Answer: Almost all raspberry types are cold-hardy enough for our region, but we also get pretty warm in our summers and many raspberries do not perform well under high temperatures, to which both their roots and their shoots are sensitive. Optimal leaf temperatures are around 65? to 70? F., while roots perform well in temperatures up to about 75? to 80? F. If the plant experiences material heat stress, photosynthesis--the process by which plants produce their energy and food--tends to shut down when the optimal air and soil temperatures are exceeded, which of course can result in reduced plant and fruit size, plus possibly a reduction in the amount of food that the plant stores to get through the winter. In particular, most types of primocane-fruiting (fall-bearing) raspberries do not perform well under high temperatures. Whether our temperatures are high enough often enough to be a problem is not clear from the literature we have seen; our best guess is that since it is not much discussed in regional articles on the fruit, that absence signals that it is probably not a major problem (which it certainly is in more southerly climes), though one Washington State University bulletin notes that blackberries and other close cousins of raspberries are not recommended for our region owing to the summer heat.
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