Chestnut Tree Pollination

Chestnut trees are a fruiting tree. Like most all other fruiting trees, chestnut trees require proper pollination to successfully set fruit. The ability of a fruiting tree to set fruit and carry it to harvest is dependent on many conditions to be met. For chestnuts these conditions are:

1. Healthy trees – stressed trees are poor producers of chestnuts

2. Genetically capable of setting fruit – some seedlings never produce fruit

3. Weather conditions during the growing season

4. Available and viable male and female flowers

5. Mature trees

6. Pollen available and viable during the period the female flowers are receptive

Cross Pollination of Chestnut Trees for Nut Production

Chestnut trees by themselves are not self pollinating. In other words, a chestnut tree all by itself usually will not produce any chestnuts with the kernel filled in. This chapter attempts to explain the somewhat complex subject of getting chestnut trees to successfully fruit with mature, good tasting nuts. Chestnut trees can be grouped into four groups as it pertains to pollination.

1. Pollen producers with viable pollen

2. Pollen producers with sterile pollen

3. Non pollen producing catkins (male flowers)

4. Non pollen producers having little or no catkins

Only chestnut trees producing viable pollen can pollinate other chestnut trees. Chestnut trees are generally not able to pollinate themselves. Isolated chestnut trees where pollen from other chestnut trees is not available rarely produce any nuts with filled in kernels. So far this is a simple example, one chestnut tree and no nuts, but it overlooks the pollination rule for chestnut trees where a cloned chestnut cultivar can not pollinate the same cloned cultivar. For example, a cloned (grafted) Marival chestnut tree can not successfully pollinate another Marival chestnut tree even though it produces an abundance of viable pollen. This rule only applies to grafted cloned trees pollinating their own clone. A seedling from a cloned tree tree can pollinate its own cloned parent. One of the best ways to demonstrate this pollination rule is with a simple logic representation of the rules. First lets equate the letters CP as a cloned pollen producing chestnut tree producing viable pollen. Then assign CN as a cloned chestnut tree that does not produce pollen. In the logic equations the first trees in the equation represents the female flower and the second tree is the male catkin tree.

CN + CP = nuts on CN tree

CP + CN = no nuts on either tree

CP + CP = no nuts on either tree

CP + CN = no nuts on either tree

Now lets introduce a second pollinator and identify the the two pollen producing trees as CP1 and CP2. Chestnuts will be produced when the following conditions are present:

CP1 + CP2; CP2 + CP1; CN + CP1, CN + CP2, CP2 + CP1

Casual observation shows that by introducing a second pollinator the combination of nut producing trees is dramatically increased. With the use of at least 2 chestnut cultivars that produce viable pollen close enough to each other, all the chestnut trees will have the potential to produce nuts. Aside from just viable issues there is a timing issue also.

Pollination Timing in Chestnuts

Chestnut trees can start blooming in the northern hemisphere as early as May and the bloom can continue until the first part of August depending on local climate conditions and latitude. The farther north the trees, the later they will bloom. The male flowers (catkins) usually start blooming within a day or two of the female flowers on the same tree. It is possible to have two different chestnut trees with both producing viable pollen, but due to bloom time differences, the pollination of the female flowers fails and no nut set occurs. Apple pollination is similar with the exception of the male and female flowers bloom at the same time (the male and female parts are present in each blossom). The similarity referred to here is where the apple is not self fertile so another type of apple with the same flowering period must be present to provide pollen for successful fruit set. Apples have 5 groups of pollination periods. These periods are 1)Early; 2) Early-mid; 3) Mid; 4) Mid-late; and 5) Late.

Because chestnut trees bloom for a longer period than apples, only 3 periods are needed to group chestnuts into. These periods are 1) Early; 2) Mid; and 3) Late. Most chestnut trees begin blooming in one period and continue into the next period. For example, Marival begins blooming (catkins release pollen or female flowers become receptive) early and continue blooming into the late period.

Chestnut tree cultivars that release their pollen in the late period can not be used as pollinators for early and mid period blooms. Marrone chestnut trees are one of the last chestnut trees to release their pollen but the female flowers have been receptive for up to 2 weeks. This period where the female flowers are receptive is difficult to predict because it is weather dependent. If the weather is too cool for the female flowers to be receptive of pollen during the receptive period, pollination will fail. The best way to demonstrate this is with a time line. In this time line example the overlaps are the important areas to understand. Lets say chestnut cultivar EC is a pollinator that starts releasing pollen the mid part of the early period and continues into the first week of the late period. On a time line it would look like this on a 6 week time line:

Now lets add the late blooming LC chestnut cultivar to the time line.

The BW period is a period where the weather is not supportive of pollination of the chestnut female flowers. In this example the EC chestnuts will not successfully set fruit. Also, because LC releases its pollen too late for the EC female flowers, the LC can not be used as a pollinator for the EC chestnut trees.

Lets go from theoretical to practical with some real data. The data comes from Missouri State University Agroforestry Department. The data is collected from their research facility in New Franklin, MO. A Chinese hybrid Dustin chestnut tree on average starts its female flower bloom on June 8 each year. The female flower on average ends it bloom on June 24. The male flower on average starts its bloom on June 7 and ends flowering on July 4. When selecting a matching chestnut tree to pollinate the Dustin, the bloom times need to overlap at least by 5-7 days. A Qing is a excellent match for the Dustin. The Qing chestnut tree starts the female bloom on average on June 8 and ends on June 24. The male flower on average starts on June7 and ends on July 4. Both the Dustin and the Qing are Chinese hybrid chestnut trees.

Now lets look at some European and compare the bloom times in New Franklin, MO with data from Northwest Washington state. One of the most popular chestnut trees grown in the Western USA is the Colossal. It is pollen sterile so there is no male bloom period to compare. The female bloom time starts on average on May 31 and stops on June 22 at the Franklin farm. In NW WA the female flower bloom period starts about July 2 and ends July 27. A good pollinator for the Colossal is Marigoule. In Franklin the Marigoule male flower starts on average June 3 and ends on June 28. This provides almost 3 weeks of overlap in the pollination period. Because the Colossal is pollen sterile the Marigoule will not set any nuts unless another pollen producing chestnut tree is introduced.

So what must a chestnut farmer do to make sure the chestnut trees on the farm produce a full set of chestnuts? There are two simple possible solutions. The first is to grow at least two different chestnut trees that share the same pollination periods and both produce viable pollen. The other solution is to plant several chestnut cultivars that produce pollen over all three pollen periods. This is what we call “covering your bases”. Based on the data collected at the Franklin, MO farm, the main issue with successful chestnut set is growing chestnut trees that produce viable pollen that can cross pollinate each other. In Northwest Washington state pollination timing is more complicated because some chestnut cultivars such as Maronni and Belle Epine bloom much later than commonly used pollinates such as Okie and Precoce Migoule. One last thing, no matter what type of chestnut tree you grow (American, Chinese, Japanese, European, and all of their hybrids), if it produces viable pollen, it is capable of providing pollen to any other chestnut tree.

Evaluating the Pollination of Chestnut Trees

How do you know if the chestnut trees are getting pollinated properly or not? The evaluation of the pollination success rate is directly related to the chestnut kernels getting filled in fully. If most of the nuts in the burrs are blanks (have no kernel meat) then pollination failure is the most likely cause. Most chestnut trees produce 2 or 3 nuts per burr. If the pollination is successful all the nuts in the burr will be filled in. If pollination was only partially successful then one or more of the nuts in the burr will not be filled in.

When there is a complete pollination failure, the burrs could have anywhere from the normal number of nuts in them but are all blanks, to as many as 7 or 8 blank nuts in each burr. This condition happens when the chestnut trees are healthy, have the right amount of moisture in the soil but the weather was too cold for nut set.

Summary

To produce a nice crop of chestnuts, a chestnut farmer has to make sure there is enough viable pollen present at the right time, the chestnut trees are not stressed, and the day time high temperatures are above 68 degrees F. Having the wind blow from the right direction or having insects move the pollen at the right time is also helpful too. The simplest approach to reducing the risk of pollination failure is to plant several pollen producing chestnut trees that cover all 3 periods of the chestnut bloom period.