· John Heider's Answer
· George - I have grown a large number of nandina (Nandina domestica and dwarf varieties) over a period of many years and found them to be one of the toughest plants you will ever find. If possible post a picture of your plants wilting leaves and the leaves that are dropping off. If you know the variety of nandina this would help also those in the nurseries that may know this perticular variety more than I may.
Being that the plants have done well for three years and now showing wilt and dropping leaves makes me believe something has possibly changed in the amount of water they may be getting. Both to much or to little water may cause this wilting and dropping of leaves.
The nandina domestica while growing in height will shed its lower older leaves leaving only a leafless stem and higher canopy of leaves as you seem to have now. I prune the higher stems out at their base which allows the new stems with new foliage to fill in and make the plant thicker with foliage. I somewhat doubt your problem is soil PH. Are the leaves turning yellow? The nandina is extremely tolerant of most soils as long as they drain fairly well. The do like a more acidic soil (PH 6.1 to 7.0).
You should see a color change in all the leaves if your soil was to alkaline for the plant. There are a few diseases and insects that can be affecting your plants. Brown or black spots on the leaves can be an indication of a fungal disease. This can cause the leaves to drop also. Other fungal diseases including Verticillium wilt which I have read about on plants but have never seen can be another disease more educated nursery personnel than I could explain if in fact your pictures show more damaged plants than I believe you have.
These are extremely hardy plants and hopefully your pictures will help with a simple solution.)