Now that the summer days are warmer and drier, the worst of the effects from this year’s powdery mildew event are over until the autumn at least. Young sprouts of big leaf maple trees seemed the hardest hit with many individual trees’ and resprout clumps’ leaves now fully shriveled and dry. Next spring will show if the roots of these trees can sprout a new batch of leaves or stems. Larger trees will fare better, even if leaf loss occurs, but if leaf/chlorophyll producing activity (from a thick, UV-blocking growth of mildew) loss continues into consecutive years, survival of larger trees is not assured.
Elsewhere along the Salish Sea’s maritime edges, tops of many species of trees can be seen losing leaf/needle density even when watered well. Leaf loss can be from too much watering, from winter drought when plants are not usually watered, or in response to heat events of the last two years. Of course, canopy decline can also be a sign of a pathogenic infection but that cannot be assumed without evidence. When targets are not identified, larger declining trees should be kept to provide partial protection to sun and wind exposure for the lower, medium-sized tree canopy which will soon be the one keeping the most UV from reaching the soil surface.
Interplanting current forest trees with edge-loving species such as shore pine and maritime juniper, as well as a larger native from just to the South and West of us, the Garry oak (Oregon white oak) may serve a landowner well in the future vs. allowing opportunistic red alder, big leaf maple, and black cottonwood growth take over areas of failing forest trees. Pathogenic fungal disease centers increasing in currently healthy, forested areas should be proactively hunted down and cordoned or destroyed if possible.
Extrapolating from an increasing need for species diversity, changing zone boundaries of the USDA plant (cold) hardiness map, general lack of heat hardiness of PNW native plants, and since cold spells will limit growth of many introduced plants from the South, careful testing should be made before transplanting trees/plants tolerant of arid or limited climates as these hardy species’ release into milder climes has not always gone well for us (see Himalayan blackberries, Irish ivy, and Scotch broom), escaping cultivation and producing invasive growth spurts in ‘wild’ areas which are now successfully outcompeting native plants on many levels, not increasing diversity and resiliency to the landscape. Once again, ivy can easily be kept from maturing and setting seed by not letting it grow vertically; cutting vines at the tree base will kill the above plant if not rooted in dead wood.
Pruning should be minimized during the hottest of months for various reasons. Keep a shade hat and hose ready for your desired plantings during drought.
Yours in trees,
ISA Certified Master Arborist TRAQ