Rhizomatous begonias comprise one of the largest if not the largest group of begonias.
They are differentiated from the other types of begonias in that they grow from stems (rhizomes) that grow along the surface of the soil. As they grow, the stems put out new roots. There are some semi-upright rhizomatous but even these will fall over and root back to the soil like the other rhizomatous if allowed. There is also a group of rhizomatous that put up upright stems from the creeping rhizome.
Begonia ‘Bushmaster’Most rhizomatous begonias are grown for their interesting foliage that comes in various colors and patterns. The majority being shades of green, black, silver and brown. Many also have interesting spirals in the sinus of the leaf or ruffled edges.
Nearly all are seasonal bloomers that require a short day period to set blooms, so are late winter to spring blooming. Even though not grown for their flowers, they do put on a spectacular display of blooms during their short bloom season. Except for the few upright types, most rhizomatous begonias make attractive mound shaped plants.
Rhizomatous begonias can live in less light than most of the other types so are the best choices for really shady areas. Many also do quite well as ground covers for shady or semi-shady areas.
Horticulturally the rhizomatous types are broken up into a couple dozen types based on leaf size and growth. For the purpose of this article, culturally there are four basic types, common type, upright type, upright jointed, and distinctive foliage. Most of the cultural items of this page apply to areas where begonias can be grown outdoors all year. Begonias can be grown outdoors in cold climates but must be brought indoors before frost in the fall.
Common Type Rhizomatous
I’m calling this type “common” for purposes of this page because the majority of rhizomatous begonias fall in this category. Most of these are easy to grow when given proper growing conditions. They come in every sized leaf from miniatures to leaves that get two foot or more across. They range in colour from plain green to black with various markings of brown, black, or red. A few are ever blooming but the majority are spring blooming. The blooms rise above the foliage. Most are white, pink, or red blooming. There are a handful that bloom yellow but these all require terrarium or greenhouse conditions. Many of this type have spiralled leaves and a few have crested margins. Most are grown for their interesting foliage. There are also dozens of varieties with silver markings. Examples of this type are B.’Cleopatra’ and B.’Freddie’.
Upright Type Rhizomatous
This type comprises only a few varieties. Many are only semi-upright and grow more in a sprawling manor. They come in all the common rhizomatous shapes and colours including some with compound leaves. Most are spring blooming. Some have the odd characteristic of losing their leaves during the winter and putting up blooms from the bare stems. Some of these such as B. crassicaulis has been in question as to whether it’s upright rhizomatous or should be moved to the thick-stemmed type. Currently, it and like begonias are considered rhizomatous. A few of the upright rhizomatous have mixed parentage so grow more upright than species of this type. There are several of this type that has crested edges. Examples of this type are B.’Madame Queen’ and B. manicata.
Upright Jointed Rhizomatous
The full description of this type is Rhizome jointed at or below the soil. I’ve shortened it to upright jointed to save words. This type grows by spreading rhizomes either on the soil or under the soil. It sends up cane like stems from the rhizome from which the leaves and blooms arise. This is different from the other types of rhizomatous where the leaves and flowers are attached to the rhizome itself. Nearly all upright jointed rhizomatous have distinctive foliage, some could almost be considered rex begonias by their colouring. Unlike the other types, most are summer blooming or ever blooming in a greenhouse.
This group can be challenging to grow. Most do best if given winter protection. They have large thin leaves which damage easily even under the best conditions. A well-grown example, however, is always a show piece. Most require staking to stay upright. During cold weather, they can lose all top growth. Unless the rhizomes are damaged by cold or dampness, they will send up fresh growth when warm weather returns. Examples of this type are B.’Connee Boswell’ and B.’Charles Jaros’.
Distinctive Foliage Rhizomatous
These rhizomatous are separated from the other types because of either their foliage colouring or texture. Many are nearly rex-like in colouring but because they don’t have B. rex in their parentage are classed this way. This group varies widely in difficulty, some being some of the easiest rhizomatous to grow, some being among the most difficult. A couple of begonias included in this class, B. gehrtii and B. paulensis, are still in debate about whether they belong here or as shrub types. Many of this type, such as B.’Silver Jewel’ are terrarium or greenhouse plants except for Florida where they are used for groundcovers. In fact, the good share of this class prefers greenhouse culture except in the warmest more humid areas. There is always constant debate as to what constitutes distinctive foliage. Most of the rhizomatous with silver leaves are not considered distinctive foliage but many think they should be. I think one reason they aren’t, is because most silver-leaved rhizomatous are no harder to grow than green-leaved varieties. However many are considered distinctive and many aren’t and the reasoning for the choices isn’t apparent to the author. Examples of this type are B. goegoensis and B. ‘Wanda’.
Credits / references:
Brad Thompson | Brad’s Begonia World