"Your Majesty, I'm afraid it will be difficult to increase domestic rubber production any further," Konstantin said after some thought.
Rubber was not so easy to cultivate. It was not simply a matter of Ferdinand planting as much as he wanted; it had requirements for climate, rainfall, and soil.
It favored high temperatures, high humidity, calm winds, and fertile soil. It required an average annual temperature of 26–27°C, could not tolerate cold, and would die once temperatures fell below 5°C.
It required average annual rainfall of 1,150–2,500 millimeters, yet was unsuitable for planting in low-humidity areas. It grew best in deep, fertile, moist, well-drained acidic sandy loam.
Its roots were shallow and its branches rather brittle, giving it poor wind resistance. It was easily harmed by cold winds, which also reduced rubber yields.
With such a long list of requirements, there were not many places in Bulgaria suitable for growing rubber. Some parts of the Balkans could barely manage it, but yields would not be high.
Ferdinand had never planned to plant rubber trees in the homeland. It was not economically worthwhile, and it would also disrupt the domestic industrial production chain.
If the homeland would not do, then only the African colonies remained.
In later generations, Africa's rubber output accounted for roughly 6% of world production. The major producing countries were Côte d'Ivoire, Nigeria, Liberia, Cameroon, Ghana, and Gabon. As it happened, all of those regions except Côte d'Ivoire lay within Bulgaria's sphere of control.
Bulgaria had begun establishing rubber plantations in Africa as early as 1923. The expansion reached its peak in 1925, after which the pace of development gradually slowed.
In other words, the coastal regions suitable for rubber plantations had already been more or less developed. To establish more plantations, they would have to push inland.