There are a few reasons summer and fall pruning is not recommended. First, if you prune in summer or early fall, the plant may send out new shoots that are young and tender and can be easily damaged or killed by a fall frost or cold winter. The second is that some of the carbohydrates and nutrients that are in the stems and leaves will be transported back down to the roots for winter storage. If you prune during summer, then you are removing some of those carbohydrates and nutrients, which will result in less food and energy being stored overwinter and available to the plant to put out new growth the following spring. January through early March, when the plants are dormant, is considered the best time to prune blueberries in Oregon. For more on pruning blueberries, refer to the following OSU Extension article: https://today.oregonstate.edu/news/prune-blueberries-yearly-more-fruit