The parents of this cross were Blue King and Nigrescens. The first is a mid-blue I sanguinea collected in Japan by Barr in the early 1900s and it is still around today Nigrescens is a bit more mysterious, but it was a dark blue I. sibirica presumably of European origin, attributed to the Belgian horticulturist and plant explorer Louis van Houtte, and has been in commerce since around 1875. It seems to have disappeared from sight now and I couldn’t find a photo of it among our “historic Siberians” slides, but it probably survives in the corner of some garden. Caesar and his brother were initially registered with the spelling "Caezar" and the names were later changed to their current form in 1949. In 1953 Caesar's Brother received the Morgan Award (which precedes the current Morgan-Wood medal and was named after the ubiquitous Mr. Morgan).
There are two remarkable things about Caesar’s Brother. Most of its contemporaries from the 1920s and 1930s have by now fallen by the wayside (including its brother, Caesar, which is rarely seen these days). However Caesar’s brother is not only still available but is very common in commercial catalogs – many pages of sources show up in a Google search. I’m guessing there are more references there than for any other individual Siberian. Perhaps this is not unreasonable since Caesar’s Brother is a vigorous and adaptable plant and has very attractive clear deep blue-violet flowers that still epitomize “Siberian iris”. Even more remarkable than its persistence in commerce and in people’s gardens, is its ability to grow and flower well where other Siberians won’t, particularly in warmer regions of the USA. Most Siberians do not tolerate hot weather well, particularly in the spring when damp, cool conditions allow them to develop and bloom at their best. In hotter climates they tend to grow and bloom short, if at all – but not Caesar’s Brother. I have seen strong, tall clumps blooming in southern California, South Carolina, and Texas where other Siberians have given up the struggle. Why is this? Hybrid vigor could be part of the answer to its longevity since it is a primary cross between I. sibirica and I sanguinea, but where does the heat tolerance come from? However it may arise, it is an increasingly valuable property in a warming world. When the city of Chicago is now planning for a climate more like Baton Rouge before the end of this century, we may be seeing the Siberian iris comfort zone heading up to Hudson’s Bay. For someone interested in hybridizing Siberians, the production of warmer-growing versions would be an excellent goal and Caesar’s Brother is an obvious starting point.
Now for some questions here at the end. Does anyone have a picture of, or, even better, is still growing Nigrescens? And, if you are reading this in the warmer areas of the US, what Siberians grow and flower well for you? They might like to meet that fine young fellow, Caesar’s Brother.