The lemon-yellow iris is one of the antique or historic irises. Its origins date back to 1830 when it was described by the botanist DeCandolle as Iris flavescens. It is now regarded as a natural hybrid and has itself become the parent in many artificial hybrids.
I. flavescens is a sturdy, tall bearded iris with medium-sized pale yellow flowers. The two specimens I am growing are more white than yellow. The one in the top photograph has more yellow in the standards and was purchased. The iris in the lower photograph has almost no yellow. It was from a garden in an old neighborhood. Both have downward pointing light colored falls with olive colored reticulations and are constricted near the base. The beards on both are also fine textured and yellow. Their fragrance is sweet but not strong.
The Cornell Extension Bulletin from 1925 describes I. flavescens as a tall, floriferous, sweetly scented yellow iris with martius yellow standards, naphthalene yellow falls that are faintly veined olive buff, and a dense, fine yellow beard.