Conifer trees may well be the source of the mites.
Oligonychus ununguis, "Food ... Most species of conifers, including spruce (Picea), fir (Abies), hemlock (Tsuga), pine (Pinus), larch (Larix), arborvitae (Thuja), and juniper (Juniperus)."
Bugguide
Oregon State reports them as being on deciduous trees.
Not their location now but for about 10 years, most of my dahlias were in a garden well protected from winds. An evergreen forest was across the road at the front of the house. Not where the dahlias were but one corner of the garden was within 50' of 5 tall trees. Pine, larch, hemlock, spruce and arborvitae grew naturally or have been planted around the 4 lot area. Especially the arborvitae would show lots of spider mites each summer ... but I may have missed similar populations because I wasn't climbing the trees
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Each year, the worst... mite problem in the dahlias was where the prevailing breeze would blow through the side yard of the home. Battling (or, not battling) the mites on about a dozen of those plants was how I learned that the mites could limit the viability of the roots, and I lost several until I was more conscientious.
Killing mites isn't necessarily that difficult. They succumb to insecticidal soaps and oils. I have been tempted to spray them on late-season beans but often just pull those after harvest whether the mites have moved in on them or not. I've never sprayed vegetables for mites.
When I began growing dahlias, I was inspired that there were such enthusiasm for the flowers in Kalispell Montana. However, Montana isn't central Mexico, home of wild dahlias. I have since learned that some varieties are able to put out only a single flower in the area's short growing season. Fortunately, this isn't quite as high as Kalispell and I don't put much emphasis on those that would struggle to open 1 bloom.
Steve
