## 3 Walnut Buds: Bloom and Pollination Events--Cross-Pollination  
Walnuts are monoecious and have male and female flowers on the same tree. Male flowers are formed in structures called catkins. They develop directly on the prior year's growth and are easy to identify by eye. At leaf out, the preformed shoot develops, and compound leaves start to grow before the pistillate (female) flowers form. This explains why walnut cultivars are indicated by both their leaf out date and bloom date. Female flowers are only receptive to pollination for a short time. In particular, pollen shed from staminate (male) flowers during anthesis is spread by wind (wind-blown pollen) and remains viable for a short time, generally up to 48 h. As already mentioned, walnut is self-compatible but has adapted a mechanism called dichogamy to reduce the degree of inbreeding. Walnuts are classified as heterodichogamous; however, the majority of commercial walnut cultivars, such as Chandler and Serr, are protandrous, meaning that the male flowers mature and pollen shed occurs before the female bloom begins. So, protandrous cultivars, i.e., Serr may be large contributors to protogynous cultivar pollination. In most walnut-growing regions, dichogamy is the reason for including pollinizers in orchards. Moreover, in certain cultivars, such as Serr, excessive pollen on pistillate flowers (too much pollen) has been identified to be the cause of pistillate flower abortion [1].  
During cross-pollination, pollen is transferred from the male reproductive organ (stamen) of one plant to the female reproductive organ (pistil) of another plant of the same species, resulting in fertilization and the production of seeds. In the case of nut trees like walnuts, cross-pollination is necessary for the trees to produce nuts. Cross-pollination can occur naturally, through the action of wind or insects such as bees, or it can be performed artificially through hand pollination, in which pollen is manually transferred from one tree to another. Natural cross-pollination can be affected by factors such as the proximity of the trees, the timing of flowering, and the presence of pollinators.  
Walnuts require cross-pollination for optimal nut production. Cross-pollination increases the genetic diversity of the trees and can lead to larger and more flavorful fruit. However, not all nut tree varieties are compatible for cross-pollination. It is important to choose compatible varieties and to plant them close enough for natural cross-pollination to occur. In some cases, artificial pollination may be necessary to ensure adequate pollination and fruit set. This is especially true in orchards where natural pollinators are scarce or when the weather is unfavorable for pollination. Artificial pollination involves manually transferring pollen from the male flowers of one tree to the female flowers of another. As mentioned above, walnut trees probably require cross-pollination with another walnut tree to produce nuts and minimize the risk of bacterial contamination. This can occur naturally, artificially with air vehicles, or through the process of hand pollination or the design of specific aerial pollination systems for walnut trees.