The begonia family is among the largest family of flowering plants with about 1500 species along with an incredible number of cultivars, mutations and opportune seedlings. The American Begonia Society classifies begonias into 8 distinct classes. Rex begonias are herbaceous perennials in zones 10-12 so they are treated as annuals or houseplant specimens in Connecticut. Rex begonias do produce delicate flowers but are usually grown for their spectacularly colored and intriguingly textured leaves. Other common names for these plants are fancy-leaf, painted-leaf or king begonias.
The Rex cultorum group originated from an Indian species, Begonia rex. These plants were noted for their dark leaves with prominent silvery grey bands. They are actually a type of rhizomatous begonia. A rhizome is a thickened stem from which leaves arise. Around 1850, this species was introduced into cultivation and rapidly became popular as a houseplant. Hybrids began to appear with varying leaf patterns as Rex begonias hybridize quite freely. From each cross, whether on purpose or naturally occurring, there may be several hundred tiny seeds produced, each capable of growing into a unique plant. Interesting cultivars include ‘Escargot’ and ‘Froggy’.
Rex begonias are a bit demanding to grow but their incredible, vibrant leaf display is well worth the effort. Leaves will maintain their dazzling color and unique patterns best when grown in bright but indirect light. East windows are fine except for maybe the hottest, sunniest part of the summer. If grown with southern or western exposures, set the plants a few feet back from the window so that the leaves are not in direct sunlight. Plants also do well under fluorescent lights. Outdoors, Rex begonias can be planted in full or partial shade in containers, hanging baskets, or beds. Rex begonias are tolerant of the juglone toxicity produced by black walnut trees.
Water and Humidity Requirements
While plants like to be kept evenly moist, it is better to underwater them slightly than to overwater them. Too much water invites root rots and certain death. Wait until the top inch or so of potting mix feels dry to the touch before watering. Avoid getting the leaves wet as this may make the plant more susceptible to a disease known as powdery mildew.
Plants also thrive in an environment with relatively high humidity. They may do well in a brightly lit bathroom or kitchen where humidity levels are often higher. In homes with dry winter air because of heating, consider grouping plants together to raise humidity levels. Plants can also be placed on trays of wet pebbles colored marbles or other decorative non-porous items. Make sure pots are sitting on the pebbles and not in the water. Brown leaf tips are typically an indicator of low humidity.
Like many houseplants, Rex begonias should be fertilized when they are actively growing. Use a dilute, quarter-strength, water soluble fertilizer once every 2 to 3 weeks during the growing season. Refrain from fertilizing from October until there is new growth in the spring.
If plants do not receive enough light during the winter months, they will often go dormant. Leaves will begin to drop from the plant and as they do, reduce the amount of water given to your begonia. Keep the potting medium barely moist and look for new growth once the days begin to lengthen and temperatures increase. Also, note that growth slows down when temperatures fall below 60°F.
Container Selection and Repotting
Shallow, wide pots are best for this rhizomatous species. The variation in leaf color, form and texture is outstanding for Rex begonias. Colors include pink, red, purple, silver, grey, cream, green and burgundy. When potting plants up, consider matching or contrasting the container to the leaf color for an even more outstanding presentation.
Once the rhizomes reach the sides of the pot, it is time for either repotting or starting new plants. Typically, plants start to diminish in appearance after a few years so propagating by leaf cuttings will provide new plants to fill the void. When repotting, use a soilless media that is light and well-draining, such as mixes made for African violets. Rhizomes can be cut into two or more pieces depending on their root systems and number of leaves. Repot the more attractive and vigorous sections.
Rex begonias generally mature at 12 to 18 inches high and wide. Young plants and miniature varieties can be used in indoor container gardens or terrariums. One glimpse of the gorgeous leaves of Rex begonias will probably be enough to convince you to give these plants a try. Check out your local garden centers to see which cultivars they have available.
Diseases and Pests
As mentioned above, Rex begonias are susceptible to powdery mildew. Initial symptoms may include spotting and curling of the foliage followed by white patches on the leaves. As the disease progresses, foliage and shoots may become distorted, buds may fail to open normally, and foliage may turn chlorotic and yellow. Some leaf drop may occur. Keeping the foliage dry and providing good air circulation are helpful in preventing powdery mildew. Low-toxicity options for treating powdery mildew include neem oil and horticultural oil.
Rex begonias may be infected with spider mites in hot, dry circumstances. Initial symptoms of spider mite feeding are small yellow or whitish stippling (spots) that appear on the foliage where chlorophyll has been removed. Leaf or needle drop may be caused by dehydration due to loss of water by such feeding. In heavy infestations, plant vigor is often reduced and unsightly webbing may be seen on the foliage. Insecticidal soap, summer oils, and miticides can be used for control of spider mites.
Rex begonia is occasionally troubled by mealy bugs. The nymphs of mealy bugs feed on the sap of the plant by piercing the outer layer of plant tissue and then they secrete the honeydew that can lead to sooty mold and is also attractive to ants. Mealybugs can be controlled by wiping the area with a cotton ball dipped in rubbing alcohol.
Despite good cultural practices, pests and diseases at times may appear. Chemical control should be used only after all other methods have failed. For pesticide information or other questions please call toll free: 877-486-6271.
UConn Home and Garden Education Center, 2019