Thank you for your question, Joan.  I'm afraid the photo is taken from too far a distance to respond confidently.  Leaf yellowing can occur because the plant is nitrogen deficient ("chlorosis").  But it can also occur because the plant is suffering from a soil problem, including nematodes and iron deficiency.  You can read about the several potentials here:  https://hgic.clemson.edu/factsheet/gardenia-diseases-other-problems/
I suggest that you take photos closer to the leaves--top and underside--and visit the Washington County Extension office when Master Gardeners are there helping diagnose the problems.  (If it is attributable to nematodes or a soil deficiency, you'll need to get a soil test.)  Take with you any information about the plant's location, fertilization history, and watering schedule.  Here is information about the Extension office:  https://extension.oregonstate.edu/washington

Good luck!