Answer: Dry leaves can be sign of several things, including overfertilizing, accidental chemical exposure such as herbicide drift, and somewhat surprisingly both overwatering and underwatering. Your goal in watering is to keep the soil evenly moist but not sopping wet. If overwatered the plant can more or less drown, and barberries, being very drought tolerant, are somewhat subject to that. Underwatering on the other hand will cause the plant to dry out and dropping leaves is a protective mechanism to counter that kind of stress.
To make things complicated, the potting mix can drain at a different speed from the surrounding soil, so you need to water, wait a few hours, then dig down a bit and see what is happening when you water. This will help you judge how much and how often you actually need to apply supplemental water. Hot summer weather can also cause additional transplant stress, making watering especially important to summer-planted material.
Using several inches of organic mulch over the root zone (don't allow it to touch the stem however) can help reduce watering needs, maintain a more even level of soil moisture and will also help keep down weeds.
Finally, planting technique can adversely affect the plant. Planting too deeply, meaning allowing the plant to settle to a level lower than the soil surface or lower than its original planting depth in the pot can also cause problems.
I should mention too that if the plant had severely constricted encircling roots growing inside the container, the roots should have been either untangled and/or cut and angled outward to encourage them to grow into the surrounding native soil. Failure to do so can cause the roots to stay entwined, stuck growing in the original potting soil and this can cause ongoing problems and eventually kill the plant.
If you think either of these situations would apply, you can lift the plant, do what is needed and reset it.
I hope this helps you trouble shoot. Good luck with your barberry.
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