Low heat units indicate late start to 2023 season

Nik Wiman

April 21, 2023

Winter is holding us tight this year in the Willamette Valley, but it may be finally losing its grip. A break in the rain and warmer temperatures in the forecast will be a relief for hazelnut growers with orchards susceptible to eastern filbert blight (EFB). Rain spreads the spores of the fungal pathogen Anisogramma anomala to the susceptible meristem tissue as the trees break bud an enter the shoot elongation phase. It has been difficult to find a break in the rain to get the necessary fungicides applied. More than 5 inches of rain have fallen since April 1 at the North Willamette Research and Extension Center in Aurora.

Its not just the rain, it has also been very cool in the Willamette Valley with low elevation snow levels and late freeze events. So how far behind are we on heat unit accumulation? The table below indicates that growing degree days (lower threshold 50o F) accumulated since January 1 are running roughly a month behind last season, and 3 weeks behind the 30-year normal on average across 4 sites in the Valley. Expect pests like budmite and filbertworm to be later (but not aphids) and it could be another late hazelnut harvest. Time will tell if we can catch up, but right now it looks like another late year.

Late blooming almonds in a research trial at the North Willamette Research and Extension Center typically bloom at the end of March, but didn’t bloom until mid-April in 2023.

Hazelnuts in the Willamette Valley are still pushing leaves from buds and leaves are expanding. Orchards have been very wet, and orchard access has been difficult this spring. Some growers have been waiting for a weather break to plant new orchards so there will be some late plantings.

Accumulated growing degree days (GDD; base 50 F) at Willamette Valley weather stations. Degree day heat units drive development in plants, insects, and other organisms that depend on heat from the environment to drive metababolic processes. The degree day calculator at USPest.org can be used to calculate GDD.