Rhizomatous begonias as a group are generally more finicky about proper potting than the upright growing types of begonias. Rhizomatous begonias are shallow rooted. In the wild, they commonly grow in rock crevasses and on the banks of streams. Since the rhizomes are very fleshy and are in constant contact with the soil mix, they can rot easily if the mix stays too wet.
Rhizomatous begonias will grow in all the various types of pots. Many can even be grown as epiphytes using various methods such as on boards with a small amount of mix covered with sphagnum moss. Most rhizomatous begonias do best in shallow pots or bowls when using plastic pots. If you encounter difficulties growing this type of begonia using plastic pots, you may do better using clay azalea pots or clay bowls. These pots are more expensive and heavier but nearly all rhizomatous begonias will do well in them. Rhizomatous begonias will also do very well in wooden pots or moss covered baskets.
When repotting, keep the rhizomes at the same level they were in the old pot, or slightly lower. Plants can be pulled apart and the rhizomes rearranged in the new pot to be more symmetrical. Put all rhizomes either parallel with the rim of the pot or centre them with the growing tips toward the rim. Plants that are lopsided can also be recentered in the new pot to make them more symmetrical.
Rhizomatous begonias aren’t usually pruned the way you prune the upright types. Since most bloom in the spring, you should wait till after blooming to do any shaping unless blooms aren’t important to you. Rhizomes that are single can have the tips pinched out (removed) to make them put out new growth along the single rhizome. Rhizomes that have grown over the edge of the pot can be cut off inside the rim. The removed portion can be used to start a new plant or can just be placed back in the pot on the soil surface in a bare spot. The piece will quickly root and become part of the plant with no extra attention. Plants that have become overgrown or have crossing rhizomes can be pulled apart and made into several new plants.
In the spring, once the plants are actively growing and putting out new leaves, remove old and damaged leaves. New leaves will quickly fill in. Damaged leaves never get better so it’s best to remove them so the plant will fill in space with a new fresh leaf. Whenever removing old leaves, try to always break the petiole of the leaf off at the rhizome. This will prevent a left piece of stem from rotting. If this isn’t possible, either because the plant is too full or they seem hard to break off from the rhizome, cut the leaf off leaving a long petiole. In a couple days the left petiole should either fall off or break off easily from the rhizome and can be removed.
Watering and Fertilizing
Rhizomatous begonias should be allowed to dry slightly before watering again. Anytime they are actively growing they should be given quarter strength fertiliser weekly. Getting water on the leaves of most doesn’t seem to be a problem. However, if you have problems with botrytis during certain times of the year avoid getting water on the leaves at those times.
Light and Heat
Rhizomatous begonias generally will grow nicely in areas that are too shady for other types. There are varieties that will grow in fairly strong light and some that will even grow in pure shade. You should be able to find a variety of rhizomatous for nearly any light condition except full midday sun. The leaf colouring can change wildly with amounts of light. Some can be nearly black during the good light of summer and change nearly green with winter sun. If your plant doesn’t have the colouring it did when you got it or isn’t showing the colouring you expect from that variety, try giving more or less light to bring out the colour you desire. Some varieties have more colouring in shade, some have more colouring in more sun. You’ll have to experiment to see what amount of light is required to bring out the desired colour for any particular variety.
Most rhizomatous will do quite well during the winter without the protection provided they don’t get too wet during cooler weather. It isn’t normal for most varieties to lose all their leaves during the winter. If you are finding plants defoliating, they are either cold sensitive or are staying too wet. Provided the rhizomes are kept from rotting, the leaves usually will return with warmer weather. Plants that seem to be prone to this problem probably should be given protection.
Pests and Diseases
Rhizomatous begonias are prone to several types of pests. Since their rhizomes are usually covered by hairs and stipules and are under the leaves, it’s a good place for many pests to hide. If you chose to use an insecticide, it’s best to use a systemic type because it’s nearly impossible to hit all the bugs directly with a spray on this type plant. A systemic insecticide soaks into the plant and kills any bugs that were missed by the direct spray.
The most common pest, as with all begonias, is the mealy bug. They should be suspected on any rhizomatous where new leaves fall off without maturing, or where rhizomes die off. You can have a major infestation without being able to actually see the mealy bugs. You can scrape some of the stipules off the rhizome to check for them.
Aphids can also be a problem. They attack mostly young leaves just coming out. The damage appears as distorted or virus looking new leaves. They can be seen on very close examination of the growing tips of the plant. Thrips also cause the same type of damage but can’t usually be physically seen so should be suspected if you have distorted or damaged new leaves but can’t see a pest. Although an actual virus can cause the damage, it’s more commonly caused by insects.
Very few rhizomatous begonias are affected by powdery mildew except for a handful of the more tender ones so this isn’t usually a problem. A more common disease, especially in cool wet weather is bortrytis. Bortrytis appears as rotted portions of the leaves, usually at the petiole. Later these rotted portions are covered with a grey hairy mould. Keeping bad leaves removed and litter from other trees and plants picked up will help keep this problem down. For bad cases or constant problems with bortrytis, there are fungicides the will protect against it. Plants that seem especially prone from year to year may do better given winter protection from the elements. Bortrytis is the main cause for rhizomatous begonias to defoliate during the winter outdoors.
Prevention: Good hygiene, adequate ventilation, and plant spacing are of paramount importance in preventing the onset of botrytis. Pay particular attention to removing stems and leaves as they drop, for they will cause the infection to start when left lying on other parts of the plant.
Of all the begonia types, rhizomatous are the most versatile in the ways they can be propagated. Nearly all will grow from leaf cuttings or rhizome cuttings. Leaves can even be cut up further into wedge cuttings or cone cuttings containing a piece of leaf with a main vein. Most will root quite easily in any sterile damp medium. Leaf cuttings should be rooted in an enclosed container to keep the conditions sterile and the humidity up. It may take a few months before you get a plant from a leaf cutting so they do need perfect conditions while rooting. Leaf cuttings can be rooted out in the open in small jars of water but this method is less reliable. Leaf cuttings will give you the most plants in the long term.
Rhizomatous begonias can also be grown from rhizome cuttings too though. Long rhizomes can be cut into sections or you can use just the tips from the rhizome. They will root faster if given an enclosed container to root in but that isn’t necessary for most varieties. Rhizomes can be rooted directly in small pots of mix. The rhizomes should be pushed down slightly make good contact with the mix. Afterwards, treat as you would a rooted plant and the rhizomes will quickly root and send out new growth.
Special Uses and Tips
Rhizomatous begonias can be used as landscape plants for shady or semi-shady areas. Many are low growing and make excellent ground covers. They do especially well with soaker type watering systems. Some get very large and can be used as specimen plants in the landscape. Nearly all have a mounding habit of growth so even the largest leaved varieties don’t get overly tall except for the upright jointed varieties.
Rhizomatous begonias can be used as hanging baskets. They do best in moss baskets or wooden hanging pots but can also be grown well in plastic baskets. In hanging baskets the rhizomes can be allowed to grow over the edges of the pot in a trailing manner. If you’ve had difficulty growing rhizomatous begonias in the past, try switching to clay pots to see if that will make you more successful.
Rhizomatous begonias are heavy bloomers. Keep fallen flowers picked off the leaves though to avoid having damaged leaves.
Credits / references:
Brad Thompson | Brad’s Begonia World