We are updating the nutrient management guidelines for the popular new hazelnut varieties being planted. Existing guidelines are outdated and were developed for Barcelona, a less frequently planted variety that is susceptible to Eastern Filbert Blight. We began collecting tissue samples in 2016 and continued year-round sampling for three years to determine the optimum time to collect leaf samples for determining nutrient levels in the trees, and whether or not there are deficiencies and/or toxicities.
Nutrient allocation to different organs in the tree varies quite a bit with catkins requiring the most macronutrients including potassium, sulfur, magnesium and sulfur. Female flowers (which develop into nut clusters) also have a high demand for macronutrients. Regarding micronutrients, catkins require high levels of boron, which could account for the yield increase associated with foliar boron applications being linked to higher yields.
Last year we took this research to the next level to study the association of non-structural carbohydrates (NSC) with tree health and yield. NSC in plants consist mainly of simple water-soluble sugars such as glucose, and are produced by photosynthesis. Structures such as flowers and fruits are NSC sinks, and NSC levels have been directly linked to flowering, fruit/nut set and yield of woody plants such as trees. They have also been shown to play an important part in drought tolerance, where higher levels of NSC allow trees to withstand more severe drought. Hazelnut bloom (which occurs during the winter and takes place before foliage emerges) is likely dependent on stored NSC. This means pollination and early nut development also rely on carbohydrate reserves. We are interested in determining a baseline for the different varieties of hazelnuts grown in the Willamette Valley, because in the end, yield will also be affected by the status of NSC in trees during key periods in the season.
This graph shows the seasonal pattern of carbohydrate content in hazelnut twigs. Hazelnuts go through more oscillations in NSC compared with other tree nuts. The drops in NSC (red arrows) appear to be related to spring bloom, nut filling and male flower (catkin) formation. Recovery periods (green arrows) are related to spring growth and post-harvest time in late fall.
Most carbohydrate reserves are located in the wood (xylem), particularly in the fall (highlighted area). A significant loss of NSC reserves from wood is observed during dormancy, which could be related to relocation of reserves to other parts of the tree such as stems or roots.
Many growers apply various types of fertilizer both in the spring to push tree growth, and in the fall post harvest. Growers often try to accelerate growth of young trees to bring them into production sooner through increased use of water and fertilizer. Excessive fertilizer applications could cause nitrates to leach into groundwater, and may be an additional cost to growers that does not improve growth and yield sufficiently to justify the expense. There is also very little research that has been done recently to investigate the optimum fertilizer composition and timing for the new varieties of hazelnuts grown in Oregon using current practices. In 2018 we began investigating different rates of fertilizer applied to both irrigated and dryland trees. We are not only looking at uptake by the trees, but also effects on growth, and eventually yield as the trees age. In addition, we are tracking a stable isotope of nitrogen in a few selected trees, to see where the nitrogen travels inside the tree after it is absorbed by the roots. To date, we have found that trees receiving a blend of urea, P, K, S and Mg grew slightly faster and had better uptake of nutrients than trees that only received nitrogen. Only adding urea actually caused excessive N:S ratios in trees, leading to S deficiency symptoms and poorer growth. Uptake was more efficient for lower fertilizer rates, meaning less fertilizer could result in less wasted money and leaching. This research is being funded by the Oregon Hazelnut Commission and the Oregon Department of Agriculture.