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Written by Dave Gs • August 20 Summer Tree Problems – What Counts, and What Doesn’t

There is a cycle to the growth of a garden through the seasons. After the silent wait of winter, spring arrives full of promise. The fresh, new growth of trees and shrubs is beautiful to see, and the perfect blossoms lift our hearts. Gardeners are filled with enthusiasm, planting new plants, and making plans. Summer arrives, and then things can start to go wrong. Newly-planted trees may die back, or even die completely. Leaves become chewed or covered with unsightly fungal growths. Some leaves may drop prematurely, and at the sight of all this a new gardener can become disheartened.

We don’t like to see our dreams of perfection showing flaws, and summer is a time when inquires about plant problems become more frequent. Sometimes the problems seen in summer can be serious, and other times they are not – so how to tell the difference? It takes time to become a ‘gardening expert’, but in this blog, at this time of year, it seems like a good opportunity to look at some of the common problems seen in many gardens in summer, and to separate the serious from the non-serious. Learning to ignore things that don’t matter – even if they are unsightly – lets us focus on the important things, and it also relieves a lot of the stress that comes from feeling overwhelmed by problems.

Leaf Drop

Leaves falling in summer is perhaps the most common thing seen in gardens. It helps to distinguish evergreens from deciduous trees in this matter. In the normal life-cycle of a deciduous tree or shrub, leaves drop in fall – and we accept that as natural. For evergreens, life is more complicated. They hold their leaves for several years, depending on what kind of plant it is, but sooner or later the leaves do age and fall. Late spring and early summer is the most common time for this, although it can happen at almost any time. With cedars or pines, and laurel too, seeing brown leaves in summer is perfectly normal, and nothing to worry about at all. Just as long as the newer leaves are green and healthy, everything is as it should be. Once the leaves are shed everything will return to normal.

With deciduous trees it is more complicated. For many trees, most of the growth occurs in spring and early summer, so that by mid-July the buds for the following year are fully formed. Sometimes – especially if you trim them in summer – they may put out a second flush of growth, but usually the work is over before peak-summer arrives. If, for reasons of mild drought, sun-scorch or superficial disease, leaves fall, this is usually not significant, and the growth next spring will be healthy and normal. Japanese maples are especially prone to this, particularly if planted where they receive the afternoon sun, and/or if the soil becomes dry. Although unsightly, and preventing good fall coloring, when this happens after July it does very little harm to the tree.

Verticillium wilt on a maple tree

There is an important exception to this with other maple trees. If you have established trees showing premature fall color and leaf drop, especially for two or three years in a row, you might be dealing with a serious maple problemVerticillium wilt. This disease has been spreading through maple populations in recent years, especially in the north-east. It is a potentially lethal disease, so if you see these signs on your maples some action is called for. Pruning the dying or dead branches does no good, as the disease is inside the tree already.

There is a simple step you can take though – not guaranteed, but often very helpful. In early spring apply a high-nitrogen fertilizer to your tree, all across the root-zone, which extends several feet outside the drip-line of the tree. The best way to do this is with root injection equipment, which a local arborist should have. You can also do it yourself by punching holes in the ground with an iron bar and dropping ¼ – ½ a cup of tree food down each hole. Use a high-nitrogen food for evergreen trees, rather than one for deciduous trees. The burst of new growth this causes can isolate the disease and prevent its further development.

Remember that we don’t want to see leaf drop in any newly-planted trees, where it is much more serious. Careful watering is needed for the first couple of seasons for proper establishment.

Dead Branches

Fire blight

When talking about dead leaves, we need to distinguish between them and dead branches and buds. To tell the difference, look closely at the branches. Are they shriveled? If you take a knife and scrape a little of the bark away, what is the color underneath? Green or white is good, brown is not. ‘Blight’ is the name usually given to this, and it can be common in a range of plants.

Trees like crab-apples, ornamental pear, edible apples and pears, and the firethorn bush all suffer from a serious disease called Fire Blight, and branches dying can indicate this. Remove these branches, after finding where the dead bark ends, and the live bark begins. Add six inches for small branches, and 12 inches for larger branches, and cut there – not exactly where the brown ends. The disease has already traveled further, and you need to remove it all. This disease is worse in a wet spring, and sadly infected trees often die over several years. Other trees like chestnut and sycamore can also suffer serious blight diseases, and you may need expert help to control them.

Something to remember is that with younger trees, as they grow, the lower branches will die quite naturally. Non-serious dead branches will always be the weaker ones, lower down the tree, or inside the crown. Their loss is just part of the life-cycle of the tree. It is when the younger growth at the top and outer parts is dead you need to look more closely.

Spots and Growths on Leaves

Mildew on leaves of a Crape Myrtle tree

Many trees are hosts to pest and diseases that disfigure the leaves but do little or no harm. Common on many trees and shrubs is a white, powdery growth that can cause leaves to also shrivel and fall. This is Powdery Mildew, and it is usually harmless, especially on established plants. There will be no sign of it on new growth next year, but it can develop each summer during hot, humid weather. Lilacs, apples, sycamore, maples and roses often show this problem

Large black spots are common on maples in some years, but again the do very little harm. Strange bumps and growths on maples and oak leaves also are harmless, if unsightly. Other leaf diseases can be more serious, and they are worth checking out, but usually they are seasonal and can be safely ignored. Removing fallen leaves and destroying them is the best way to break the cycle of these problems, and if done thoroughly you will see a big decline after a couple of years.

There are many potential tree and shrub problems, but most of them are harmless, so while checking is a good idea, often very little needs to be done. The good news is that most of our plants will thrive if they receive some basic care.