How to Plant and Grow Rex Begonias

Cherished for their beautiful leaves and diversity of forms rather than their tiny blooms, rex begonias are an amazingly colorful group of begonias perfect for growing at home. Derived from Begonia rex, the extensive group is now referred to as Rex Cultorum or simply “rex begonias” due to the complexity of their heritage. 

Rex, meaning “king” in Latin, is a perfect name for the group as their popularity has ensured that they’ve reigned over the trade since their introduction. Because the group is made up of many generations of hybrids, rex begonias include a variety of leaf shapes, sizes, and colors from many other species.

Although rex begonias have a reputation for being somewhat challenging to grow, this basic guide provides what you need to know about general care and some tips on how to grow rex begonias successfully. 

All parts of rex begonias are toxic to dogs, cats, and horses when ingested. The part of the plant that is underground is the most toxic.

Where to Plant Rex Begonias

Like most begonias, rex begonias are tropical plants requiring warm, humid conditions to thrive. Outdoors, rex begonias can be planted in containers in bright or shady spots in regions with ample humidity. They do well outdoors in USDA Zones 7-12 within a temperature range of 60°F-80°F, but they must be brought inside when the temperature drops.

Indoors, these begonias do well in east-facing locations or south-facing windows just outside direct sun. Areas in the house with higher humidity, such as bathrooms and kitchens, will help provide them with enough ambient moisture not to develop burned leaf edges. Keep the begonias far away from fans, heaters, and air conditioners. 

How and When to Plant Rex Begonias

Rex begonias can be planted or repotted indoors at any time of the year, but they do best while actively growing during the warmer spring and summer months.

Begonias can be moved outdoors during the summer months while temperatures remain well above 60°F and humidity is high. Rex begonias are not cold or frost tolerant and will quickly die back after even a light frost. 

Plant the rhizomes in well-draining potting soil, adding compost or perlite to improve drainage.

Rex Begonia Care Tips

Light

For the best color and health, give your rex begonias plenty of bright, indirect light. Direct light will burn most rex begonia leaves, while too little light can lead to overly long, weak leaves and stems. An east-facing location in the house with morning sun or just outside the reach of direct light in a south-facing location is ideal. Outdoors, morning sun or dappled light under a tree is preferred.

Soil and Water

Rex begonias are vulnerable to overwatering and can quickly rot if not planted in soil with excellent drainage. Use a quick-draining soil or add perlite, sand, or small bark chips to aid drainage. While drainage is essential, rex begonias prefer constant moisture with slight drying between waterings during their growing season.

Water your begonia thoroughly and allow any excess to drain away from the roots. Water plants when the top layer of soil begins to dry or the leaves start to flag (lightly wilt). 

Some rex begonia cultivars go through a slight dormancy period toward the end of the season as temperatures begin to cool. If your plants show signs of dormancy by slowing growth and dropping leaves, cut back on watering until growth resumes.

Temperature and Humidity

Rex begonias prefer temperatures similar to those people enjoy, making them great houseplants. Daytime temperatures in the 70s or low 80s and temperatures decreasing by about 10°F at night are ideal. Humidity is key to keeping begonia leaves full and unblemished by unsightly brown edges and deformities. Keep humidity above 50% throughout the growing season. 

Fertilizer

During their growing season, rex begonias benefit from a constant supply of a well-balanced liquid fertilizer applied every other week, following the instructions on the product packaging. However, toward the end of the growing season, when plants slow in growth, cease fertilizing your plants to avoid overfeeding them. Resting begonias do not need to be fertilized and may even show signs of burn along their leaf edges or root rot if overfertilized. 

Pruning

While rex begonias do not require pruning, some touch-up trimming can be beneficial to redirect plant growth or remove dead leaves. If the plants begin to grow out of their pots, they can be pruned back and the cut off portions can be replanted, forming clones of the original plant.

Potting and Repotting Rex Begonias

When potting rex begonias, use a ceramic or terra cotta pot with drainage holes. Fill it with a quick-draining soil or add perlite to commercial potting soil. The container should be only 2-3 inches wider than the plant's root ball. Your begonia can stay in the container long as the rhizome isn't touching the sides. Once it grows large enough to touch the sides, it is time to repot your plant into a new container a few inches larger with fresh planting medium.

Pests and Problems

Begonias of all kinds are prone to a variety of pests, including mealybugs and aphids, as well as fungal infections and root rot. 

Indoors, mealybugs are one of the most common houseplant pests to affect rex begonias. Isolate infested plants and manually remove the insects with a fingertip or cotton swab. Next, spray the plant with neem oil or insecticidal soap. Multiple applications may be necessary to eliminate mealybugs and other pests. 

Root rot is a common problem for many begonia species and hybrids. To avoid root rot, plant begonias in loamy soil with excellent drainage. Perlite, sand, and ground orchid bark can be added to the soil to improve drainage, porosity, and airflow. Planting in terra cotta clay pots can also help to avoid overwatering. 

How to Propagate Rex Begonias

Rex begonias are relatively easy to propagate through leaf cuttings and division. However, harvested seeds are unlikely to resemble the parent plant, so seeds should only be sown when experimenting or creating new hybrids.

Leaf Cuttings: To propagate rex begonias from leaf cuttings, remove a leaf from the parent plant and use a sterile razor to cut through the underside veins on the leaf about a quarter of the way from the petiole (stem). New plants will grow from these cuts, so take care not to smash or otherwise damage the surrounding leaf.

Place the cut leaf top-side up on the surface of a sterile seed sowing mix, perlite, or sphagnum moss in a small pot. Place a few small stones on the leaf near the cut veins to maintain contact between the leaf and the planting medium.

Cover the top of the pot with plastic wrap to maintain humidity. Keep the pot in a warm, bright location. The temperature should be maintained above 70°F degrees. A horticultural heating mat can provide additional heat. Although bright light will help plantlets begin to form, direct sun should be avoided. Artificial grow lights can provide ample light without burning the leaf or new plants.

New plants begin to form in about three to four weeks. Be patient as this process can take quite a while. New plants will form leaves, rhizomes, and roots. At this stage, the plantlets can be divided and planted in individual containers.

Divisions: Turn one rex begonia into two by dividing the rhizomes. Remove the plant from its container and use your hands to pull the rhizomes and roots apart. Immediately replant the two sections in containers filled with potting soil. These plants can be divided at any time of year, but they respond more slowly in the winter.

Types of Rex Begonias

Although there are hundreds of varieties of rex begonias, here are a few that show the diversity of forms and colors.

'Escargot' Rex Begonia

Begonia rex 'Escargot' is a relatively common variety that is rightfully popular due to its wonderfully swirled green and silver leaves, appearing almost like the shells of its namesake. It grows 6-12 inches tall and wide.

'Fireworks' Rex Begonia

Begonia rex 'Fireworks' is another exceptionally colorful plant. 'Fireworks' produces heavily textured leaves with deep plum purple, green, and silver. It grows 10-16 inches tall and 18 inches wide.

'Shadow King Lava Red' Rex Begonia

'Shadow King Lava Red' is a bright red leaf with a distinctive shine. It grows 8-12 inches tall and spreads 15-18 inches wide.