage is the main culprit and may cause black rot in the roots which spreads up through the plant. Powdery mildew appears white and powdery in small round patches on the leaves. It most often develops when the temperature is low and humidity is high. Good air circulation, ventilation, and watering in the morning will help prevent powdery mildew from developing. Treat with a fungicide, if necessary.Leaf spot is usually a round, dark spot surrounded by a yellow halo. These spots are most prevalent in the summer when the temperature is high and the humidity is high. Avoid splashing water on the leaves to help prevent this problem. Treat with a fungicide if it does occur.

Now that you know the basic principles of good culture, you, too, can enjoy the beauty of begonias.