Answer: Yellowing can be caused by overwatering or mineral deficiencies. Clematis need evenly moist yet well drained soil, meaning not saturated or sopping wet. Since you have grown it successfully for three years in that spot I would tend to think it is soil or nutrient related. (Unless something has caused a change in the soil moisture level where it is planted causing it to be too wet.) You could try a foliar feeding with a water soluble fertilizer such as 10-10-10 plus minors (follow label instructions) and see if that helps. Also top dress with a good quality compost and use an organic mulch to try to provide minor nutrients on an ongoing basis. Also check the soil pH, it should be close to neutral or 7.0 for clematis. You might also want to consult with your local county extension and/or professional nursery staff for help in testing the soil and trouble shooting the discoloring. They may appreciate a foliage sample enclosed in a plastic bag and/or some photographs showing the overall plant and some close-ups.
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