Begonia thiemei

The genus Begonia comprises over 2000 described species, found in tropical and subtropical regions worldwide, inhabiting all imaginable habitats from deserts to rainforests. Some are very rewarding, easy, and fast-growing houseplants, while others are highly specialized jungle dwellers that can only be cultivated in terrariums. It is therefore almost impossible to provide general care tips for begonias that apply to all species.

However, a rough classification can be made based on growth habit, size, and leaf color:

• Species with purely green leaves usually come from open, sunny habitats and are better suited as houseplants.
• Species with very colorful, curiously shaped leaves are usually more suitable as terrarium plants. A notable exception are the very colorful so-called “Rex Begonias.”
• Large, upright-growing species (cane begonias) are usually more robust than small, creeping species (rhizomatous begonias).

Depending on their group, they should be cared for accordingly. More on this in the other tabs.

This begonia originates from the undergrowth of rainforests and is accordingly adapted to semi-shady to shady conditions. It grows best in a protected environment with high humidity, such as a plant cabinet, terrarium, bottle garden, or grow tent.

When cared for under grow lights, moderate lighting is sufficient. At 2000 to 5000 lux or 35 to 100 PPFD (µmol/m²/s), it grows well, and the leaves develop ideal coloration.

It prefers a humidity of 70% or more and still thrives at over 90% RH. The leaves should not remain constantly wet and ideally should not be misted. Plant pathogenic fungal spores can germinate more easily on wet leaves and penetrate the plant.

It should always be kept above 20°C, including the pot and substrate. Temperatures above 30°C should be avoided.

This begonia likes a fine, but airy and consistently moist substrate.

In nature, this plant grows in very nutrient-poor soil. In the rainforest, there is little to no humus, as one knows it from native forests. A thin layer of leaves/detritus, only 1 to 3 cm thick, is usually followed directly by dense clay soil or rock. The roots of the plants creep along the surface under the thin leaf layer and almost never penetrate deeper than a few centimeters into the soil.
Therefore, this plant should be potted in a loose, only slightly acidic (pH ~ 6 to 6.5) and very airy mix. We recommend our houseplant mix or a mixture of sphagnum and perlite in a ratio of 3:1 for this plant. It also grows well in a purely mineral substrate like our ASP mix.

The pot for this plant does not have to be particularly large. A pot that is too large will cause the substrate to be dry at the top but still wet at the bottom. Then one quickly thinks one has to water and drowns the plant, which is actually still moist enough. Even when planted in a terrarium, no deep substrate layer is needed. The roots usually do not penetrate deeper than about 5 cm into the substrate.

Begonias can be kept in surprisingly small pots and can also remain in fully root-bound pots for a long time. Well-established, healthy plants tolerate repotting well and can also have their roots trimmed by about 30% when repotted or divided.

This begonia prefers soft water and should therefore be watered with distilled water, rainwater, or water from a reverse osmosis system. It should never completely dry out.

It is a rather slow-growing species that does not require too much fertilizer. Standard complete fertilizer (N-P-K + trace elements) for houseplants is sufficient. We recommend reducing the manufacturer's recommended dosage by about a third (which corresponds to about 300 µS/cm conductivity for fertilizer) but fertilizing weekly. The conductivity can be determined with a TDS meter. For stronger leaves and stems, a potassium-rich fertilizer (e.g., cactus fertilizer) can be used instead of a standard fertilizer.