How to Determine the Harvest Time of Walnuts? A Detailed Guide to Harvesting Methods
Timely harvesting of walnuts is very important. If harvested too early, the green husk is difficult to peel off, the kernel is not plump, the yield decreases, the kernel yield rate is low, the oil yield rate during processing also decreases, and the nuts are not resistant to storage. According to measurements, harvesting 1 day early on average reduces the dry fruit weight by 0.49% and the kernel weight by 1.17%; harvesting half a month early reduces the yield by 10.6%. If harvested too late, the fruit easily drops off, and at the same time, after the green husk splits, staying on the tree for too long increases the chance of mold infection, leading to a decline in nut quality. This is especially true for some paper-shell type varieties, which are even more affected, resulting in reduced edible value. Therefore, timely harvesting is an important step in achieving a bumper harvest and ensuring kernel quality, and harvesting must be done at the right time.
The maturity period of walnut fruit varies depending on the variety and climatic conditions. The maturity period of early-maturing varieties and late-maturing varieties can differ by 10~25 days. Generally speaking, the maturity period in northern regions is mostly from early to mid-September, while in the south it is relatively earlier. The maturity period of the same variety differs across different regions, and within the same region the maturity period also varies: plains mature earlier than mountainous areas, low-altitude areas mature earlier than high-altitude areas, sunny slopes mature earlier than shady slopes, and dry years mature earlier than rainy years.
When the green husk of the walnut turns yellow-green or light yellow, the fuzz decreases, cracks appear at the top of some fruits, the green husk is easy to peel off, a small number of mature seeds have already fallen off naturally, and the inner shell has completely become bony, this is the best harvest time for walnut fruit. At present, harvesting in production is mostly too early, and this should be corrected.
Harvesting methods
There are mainly two walnut harvesting methods: one is manual harvesting, and the other is mechanical vibration harvesting. China's labor resources are relatively cheap, so manual harvesting is the main method, while developed countries in Europe and America have already replaced manual labor with mechanization in walnut harvesting, greatly improving harvesting efficiency and saving harvesting time.
1. Manual harvesting. Manual harvesting is relatively common in China. Before harvesting, how to pick should be decided according to the terrain where the walnut tree is located and the size of the canopy. For those with inconsistent maturity periods, harvest in batches, and strictly harvest and process separately by variety.
① On flat ground or when the canopy is small, you can pick directly by hand, put the harvested fruits into baskets, cloth bags or bamboo baskets, and after the picking process is finished, gather the fruits onto a transport vehicle and take them to the processing site to remove the green husk.
② On hillside land or when the canopy is large, manual picking is very inconvenient. Generally, plastic sheeting is first spread within the walnut tree basin, and then a bamboo pole or a long wooden pole with some flexibility is used to strike the branches where the fruits are located or directly knock the fruits down. When striking, it should be done from top to bottom and from inside to outside along the branches to prevent the branches from splitting, but be careful not to use too much force, so as not to damage the branches and buds and affect the next year's yield.
2. Mechanical vibration harvesting. Mechanical vibration harvesting means using machinery to vibrate the trunk, shaking the fruits to the ground and then collecting them. It has the characteristics of saving time and labor, high efficiency, and low cost.
The machinery for mechanical harvesting includes a vibration fruit-dropping machine, a sweeping and row-gathering machine, and a picking and cleaning machine. The operating procedure is to first use the vibration fruit-dropping machine to shake the walnuts to the ground, then use the sweeping and row-gathering machine to gather the walnuts on the ground into rows, and finally use the picking and cleaning machine to pick them up and simply clean them before boxing. Since the maturity period of fruits on the same walnut tree is not completely consistent, when using mechanical harvesting, 500~2 000 mg/kg of ethephon must be sprayed on the tree 10~20 days before harvest to promote ripening so that they mature uniformly. The advantage of this method is that the green husk is easy to peel off and the fruit surface is less contaminated, but its disadvantage is that using ethephon to promote ripening often causes a large number of leaves to fall off and affects tree vigor.