Alocasia macrorrhizos 'Black Stem'

Arrowhead plants (Alocasia) are a very diverse group of aroids (Araceae) from Asia. Some of them are very rewarding, easy, and fast-growing houseplants, while others are highly specialized jungle dwellers that can almost only be cared for in a terrarium. They can be roughly divided into three groups:

• The “Swamp Dwellers” – Alocasia, mainly from mainland Asia. They have large, leathery, green leaves and thickened rhizomes, some of which are eaten as vegetables. They are fast-growing, like plenty of water, light, and fertilizer, and are generally the easiest Alocasias.

• The “Jewel Alocasias” – Alocasia, mainly from the islands of Southeast Asia, especially Borneo. They are small to medium-sized, often with hard, patterned, or metallic-colored leaves. They usually grow slowly and often in the shade in nature. Some are easy to care for, but others depend on high temperatures or humidity to thrive.

• The “Oceanians” – Alocasia, primarily from New Guinea, Northern Australia, and the Pacific Islands. A small group, with only a few species in cultivation. These are also swamp dwellers with fast growth and considerable ultimate size, but they are not as robust as the Asian species and require consistently high temperatures.

Depending on the group they belong to, they should be cared for somewhat differently. More information can be found in the other tabs.

This Alocasia originates from the undergrowth of rainforests, and as such, it is adapted to semi-shady to shady conditions.

As a houseplant, it should be placed where it receives bright but indirect light, for example, next to a south-facing window or directly at a north-facing window. It also tolerates a few hours of morning sun well, but must be protected from intense midday sun, especially in summer. In winter, it can be placed in a much brighter spot, ideally directly at a south-facing window.

When cared for under grow lights, e.g., in a plant cabinet, terrarium, or grow tent, medium illumination is sufficient. At 2000 to 6000 lux or 35 to 110 PPFD (µmol/m²/s), it grows well, and its leaves develop ideal coloration.

Supplementary lighting to help it get through the dark winter months is very beneficial for this plant. Otherwise, it usually loses many or all of its leaves in winter and enters an (unwanted) dormant phase.

It likes a humidity of 60% or more and still grows well at over 90% RH. The temperature should always be above 20°C, even in the pot. Due to evaporation, the substrate is usually cooler than the measured air temperature. For growing young plants or for winter care, we therefore recommend a heating mat.

This Alocasia prefers a coarse to very coarse, airy but 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 go deeper than a few centimeters into the clay soil. Therefore, this plant should be potted in a loose, only slightly acidic (pH value of approx. 5.5 to 6.5) and very airy mix. For this plant, we recommend our Aroid Mix or, for larger specimens from 15 cm pot size, pure coconut chips.

It also grows well in a purely mineral substrate, but we recommend not repotting it immediately after arrival, as this can stress it a lot. Most of our plants are already potted in the substrates we recommend and can remain in them.

The pot for this plant does not need to be particularly large. A pot that is too large leads to the substrate being 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. Some experimentation is required for the correct pot size. As a guide: if you can water every 5 to 10 days and the substrate is almost completely dry in between ("ironing moist"), the size is well chosen.

Alocasias can be grown in surprisingly small pots and can remain in fully root-bound pots for a long time. They do not like to be disturbed at the roots, so when repotting or placing them in a terrarium, they should simply be transferred with the entire old root ball.

We recommend not watering alocasias after repotting, but letting them stand for about 7 to 10 days until they have rooted into the new substrate, and only then start watering again. If they appear dehydrated during this time, it is better to temporarily place the plant in a bag or a transparent box to prevent it from drying out, rather than watering it.

This Alocasia prefers soft water and should therefore be watered with distilled water, rainwater or water from an osmosis system. If it is watered with (hard) tap water, regular flushing with soft water every 3 months or so helps to prevent limescale accumulation in the substrate.

It should never dry out completely, but how moist it likes it depends heavily on the (substrate) temperature. In summer and at temperatures above 25°C, it can be kept consistently moist to wet. If it is kept as a houseplant (especially on a windowsill), it should be watered sparingly in winter, as the combination of colder temperatures and moisture can weaken it and lead to rot. We recommend placing it on a heating mat in winter, then it can continue to be watered normally.

This is a rather slow-growing species that doesn't need too much fertilizer. As a houseplant, it can be fertilized during the growing season from approximately April to October with a good complete fertilizer (N-P-K + trace elements) for houseplants, such as Wuxal Universal Fertilizer approximately every 2 weeks. We recommend halving the dosage indicated by the manufacturer. This corresponds to approximately 500 µS/cm conductivity for fertilizer. The conductivity can be determined with a TDS meter.

In the autumn and winter months, fertilization should be reduced, as the plants do not receive enough light to utilize the nutrients. A very weak fertilization of approx. 100 – 200 µS/cm monthly is sufficient here.

When cultivated under grow lights, they can be fertilized year-round with approx. 500 µS/cm every 2 weeks.

If they are only watered with soft water (e.g. when kept in a terrarium), a calcium and magnesium fertilizer (so-called CalMag fertilizer) should also be used.

For even better leaf coloration, especially when the plants receive a lot of light, regular foliar fertilization with an iron fertilizer is recommended.