The Shoestring Gardener
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Tuberous Culture

Potting

Tuberous begonias will grow in a variety of pot types. The most commonly used pots for tuberous are plastic. Some species such as B. boliviensis, which has a very large tuber, may do better in a clay pot to avoid any chance of over watering. Although tuberous begonias can be started directly in the pots they are to grow in, they usually are started in flats or smaller pots first. Most will require at least an eight inch pot but starting them directly in that size is more risky than starting them first and transplanting. Large numbers of tubers are usually started in shallow nursery flats filled with mix or oak leaf mold.

Mike Flaherty (left) visiting Mike Stevens and his Tuberous Begonia collection, photo by Mike Flaherty.
Mike Flaherty (left) visiting Mike Stevens and his Tuberous Begonia collection, photo by Mike Flaherty.
If you’re starting a small number of tubers, you can substitute four inch pots. The tubers shouldn’t be placed into flats or pots for starting until the eyes on the tubers have started to sprout. This is a sure sign they are ready to grow. Planting sooner runs the risk of rotting tubers.

The top of the tuber is the side with the depression in it. When starting them, cover them shallowly and keep slightly moist until you have shoots a few inches tall. Once you have shoots, they can be transplanted directly into eight inch pots.

When transplanting, plant them an inch or so deeper than you started them at. This will cause more roots to form above the tuber and make for a sturdier plant.

If planting the pendula hanging type tuberhybrida, it’s best to use at least three tubers per basket to make a full plant. Tuberous begonias can be grown easily in beds but the tubers should be started first before planting in the beds. They should have soaker type watering or carefully watered with a wand to avoid wetting the foliage. Overhead or spray watering may cause mildew problems or damage the blooms, at least in many growing conditions.

Pruning and Staking

Tuberous begonias don’t require pruning. Some of the species and pendula type tuberhybrida should have the tips pinched out when they are a few inches tall to promote fuller growth. The tuberhybrida are not usually pinched. Pinching these will force branching but reduce the flower size. Most of the upright growing tuberous require staking to keep the stems from breaking off at the tuber. It is best to insert a stake when planting the tuber so you can place the stake next to the tuber. If you wait to stake later, make sure you insert the stake or stakes for multiple stems, far enough away from the stem that you don’t push it down through the tuber. It is best to use the plastic stretch type tie since the stems are soft and can be damaged if wire ties are used. For some very full growing tuberous varieties I cut down round tomato cages and put them into the pot when the plant is young. As the plant grows up it will fill in and be supported by the cage so doesn’t usually need ties. If growing for a show however, you need to use proper stakes.

Watering and Fertilizing

Tuberous begonias usually require more water than some other types but should be allowed to dry slightly before watering. They will let you know if they are improperly watered by dropping flowers or wilting. They should be fertilized weekly with quarter strength fertilizer all during their active growing period. Stop fertilizing when plants shows signs of winding down or a month or so before you expect them to do dormant. Double flowered tuberhybrida will put out single or undersized flowers if not fed properly. Avoid getting water on leaves and flowers except for occasional necessary rinsing for grooming purposes.

Light and Heat

Tuberous begonias require good light in order to grow or bloom properly. Morning sun or filtered light all day is preferred. They won’t grow in extremely hot or dry areas. They grow best in coastal areas of California and northern climates. They either won’t grow or won’t grow well in most areas of Florida and Texas where other types of begonias are commonly grown because they can’t tolerate the high temperatures.

Pests and Diseases

Since tuberous begonias go dormant part of the year, they shed most of their pests at that time so aren’t usually pest prone. The only pest that is a common problem are thrips. Any time you see distorted new leaves or flowers you should suspect thrips. They can sometimes be seen moving quickly if you pull flowers apart but are extremely small. Thrips can only be controlled with an insecticide. Routinely spraying tuberous begonias is recommended to avoid a thrips problem. Waiting till you see damage will usually result in having bad looking plants for the rest of the season. They will survive but once the damage is done they probably won’t look pretty again that season unless caught early. Since thrips hide in the buds and flowers, a systemic insecticide works best since it’s nearly impossible to hit them directly with any other type spray. The tubers can sometimes have mealy bugs or borers on them so should be sprayed or dusted before storage.

The Shoestring Gardener Hundreds of Eco-Friendly tips

Tuberous begonias can be mildew prone in various locations or during poor weather conditions. Some may require a fungicide to keep mildew in check. Some fungicides may discolor or damage flowers so should be tested on one plant first. If you notice stem rot or rotting leaves this is a more serious fungal disease. Remove damaged stems and leaves and start routinely spraying with a fungicide that specifically says it treats other fungal diseases than powdery mildew. Mildew spray don’t usually treat this type of fungus disease. Also try to start watering in the morning so the inside of the plant and the leaves have time to dry before night time, and make sure to let plant dry out before watering.

Propagation

Tuberous begonias can be started various ways. They can be grown from seed, from stem cuttings, from tuber sections or from leaf cuttings. Some that form bulbils from leaf nodes can also be started from those.

Seed should be started early enough in the season so plants have time to get some size before moving outside. Plants grown from seed started early enough will perform nearly as well as plants grown from tubers but usually aren’t their best till their second year. After the first year, the tuber may only be pea sized so you may have to look carefully after they go dormant to find the little tuber. The tuber will increase in size every year.

Tubers can be sectioned like potatoes with at least one eye per section. These sections should be allowed to heal before planting and dusted with a fungicide. Since many tubers are very expensive I don’t recommend sectioning the tubers. If you’d like to try that method of propagation, practice with cheaper tubers before trying it on any expensive tubers. Most experts advise against sectioning tubers as the cuts never really form a thick skin so can rot at that point, even years later. This method should really only be used to save tubers that have rotted sections and even them you should take stem cuttings later to start a new tuber. Some species such as B. sutherlandii form clusters of small tubers. These can be separated into individual plants each year.

Tuberous begonias grow easily from stem or leaf cuttings. Young growth works best and cuttings should be taken early in the year. The plant grown from the cutting needs time to grow it’s own tuber before it goes dormant the next fall, so the earlier the better. Tuberous begonias don’t root well in water, except for the smaller stemmed types like the caudex forming types and B. sutherlandii. Most tuberous stem cuttings will start directly in potting mix. They may look wilted initially but most will eventually root and recover. It’s best to keep them in shade until they have rooted. Leaf cuttings should be started as you would rhizomatous begonias. In my experience leaf cuttings will look funny the first year as they usually just spend their energy making a tuber. The second year the tuber will usually put up stems.

Special Uses, Tips, and Winter Storage

You can find tuberous begonias to fill nearly every niche except full sun or deep shade areas as long as you have a climate where tuberous begonias will grow. Some begonia growers have collections exclusively of tuberous begonias. They put on a magnificent show during the summer and fall and since they are dormant for the winter, the grower gets a rest too. In Southern California tuberous begonias aren’t commonly entered in shows so this is a good category for new growers to try in those areas.

A few varieties such as B. sutherlandii, it’s hybrids and also B. dregei varieties can be kept growing through the winter by growing in terrariums or under lights. Most tuberous varieties however will go dormant and must be stored for the winter. In late fall your plants will start to wind down and foliage become yellow. Once it’s apparent they are stopping growing for the season, stop all watering and put in a location where they won’t get rained on. Eventually the plant will die back completely.

You can store the tubers in the pots they grew in but it’s better to remove all the soil mix and dry the tubers further in open flats or boxes. Tubers planted in beds should also be dug up and dried. Many growers dust the tubers with a fungicide or insecticide powder at this time to eliminate any pests or diseases before storing the tubers. They can be stored in cardboard boxes or paper bags. Some growers store them in sawdust, vermiculite or peat but that isn’t necessary. Whatever they are stored in should be dry and be able to breathe. Plastic bags aren’t recommended since they hold moisture and cause diseases that can do damage to the tubers.

Tubers should be stored in a cool dry location. Storing them in overly warm areas may cause them to sprout prematurely, too cold may cause rotting. Tubers should be inspected routinely over the winter to check for rotting or insect damage. Prompt attention may save tubers with problems if caught early. Rotted parts can be cut out and the remaining tuber treated with a fungicide. Storing each tuber in it’s own paper bag will keep potential problems from spreading to other tubers. When you’re almost ready to plant the tubers, you might move them to a warmer place to help them sprout out sooner.

If you’re really lazy and live in a temperate climate like southern California, you can leave the tubers in the pots they were growing in and just tip them on their sides for the winter to keep them dry. In the spring when they should be starting to sprout, turn the pots up again and start watering. This method isn’t as good as proper storage but better than doing nothing and most tubers will usually come back though you will lose a few. See tuberous page one for more info on winter storage and restarting the tubers in the Spring.

Credits / references:
Brad Thompson | Brad’s Begonia World