Answer: Most wisteria grow best in full sunshine, in well-draining, fertile soil, and most are vigorous growers once they become established. It's possible that neither of your plants are getting as much sunlight as they need. The yellowing leaves indicate an insect or disease problem, possibly because the plant is under stress. When the leaves fall, rake them up and destroy them. Prune after bloom, cutting the new growth back to three buds. This will encourage the development of new flowering wood. Apply ample water to both plants next spring and summer, and feed by spreading a thick compost mulch around the root area. (Too much nitrogen fertilizer will produce vigorous top growth at the expense of blooms.) If the leaves become yellow next season, carefully inspect for insects or leaf spots that indicate disease. If you need help identifying a pest, take a sample of your plant to your local extension office, or to a Master Gardener Clinic. Contact OSU Cooperative Extension, 211 SE 80th Ave, Portland 97215-1597. Phone (503) 725-2000.
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