Rex begonias require the same basic care as the other types of rhizomatous in regards to pots. They do best in clay pots or wooden pots, at least in California. They will grow in the same potting mix as all other begonias. They should be repotted and revamped yearly. They don’t seem to be long lived plants unless refreshed yearly. Most have large leaves but avoid using pots that are too large. Like with the other rhizomatous, they do best in shallow pots. Rex begonias will do very well when grown in beds in shadier locations.
Rexes don’t require much pruning if revamped yearly but overgrown plants can be trimmed back to within the bounds of their pot. They can be pinched to encourage fuller growth but you should avoid this on the larger leaved varieties. They could become too crowded and not have room for leaves to open to full potential. Plants with one large rhizome, however, may benefit from pinching to force out more size growing rhizomes.
Watering and Fertilizing
Avoid keeping rexes too wet. Allow the mix surface to dry slightly before watering again. Rex leaves are delicate and disease prone. You should avoid overhead watering or getting water on the leaves except for an occasional rinsing for grooming purposes. Only do this during a time of day where the leaves will dry off quickly. Avoid getting the leaves wet during hot weather or leaves my burn, even in shady areas. You should fertilise weekly with quarter strength fertiliser any time plants are actively growing. Rexes planted in beds do best with soaker type watering. I have seen nice beds of rexes where they were watered with sprayers, but leaves can be damaged with this type of watering, especially during cool weather.
Light and Heat
Although rexes bloom, they aren’t grown for the blooms. In fact, many rex growers keep the blooms removed. For this reason, rexes can grow in shadier locations than some other types that need good light to bloom. Although pure shade isn’t recommended, you can use rexes in those areas that aren’t suited to your blooming types. Filtered light most of the day is preferred.
It is a common urban legend that rexes rest for the winter and go into semi-dormancy. I think this is just a myth. Rexes do have the habit of defoliating and not growing during the winter months. This is more due to an intolerance of cool wet conditions. They drop their leaves as a survival method but not by choice. Most rexes will continue to grow and stay nice if given protection for the winter. Many growers move them into greenhouses or put up temporary shelters for the winter. As long as you keep them from staying soggy wet from winter rain, even if they defoliate, the will recover with warm weather. Miniature rexes should always be brought indoors or into greenhouses, they are almost impossible to winter over outdoors in California. It may be different for Florida growers.
Rex begonias are not very heat tolerant. In areas that get very hot, they may require special conditions if you are to be successful with them. Growing close to the ground as mentioned will help but you may also need some type of misting system to help keep the air temperature down.
Pests and Diseases
Rex begonias are prone to the same pests as most other begonias. The main one being mealy bugs, the scourge of all begonias. As a group, they are disease prone however. They are especially prone to powdery mildew and most will require some type of routine spraying. Unlike most plants where mildew appears on top of the leaves, rexes can have the undersides get covered with mildew too. When grown low to the ground, this will also help with mildew problems. If they seem especially prone in one area of your yard you can also try moving them to other areas to see if other microclimates in your yard are more satisfactory. Rex hybridizers are continuously working on new varieties especially ones that might be less mildew prone or heat sensitive. If you haven’t done well with older varieties, you may look into trying more recent introductions.
Rex begonias can be started in all the same ways as the other rhizomatous begonias. Refer to section on rhizomatous for more information. If you don’t care about named varieties you can also find many sources of rex seed to grow your own. If colour and quantity are what you’re interested in, seed can be the best way to go. Many rex hybrids are circulated without names anyway.
Special Uses and Tips
Most rexes do best when grown on or close to the ground. Most rex growers grow them in pots either sitting on the ground or on very low shelves. It’s believed they prefer the coolness and greater humidity at that level. I think that is true. Regardless of the reason, the do grow best there. You may be able to grow them in baskets in a greenhouse but it wouldn’t be recommended for outdoors because of the mildew problem you’d have.
Credits / references:
Brad Thompson | Brad’s Begonia World