Post #17 Pelargonium Substrate

One of the most often asked questions is about the substrate that might be the “most suitable” for pelargoniums (and monsonias). I use inverted commas as there is probably no such thing as the most suitable potting mix: it all depends on the climate in one’s area, particularly heat, on the watering regime, and of course on the pot size and pot material.

I grow pelargoniums in a south-facing greenhouse in a central European climate. The recipe below was developed on the basis of several considerations:

  1. Peat-based potting mixes get very dry in the summer and provide no support to plants and are difficult to moisten once dry. Due to this, I lost many plants during summers. The situation is different during wintertime: due to low temperatures the greenhouse needs to be heated and in order not to lose heat, the windows are shut. During the intensely overcast periods in December and January, the greenhouse barely heats up during the day, meaning that I needed a quick drying substrate so that the plants are not too wet, which would lead to etiolated growth and even mould. Loam-based substrates seemed preferable for this reason.
  2. A nutrient-poor substrate was preferred, as I like plants to grow slowly so they resemble those in the habitat. This meant a low proportion of organic material.
  3. As the pots are carried by benches and these occasionally need to be moved, a lightweight aggregate as the basis seemed like a good idea. Pumice (3-8 mm) seemed like the material of choice though I imagine other nutrient-poor aggregates might work as well, as long as not alkaline (such as limestone), e.g. perlite or vermiculite, but these tend to float and stick to plants.
  4. I don’t repot often (typically only every 10 years, even less) as many geophytic species really dislike being disturbed, so the susbtrate should also be reasonably stable, i.e. not disintegrate with time, as would be the case with expanded clay (hydroton).

Based on the above, mixing my own substrate seemed like the only option. The following photos illustrate the process.

Fig. 1: The marl rock of which the hill behind the house consists, slowly degrades into clayey soil.   

Fig. 2: The leaves and the humus-rich layer are removed, revealing the nicely degraded loam. This should have a pH of about 6-6.5, which is acidic enough for most plants to be able to extract nutrients.

Fig. 3: The loam is cleaned of organic remains and mechanically disintegrated.

Fig. 4: The mix consists of 50% pumice (3-8 mm), 40% loam, 10% commercial potting mix with a low content of peat (e.g. John Innes No. 1). 

Fig. 5: The well-mixed substrate.

Fig. 6: I mostly use square plastic pots – efficient in terms of space, and lightweight. I occasionally use trays as above, but the majority of the plants are in pots of 15 x 15 x 25 cm. The extra depth is important so that geophytes can develop a good root system. The bottom layer (3-4 cm) consts of pure pumice for drainage, followed by the substrate.

Fig. 7: To keep the necks of plants dry, especially the geophytes, the topsoil layer consists of 50% loam 50% pumice. The loam washes down into the pot after the first few waterings, leaving a top layer that dries very quickly.

The substrate described above enables good control of water content. As it dries out quickly, summer growing species need to be watered more often, however, as the pots are plastic and deep, they do retain some humidity so that a week or two of absence in the summer heat (the greenhouse heats up to 40 deg regularly) does not mean a catastrophe. The summer growers get water once a week or less, and the winter growers once every three or four weeks or less, during their respective growing seasons.

I use the above recipe for almost all the species, with the exception of the large, shrubby species of section Pelargonium, where I use more organic matter, section Campylia, for which I use pumice:peat mixture 1:1 only, as these mostly come from wet fynbos organic-reach but nutrient-poor habitats. There are few exceptions, such as Pelargonium sibthorpiifolium, which I grow in almost pure quartz sand.

By Matija Strlic, Ljubljana, Slovenia.

Citation and Copyright

© The Author. This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
ISSN 2464-014X.

Cite as: M. Strlič: Pelargonium Substrate, Pelargonium Notes, #17 (2021).

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