This crop is adaptable to areas with as little as 20 in. of annual rainfall. Sufficient moisture must be present to establish good stands and permit the first flower heads on each stem to mature. But rainfall levels that allow secondary flower heads to develop will result in poor uniformity of seed maturation and seed shattering during subsequent rainfall or harvesting. The effort to develop this species as a new crop must concentrate on production areas where a short rainy season occurs during a period of four, or at most five months, to promote good growth and flowering. The subsequent period for seed development and maturation should have one or two months when there is very little or no precipitation.
Dry conditions promote seed retention.
It seems that natural selection has favored a plant type in vernonia that does not disperse seed at maturity, but rather will retain it until rainfall is adequate for germination and growth of the seedlings. Rainfall pattern is evidently more important than total amount for maximum productivity of vernonia. Porous, well-drained soils are required to grow this crop successfully. Field trials in Zimbabwe on well-drained soil found that plants grow erect with a single stem until the first flower head appears, after which lateral branches develop. However, on poorly-drained soils the terminal growth stops before flowering and the upper portion of the plant dies.