Cultivation of Tuberous Begonias

The tuberous root system of these begonias enables them to become dormant over winter. This means tubers can be lifted and stored. When the growing season approaches, they can then be potted either in containers for outdoor display, or placed in the garden once the weather is warm enough for flowering to begin. So, the environments in which tuberous begonias are grown include the garden, the greenhouse and shade house, and conservatory. Wherever they are, matters such as air circulation, humidity, water and light need to be considered. Arguably the most important of these is air circulation.

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Air circulation

Tuberous begonias prefer cool to moderate temperatures and will struggle at anything constantly above 86°F (30°C), whether indoors or out. Furthermore, the one thing they thrive on above all else is good air circulation; therefore, plants must not be overcrowded. Good spacing is critical to prevent stagnant air, for this predisposes the plants to fungal diseases such as botrytis and powdery mildew, especially toward the latter part of the season when night temperatures drop and humidity is still high.

While they enjoy good airflow, Tuberhybrida may be damaged by excessive wind and marked by heavy rain. The best results, particularly worth the effort with the better quality plants such as named begonias, will be achieved with some form of protection. If you are growing indoors during the flowering season, obviously protection from the elements won't be a concern. But if your collection is very large and you want perfect blooms, you will need some form of specific shelter designed to house plants. The choice of this shelter depends largely on the climate. A greenhouse will give full protection from the elements and, if heated, will protect also from unseasonable cold, although this adds to the cost. Thus, in some colder areas, a greenhouse may still be the preferred location to grow tuberous begonias.

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The use of a greenhouse presents several problems. On hot days, some means of forced ventilation is desirable, as well as shading (such as blinds) to prevent burning. Ideally the ventilation should be automated; otherwise, someone should be available to watch for sudden temperature fluctuations. Unfortunately, the smaller the space the harder it is to control the environment.

A shade house has the advantage of providing excellent ventilation, while judicious placement of the shade cloth filters the wind and provides protection from direct sunlight. If the shade house is built with some form of permanent roofing material such as corrugated plastic, there will be maximum protection against rain, yet at the same time a good amount of light.

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Begonias love a humid atmosphere whether indoors or out, but it must be associated with warmth. Warm and damp are fine, but cold and damp spell disaster. During spring and fall, humidity is easily achieved-the difficulty for plants outdoors is often the lack of warmth because of the cooler evening and night-time temperatures. In summer the opposite applies-plenty of heat, but a lack of moisture. During the hottest part of the day, watering the soil around the plants (not the plants themselves) will help. Indoors, placing the containers on or next to a tray of pebbles half-filled with water will also put more moisture into the air (check. the water level regularly). Maintaining reasonable humidity is often the most difficult aspect of growing begonias in a conservatory, as most other occupants prefer a drier environment. If a shade house or greenhouse is used, watering the floor will rectify any humidity problems. Do not extend this past mid-afternoon, to allow for a reduction in air moisture with the onset of cooler evenings.


Like all begonias, Tuberhybrida require good light, but whatever their situation, they do need some shade. They will tolerate direct early morning or late afternoon sun, but will burn badly if exposed to the extreme heat around midday. Be careful where you place your begonias. A situation too near a south-facing window in summer can cause extreme temperatures. Good light levels will ensure that the plants flower well, thus producing the vibrant, colorful blooms they are renowned for. Poor light will inhibit flowering, resulting in small blooms with poor color and the plants will become tall and spindly.

Growing in difficult conditions

The key thing to remember is that the basic requirements of the plants remain the same; in anything less than the ideal situation, the grower should be seeking to modify local conditions to suit.

Cold zones
All begonias are susceptible to frost. However, they can still be grown in the harsher areas of the globe, if given protection.
If the dormant tubers of tuberous begonias are lifted before the ground freezes and brought on indoors to start flowering, they can be planted outside for the short and intense flowering season, if the necessary 60°F (15°C) is maintained during this time. Experiments have shown that by giving a plant sufficient light and some warmth, they will continue to grow and flower.
Of all begonias, tuberous begonias are the best suited to colder zones because of their winter dormancy. In such areas, the warm summer months are greatly reduced. The plants, whether for container culture or the garden, need to be grown in a protected environment, especially at night, for an extended period until conditions are frost-free.
The use of a heated greenhouse, plastic tunnel house, or other protective structure that can be heated, during the colder months, is a great advantage, but use can also be made of rooms within one's home. In this case, additional lighting may be needed, especially if the plants are growing in a heated basement.
If grown close to a south-facing window, they would have adequate light, but protection from burning with the use of sheer curtains may be necessary. In all these situations, additional ventilation with fans should be considered.
As soon as conditions are warm enough, the plants should be gradually hardened off for several weeks by placing them outside during the day and returning them to cover at night. After this, varieties intended for garden beds can be planted out and the containers safely left outside. Although the summer season is very short, the greater length of the daylight hours helps to counteract this by intensifying both the color and the quality of the blooms.
Hot zones
Such areas, being frost-free, may provide suitable conditions, at least in terms of temperature, for the non-tuberous begonias.
But tuberous varieties are not very tolerant of extreme heat and generally do not grow well if the daytime temperature constantly exceeds 86°F (30°C). That is not to say they cannot be grown successfully in hot conditions, even in areas where the daytime temperature may reach as high as 105°F (40°C).
In these locations, extra shading needs to be provided and, of course, excellent ventilation.
Some growers also use humidifiers .Extra care is also required with watering to prevent the plants from drying out too quickly.

Fertilizers and soils

The letters NPK represent the three main nutrient compounds of a fertilizer. N stands for nitrogen, essential for cell extension, protein building and photosynthesis, i.e., plant growth; P is for phosphorus, which plants need for photosynthesis, the development of roots and the production of flowers; and K is for potassium (potash), which protects from disease and improves the color of flowers.

Fertilizers, when added to a growing medium, supply nutrients (NPK and other trace elements) essential for the health and growth of the plant. There are two main types: organic fertilizers, which are derived from plant and animal remains and often referred to as manures; and inorganic, which are manmade and manufactured by chemical processes.

Organic fertilizers help improve the physical condition of the soil by the addition of humus and micro-organisms. The nutrient content will vary enormously, depending on the source of the manure. Inorganic fertilizers provide nutrients that are usually water soluble and therefore quickly available. They are in higher concentrations of NPK and are able to be applied in measured doses.

A complete fertilizer will contain these three major nutrients, and the relative proportions should be stated on the container. Flowering plants like begonias need a relatively high proportion of potassium (K) to that of nitrogen, otherwise fleshy growth and small flowers will result. This may be qualified slightly in that, early in the begonia season, before flowering, a fertilizer with a somewhat higher proportion of nitrogen (N) is desirable to get good plant growth established.

Slow-release vs. standard fertilizers

In the past, a base fertilizer was added to the mix and liquid feed was used during the season. This is certainly the direct, hands-on approach and one that arguably gives more control to increase, reduce or change the nutrients given to the plants. The result of such an approach, though more demanding and troublesome, should be better plants and flowers. At the level of serious competitive showing, this method may be preferred.

A relatively recent introduction in the world of horticulture is the slow-release type of fertilizers, which has eased the burden of working out a balanced feeding program. A number of these products are on the market, with the proportion of ingredients varied according to intended use, e.g., for shrubs, container plants or bulbs. In addition, the duration of their effectiveness can vary considerably, some being designed to release their nutrients in just three months and others for up to two years. Some also contain the trace elements essential to plant health. Although virtually foolproof, excess use of these products, as with any other fertilizer, will result in problems with the plants, so always refer to the manufacturer's directions.

Foliar feeding can be done two to three times per week, but must be stopped when no further flower buds are to be removed. Failure to do so will result in damage to the buds and resulting flowers. Spraying in full sun should be avoided, as scorching of the leaves will result. An alternative method is to water in extra food once a week along with normal watering. In this case, a high-potash fertilizer is recommended.  Always avoid giving fertilizer to dry plants, for this may cause root burn.

Potting mix

A primary requirement of a good mix for begonias is that it is open and free-draining. It is essential that water, as it drains from any mix, is replaced in the tiny spaces by oxygen. Oxygen is an essential ingredient to the life of any plant. It is necessary also that the particles of the mix are able to retain moisture, another essential requirement for healthy plants. Finally, the mix should contain a balanced supply of nutrients and be free of disease-causing organisms.

In the past, loam-based mixes were used for begonias. These generally consisted of topsoil (including grass and roots to a depth of about 2 in/5 cm) and cow manure rotted in layers for a year or more to make loam, and then put through a coarse mesh, before mixing with peat, grit (to improve drainage) and fertilizers. These loam mixes were relatively heavy and slow to drain.

The introduction and popularity of plastic containers with lower evaporation levels made watering even more difficult when using such mixes. Furthermore, good loam has become much harder to find. Labor costs have also played a big part in the demise of commercial production. Nevertheless, a few growers still prefer loam-based mixes and, provided they have a good understanding of the different approach to watering, achieve good results.

Today most mixes are soilless, made from a combination of various ingredients, most commonly with peat as the basis and substances added to improve drainage. However, peat is a non-renewable resource.


More than 90 percent of the gross weight of plants is water, so it is essential that a correct balance be maintained to ensure good plant health and quality. Water is essential for plant respiration, photosynthesis and the general metabolism of plants. It is also an active diluent of unwanted products. One of the first signs of insufficient water is the lack of turgidity in the foliage and stems, which, if ignored, progresses to leaf scorching and finally the death of the plant. Nevertheless, though our plants die without this essential element, more plants-especially houseplants-are killed through over watering than under watering.

Water quality

Water quality is very important. Contamination, such as with spray residues, will cause problems, as will hard water in areas where the catchment is in a limestone region. Deposits may be left on plant leaves following watering or spraying. More damaging, however, is the buildup of calcium, which will raise the pH level of the potting mix over the season. If this occurs, the plants will not be able to take up sufficient quantities of trace elements or phosphorus. To prevent these problems, collect and use rainwater, which is extremely soft. It will also be pure, unless a source of pollution affects the collection roof.

Deciding when to water

A plant's need for water is a matter of the individual grower's' personal judgment and experience. Watering requirements depend on many things: the size of the plant, the number of flowers it is carrying, the size and type of container used (clay, plastic or moss-lined basket), the type of mix, the temperature and the wind. A useful check is to pick up the container. If it is light, then it is dry; if not, no harm will result from leaving watering for another day. A moisture meter is a handy accessory.

Begonias thrive on being allowed to dry out a little before further water is added, because plants require oxygen as well as water to perform well. When the plants dry out a little, oxygen is drawn down into the spaces left in the mix as the water either drains out or is used by the plant.

When possible, it is good practice to water plants in the morning to diminish the risk of fungal disease. At all times, it is preferable not to get water on the flowers and foliage, since the flowers will damage easily, and wet foliage will burn if touched by the rays of the sun.


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