Q. I have two Japanese pear trees, don’t know the particular variety, but they have borne fruit over past years in abundance. However, last year I only find, that together, they only bore four pears. Are others having the same thing happening in this area or is my experience a rare or odd event? Oscar Burlison – Tacoma, WA
A. What you are experiencing is something called “alternate bearing” or sometimes “biennial bearing”. The majority of fruit and nut trees express some degree of this cyclical cropping pattern. There is actually an alternate bearing index that grades cultivars based on their propensity to bear more heavily on alternating years. This shows that there are genetic factors that contribute to this pattern. What happens physiologically in a fruit or nut tree is that a very heavy crop in one year depletes the carbohydrate reserves of the tree. This carbohydrate reserve is the energy surplus that left at the end of the growing season, which is stored as carbohydrates in the root system. This is the difference between the energy reserves from the year before added to the energy absorbed by the leaves during the season and the energy usage from growth, repair and reproduction. An example of carbohydrate storage that most of us are familiar with is the Sugar Maple. This is where real maple syrup comes from. In the fall surplus carbohydrates are stored in the roots. In the spring the carbohydrates are mobilized and converted from complex carbohydrates to simple carbohydrates like sucrose. If you were to harvest too much sap in the spring the tree’s production would decline and the tree could eventually die. Fruit trees have hormonal feedback systems that kick in when the carbohydrate levels drop below a threshold. The result is fruit abortion. The reason the tree seems to flower just as well in the weak year is because the flower buds are being formed early in the season for the next year long before the plant assesses its carbohydrate status. What you can do is thin the fruits of your tree this year so that you even out production. Thinning early is better since less energy goes into the fruit that will be removed. About 2-3 weeks after blooming is ideal. Remove fruit that are closer than 6 inches away from one another on the same branch. If you have a strong alternate bearing tree then you will need to thin each year to keep it on an even keel. Some other benefits of thinning are larger individual fruit as well as of branch breaking due to fruit load.