|I have two established Waxleaf Ligustrums which have 3 tiers of growth. Basically, the 'top' portion of the plants have lost 80% of their leaves during the past 12 months. The 'top portion' of leaves turned brownish yellow, became very dry then fell off. Any new growth turns brown then dies. Any advice which I have received locally has not helped resolve the problem. For example: too much water, not enough water, add gypsonite to the soil around the roots, thin out the dead growth, etc.The middle & lower portions of the plants have green leaves but minimal new growth.About two years ago, I placed large wood chips under the plants. Since then the plants stop flowering and I started having problems. Also per the advise of my local Home Depot for the past six months, I soak the plants with the hose every 10 days I am not sure what the problem is....wood chips under the plants, fungus, water/soil problem, insects, etc.|
Generally, when top foliage yellows and dies it indicates a problem with the roots. As roots die off, yellowing progresses down the tree or shrub until there's nothing left. You can check to see if the tops of the shrubs are still alive by scraping the bark with your thumbnail. If you find green tissue beneath the bark, the stem is still alive. If you find brown tissue, that part of the plant is dead. Sometimes dead stems can be pruned away and new growth will emerge - if you correct the underlying problem causing the distress. Roots will rot when kept too moist (they suffocate). Placing a thick layer of mulch over roots that have previously only had soil on top might be enough to block evaporation and hold too much moisture in the soil. This can lead to root rot. Also, wood chips, if they're fresh, can rob nitrogen from the soil, leaving shrub and tree roots without a source of food.
Watering infrequently but deeply is a good practice. It forces roots to grow down into the soil rather than up close to the surface where they are subject to injury.
Finally, some plants simply grow old and die. This may be what's happening to your shrubs, or they may be under more stress than they can handle through a combination of drought, excessive moisture, heat, cold, and/or a thick layer of mulch. Try scraping the bark to see whether or not the tops of the plants are still alive. If not, prune down to live material to encourage new growth. While you're waiting for that to happen, continue to water deeply until fall rains take over for you. I think I'd remove the mulch material out from under the shrubs, just for good measure. If the plants don't perk up for you by next spring, you may want to replace them with healthy new specimens.