|The tree(shrub) is about 15
|Since you live in Portland where winter ice storms are common, I'm hoping your lemon tree is in a container and not in the ground. Plants in containers tend to need to be watered more frequently than those in the ground and because of the frequent watering, fertilizer can be quickly leached from the soil, resulting in a need to feed more frequently. This can cause a problem because it's hard to know how much has leached out, how often and how much you need to feed. You can burn the roots of a containerized plant by applying too much too often. I've solved this problem by using a water soluable acidified fertilizer (such as Miracle Gro or Peter's) and diluting it to half-strength so I can feed every 2-3 weeks throughout the year. Citrus trees will grow and flower (and bear fruit) nearly all year around so feeding every 2-3 weeks makes sense to me.
Citrus trees perform best when given all day sunshine. If that's not possible, 4-5 hours of sun is acceptable. Plan to move your tree indoors in early September, when nighttime temperatures creep down to about 50F. I move my lemon tree indoors before I turn on the heating unit for the season. This helps the tree adjust to indoor temperatures and lower light levels before it has to also deal with dry air. When it's inside, try to provide some sunshine, or supplement a bright location with fluorescent or incandescent light (leave it on for 14 hours a day). This will help your tree continue to grow and flower during the winter months and there is nothing quite as fragrant as a citrus tree in full bloom in the middle of winter!!
Best wishes with your new lemon tree.