Though Trichocereus knuthianus is still considered a correct species with the name Echinopsis knuthiana, it is closely related to Trichocereus cuzcoensis and Trichocereus peruvianus. There was never any good DNA testing done to verify or question this, which is why it is on our short list of species that we´ll test for our upcoming book on the genus Trichocereus. In my opinion, it is a relative of Trichocereus cuzcoensis. We will find out how related exactly once we´ve ran some tests.
Trichocereus knuthianus was discovered and described by Curt Backeberg on the upper course of the Rio Marañon. The plant was named after Graf F.M.Knuth, who was Curt Backeberg´s co-author in the book KAKTUS ABC and a financier for some of his trips.
Most plants that are available on the open market were originally sourced from Friedrich Ritters Seed list. Ritter´s collection number of Trichocereus knuthianus was FR567, sometimes also labeled as Trichocereus knuthianus f. pachanoi or Trichocereus knuthianus FR 677! This strain can be found in collections throughout the World though it is most common in Australia.
Ritter wrote that it´s probably synonymous with Trichocereus tarmaensis and I agree to a certain degree. In addition, he shared the opinion that Trichocereus knuthianus is actually a variety of Trichocereus cuzcoensis, which is most likely correct as well. Trichocereus cuzcoensis only grows in Cusco, but its close relatives grow all over Peru, e.g. Trichocereus tarmaensis, Trichocereus schoenii, etc.
Curt Backeberg, on the other hand did absolutely not agree with Ritter´s attempt to nullify Backeberg´s species, as it would mean that his newly described “species” would´ve become invalid. Generally speaking, Friedrich Ritter knew that plant very well .
Trichocereus are relatively easily grown in the UK, especially due to the fact that they do not require massive amounts of sunlight. As most cacti grown in the UK, they are best grown in a greenhouse or conservatory (grow tents are also used for younger plants), South facing is a distinct advantage.
Watering from Spring (April) to beginning of Autumn (End September). Depending on what kind of summer we get will depend on how frequent you need to water, but, less is better with cacti. During the winter months, little to no watering is required or the roots could rot (they do not like to be wet for long periods).
In warmer climates, you could go for a more nutrient rich mixture (50% organic / 50% inorganic). But, in the UK, it is best to err more on the side of inorganic (60% – 70% inorganic) to ensure the pot is drained quickly.