Perhaps you’ve inherited a mature Douglas fir canopy. Or, maybe your garden was sunnier a few years ago, but as nearby trees filled in, the light equation shifted. Many gardeners complain about gardening with trees, but certified arborist and horticulture educator Christina Pfeiffer wants us to celebrate this relationship. Pfeiffer, co-author with Mary Robson of “Pacific Northwest Month-by-Month Gardening,” says that trees are ecological powerhouses — of all our garden plants, they support the most life above and below ground. The time they take to mature makes an adult tree canopy a gift, not a barrier to beautiful gardens. Rather than fight the canopy that makes it harder for sun-loving roses, dahlias and tomatoes to bloom, shifting to a plant palette that wants to grow under the trees will foster a healthier, easier-to-maintain garden. Instead of classic English border, think magical woodland.…