While this study has dealt exclusively with the Persian species, Juglans regia, the habits and principles involved apply equally to all walnut species.
In most plants the reproductive function inheres in a single bisexual flower, consisting of both male and female elements. In walnuts, as well as most other nuts, the male and female functions are performed by unisexual flowers of very different type and appearance.
Both the male or staminate flower and the female or pistillate flower spring from buds that are formed in the axils at the base of leaves of the previous season's growth. In the Persian walnut they may be detected as early as July. The staminate bud that forms the pollen-producing catkin of the next season, can be distinguished by its checkered appearance, something like a tiny pine cone. They occur in the axils of the lower leaves of the shoot of the current season.
The pistillate bud, which produces the nut, occurs at or near the tip of the growth of the current season. It can usually be distinguished from leaf buds by its larger size and plumpness.
When these blossom buds develop the following season, the male or staminate blossom assumes the form of a catkin, which elongates rapidly a few days before maturity. As the pollen is shed, beginning at the stem end, the pale yellow-green of the bursted pollen capsule turns dark or black, proceeding to the tip of the catkin. This change readily shows that pollen is shedding, which may be confirmed by touching such a catkin with the tip of the finger, and noting the yellow pollen that adheres, or rises in a tiny cloud.
Making note of the date when a given variety begins shedding pollen, and the date when all catkins on the tree have opened, gives the period during which that variety is effective as a pollinizer.
The female, or pistillate flower, does not, like the catkin, spring directly from the wood of last season's growth, but occurs at the end of the new growth of the current year, being preceded by a number of leaves which nourish the young nut to maturity.
The pistillate blossom assumes the form of one or more tiny nutlets with little sharp-pointed tips. When the blossom has become receptive to pollen, each tip has separated into two separate pistils which spread apart and present fresh, slightly sticky surfaces, which are known as stigmas. This is the time that pollination can take place, which period continues until the stigmas have lost their freshness and stickiness. This period marks the time during which pollination can occur.
In many cases Persian walnut trees remain barren when planted alone, not because of incompatibility between the pollen and the pistillate flower, but because pollen shedding and receptivity do not occur at the same time. Sometimes pollen shedding is over before pistils are receptive. Such blooming is termed protandrous by botanists. In about an equal number of cases the pistils lose their receptivity before pollen is shed. Such blooming is termed protogynous. There are quite a number of varieties, however, that mature both types of blossoms simultaneously, in which the variety is self-fertile and will produce crops, even if isolated from other trees of the species. Of these Hanson and Bedford are representative. On some other trees there is some overlapping of the shedding and receptive periods, enough to produce partial, but not full crops.
Warm weather hastens blooming; cool cloudy weather retards it. A warm spell may start blossoming early, but if broken by a cool wave the period of bloom may be greatly extended.