Weak or Yellowing Foliage The presence of pest, diseases, or physiological problems can reflect on leaf colour or size. A tree is considered to have weak foliage/yellowing leaves if it has thin foliage, is off-colour, or has smaller leaves than what is normal. Looking from the side, the overall crown can be more transparent or lighter green to yellow (Figure 1).  However, keep in mind that some tree species such as Honey Locust, Black Locust, and Ashes normally have more transparent crowns than species such as Maples, Elms, Beeches, etc. (As a rule of thumb, species with compound leaves have more transparent crowns.). Similarly, some cultivars have different leaf colour or shape than what is typical for the species. For example, the youngest foliage of Honey Locust 'Sunburst' has lighter green (almost yellow) leaves compared to the older leaves or those typical for other Honey Locust. Figure 1 Weak or yellowing foliage may be a symptom of a number of problems. It may be caused by pests or disease in parts of the tree other than the leaves. In addition, drought, confined root spaces, soil compaction, girdling roots and poor nutrition can cause of weak foliage. In such cases, maintenance measures such as soil aeration, fertilizing, and watering might help the tree to recover. When examining a tree for weak foliage or yellowing leaves only the portions of the crown not affected by die-back, topping, or pollarding are considered. A tree may have lost a large portion of its crown, but the remaining parts may be healthy. On the other hand, a tree may have a full crown with no history of topping, pollarding, or storm damage, but the leaves on the crown tend to be either small or chlorotic (yellow). The difference between crown loss and symptoms of a weak crown is that a tree may recover from the latter. However, crown die-back, topping, and pollarding, while perhaps compensated by regrowth in other parts of the crown, represent a long-term loss of photosynthetic area. Tree ProblemsCrown Defoliation