Inspect the Central Leader
Most landscape trees should be pruned to have only one central leader (the vertical stem at the top of the trunk). This leader adds strength and stability to the tree structure and creates the tree’s upright and straight appearance. For some trees, more than one leader may eventually cause the tree to split, creating a wound for insect or disease infiltration. Some trees, however, can exist (and thrive) with more than one leader. These species include fruit trees like peach, nectarine, cherry, and plum trees, species that contain several trunks (each trunk should have one main leader), and some topiary and bonsai forms, like an espalier, which are pruned and trained to take on a certain growth pattern. Though multi-stem trees are not necessarily unhealthy, they should be evaluated for included bark in any unions, which can compromise the tree’s structural health.
Check for Yearly Growth
A healthy tree will produce new growth every year on both its trunk and its branches. Inspect the progress yearly by checking the distance between the current season’s buds and last year’s (evidenced by scars on the branch). The appropriate growth varies by species, so check with your local garden center and understand the specs for your variety to know what to expect. Growth rings on the trunk can tell you how much your tree has grown over the past year. However, the only real way to gauge the age of the tree is to cut it down to reveal the rings. Still, a healthy tree’s trunk will expand in thickness each year. Take out a tape measure and gauge the growth of the trunk. Even a minute expansion can be a good indicator of health.
Prune Dead or Broken Branches
Each year, prune dead and broken branches as soon as they appear. Dead branches left in place provide an invitation for insects and diseases to move in. Test any suspect branches by scraping the branch with your thumbnail. Living branches will be green underneath, and dead ones will show brown. You can also test the branches by gently bending them. Living branches will be supple and bend easily. Dead branches will snap with increased pressure.
Inspect the Trunk Health
With the exception of certain trees (like birch, eucalyptus, and maple trees), the bark of your tree should not come loose or peel unless your tree is at the beginning of growing season. The bark should be free of fungi or moss, as well. Take care when using garden equipment around trees, as damage to the trunk, like a nick or a gouge, can leave an open wound for insects and disease to attack. Check your tree for large cracks or holes and cover them with a tree guard if they are substantial.
Tend to Bare Patches
If you have an evergreen tree like pine, spruce, or hemlock, watch out for sections of the tree that become bare or free of needles. Common causes of bare patches include a lack of nutrients or water reaching certain branches, damage due to animals eating the needles, improper pruning practices, pesticide damage, insect infiltration (from the pine beetle, bark beetle, or pine weevil) and disease (like canker, rust, or shoestring disease). A deciduous tree, on the other hand, will grow its leaves in the spring and then shed them in the fall, remaining bare all winter long. This seasonal manifestation should not alarm you, as it’s a natural process.
Check for Proper Leaf Color, Shape, and Size
A good indicator of good tree health is the appearance of the leaves. Make sure the tree’s leaves contain the right color hue for the season. In most deciduous trees, this means green leaves in the spring and summer, and yellow, orange, or red leaves in the fall. On evergreens, green needles year-round is a healthy sign. Unless a tree naturally produces yellow or variegated leaves in the spring and summer, be wary of this color. It could be an indicator that the tree is not getting enough water or nutrients. Also, the tree’s leaves should not be stunted or irregularly-shaped. This characteristic can be a sign of nutrient deficiency, insect damage, pesticide damage, or disease.
Know the Signs of Disease
If your maintenance practices are top notch, poor tree health can be due to insects or disease. Warning signs of either invader include visible insects, a lack of fruit or flowers on a fruit tree, distortion in the leaves, holes in the bark, irregular growth on the branches, and oozing sap on evergreens. Another common sign that a tree is stressed is wilting when the leaves and stems lose their rigidness and begin to droop. If this happens, first, check your watering. Are you watering too much or too little? Additionally, your tree may be getting too much sun for its species. Wilting can also indicate over-fertilization or a tree that is root-bound.
