Walnut pollination

No bees needed here. Two walnut flowers seen in foreground with catkins behind.

It is pollination time for walnuts! Our sinuses are plugged and we go about sneezing all day. We are asked occasionally how the walnuts get pollinated. Do they need bees? What color of flowers do they have?

Walnuts have both male and female flower parts on the same tree. The pollen does not have to be transported by bees but is simply shed from the male flowers, called catkins. The pollen is physically very small and light and is moved throughout the orchard by wind movement. Pollen can travel quite some distance. Studies have shown that in certain orchards the pollen came from trees over a mile away.

With walnuts, the time of pollen shedding does not always overlap well with the time of female flower receptivity to pollen. Hence, although most walnuts are self-fertile, they sometimes require another walnut variety that comes out later for pollination. This condition is referred to as dichogamy.

The female flowers are very small, with two feathery leaves open to receive pollen. They are not a typical flower with petals, and do not have a flower color as such, being green with a slight tinge of pink. When the flower is pollinated, a nut will develop in that place.

Nearly all commercial orchards are co-planted with a pollinator variety to ensure the main crop gets enough pollen to set nuts. In fact, at this time of year the pollinators can be readily identified in an orchard by the fact that the other trees will be putting out some leaves while the pollinators do not yet have any. In some orchards, entire rows of trees are put in as pollinators, which now appear as rows of leafless trees.

The pollen shedding characteristic of walnuts is a very efficient way to spread pollen. It is even sometimes over-efficient. Certain walnut varieties are very susceptible to being over pollinated. When that happens, the flower withers and falls off. This is bad because where there is no flower, there is no nut. In some orchards overall production can be decreased by half or more because of this problem. There are some steps that can be taken to mitigate the effect of over pollination and there are some chemical treatments which have proven effective.

All kinds of plant pollen are present all the time. There is really no way to get away from pollen altogether. People used to move to desert areas to get away from pollen, with the idea that desert plants are sparse and thus produce generally less pollen. That does not work anymore now that so many people live in the desert and have taken their favorite plants with them.

Walnut pollen has been identified as one of the worse offenders when it comes to people being allergic. In areas where walnuts are commercially grown, such as Red Bluff, there is an enormous quantity of walnut pollen in the air during the months of March, April, and May. Continual exposure year after year eventually leads to allergic reactions in most people.

There is really no easy solution for the pollen exposure problem. Most of us can not take a three month vacation. So we just have to make the best of it. For us taking an allergy pill and wearing a dust mask while in the orchard helps alleviate some of the reactions.

Our October 19, 2007 blog discussed walnut varieties.