KEEPING SHADE TREES HEALTHY

CURTIS MAY.

The formula for keeping shade trees healthy has three parts : Selecting kinds of trees that are adapted to your locality; planting them in good soil; and following a program of soil maintenance, watering, pruning, and treatment for diseases and insects.

The kinds of shade trees that will grow well in the different regions are discussed in preceding articles. A few general considerations need to be repeated here : Generally speaking, the species that grow naturally in any region are adapted to the climate of the region and can cope with native pests. If they also can withstand the artificial conditions imposed when they are planted for shade trees, it is advisable to use them, provided they are of the proper form and size. Some trees grow satisfactorily outside their natural range, it is true, and many species introduced from other continents succeed well in various parts of the United States. Before one makes extensive plantings of the introduced species, he will do well to check their usefulness, hardiness, and values. On those points, other sections of this book, arboretums, experiment stations, nurserymen, and garden publications give a wealth of information.

Your properly chosen tree has been planted how does it grow?

If it grows well, leave it alone. If it does poorly, one or several remedies may be needed. Some of the symptoms of disease and decline in trees are so specific that the cause can be diagnosed accurately and easily. Other symptoms can develop from a number of causes. Five early warnings that all is not well are sparse foliage; leaves that are paler green than normal; die-back of the tips of the twigs; drying and loosening of the bark; and abnormally slow growth.

Look first to the soil its fertility, drainage, aeration, and moisture. The trouble might be that the roots are not developing as they should because the soil is heavy clay, airless and poorly drained, such as the soil often is that is excavated in the construction of a basement. If so, fertilizer, organic matter, and loosening of the soil are needed.

Most of the roots of trees do not grow deeply into the earth. Unless the soil is gravelly or sandy, the bulk of the roots of most kinds of trees is likely to be found in the upper 3 feet; commonly they penetrate even less deeply. When the minerals in the soil mass in which the roots are growing become inadequate to maintain good growth, the tree begins to show signs of decline. Annual depletion of the mineral elements by taking away all grass clippings and fallen leaves may not affect the tree for several decades of its early life, but after 30 to 50 years signs of mineral depletion often are evident. Street trees are even more likely to decline from lack of soil fertility than lawn trees, because the soil mass in which their roots can develop well is likely to be even more restricted than it is for lawn trees. The need for additional minerals in the soil can be corrected by applications of fertilizer.

THE FERTILIZER should be applied regularly, preferably in the spring, just about the time growth begins. It can be applied safely until about the middle of the summer in most parts of the country even later in the Deep South. Or, the fertilizer can be put on in the fall after the trees have lost their leaves. Evergreens, however, should not be fertilized in the late fall.

The amount of fertilizer to be used without danger of causing injury varies somewhat with the kind of tree, the condition of the soil, and the time of application. A safe dosage is 2 pounds for each inch in diameter of the trunk 3 feet above the ground line. Wherever possible, it should be put into the ground to a depth of 15 to 24 inches to encourage deeper root growth.

When fertilizer is spread on the surface over a period of several years, the tree tends to develop roots at the surface of the soil, which interferes with mowing the lawn, and during drought periods cannot obtain sufficient water.

Maples and elms tend to produce many roots near the surface even under the best conditions, and fertilizer on the soil encourages the habit.

If a chemical fertilizer is used, it should be thoroughly soaked deep into the soil; otherwise it might cause some burning of plants, and the roots, which take up nutrients only in solution, will be able to get it slowly or not at all.

The practice of raking and burning the fallen leaves each autumn and removing all grass clippings eventually reduces the fertility of the soil to such a low point that trees may not get enough of the mineral elements they need. In the forest the decaying fallen leaves provide a protective mulch that conserves natural moisture and tempers summer's heat and winter's cold. They return to the soil the mineral elements necessary for tree growth. Grass under trees may rob them of needed minerals. Top dressing the lawn does not meet the requirements of the tree, but a heavy application of well-rotted manure over the root area of the tree is usually beneficial.

A SECOND FACTOR that sometimes affects the growth of shade trees is the lack of aeration of the soil. Clay soils that bake hard in summer and that are heavy and sticky when wet are not favorable for good tree-root growth. Such soils can be made looser by the addition of sand or some other suitable material and organic substances, such as manure, peat, compost, thoroughly rotten sawdust, decayed leaves, and so on. The materials can be worked into the top 4 to 6 inches of soil. Ordinarily for that purpose it is better to use a fork than a shovel, because a fork injures the roots less. A surface mulch of organic material is often helpful; it helps to conserve water and provides insulation against overheating.

For the decomposition of newly fallen leaves and sawdust, additional nitrogen is required. Unless nitrogen is added, the addition to the soil of fresh leaves or sawdust may actually reduce the amount of the essential nitrogen available to the tree roots. Moreover, fresh manure, newly fallen leaves, and similar materials mixed into the soil or buried in it may release, during decomposition, injurious substances that are poisonous to the trees. However, leaves and sawdust can be used as surface mulches.

Trees often get too little water: Many street trees grow in places where the area of soil exposed to rainfall is small; lawn trees have to compete for water with grass and other plants. Moreover, the drain pipes that honeycomb the soil in cities remove from it, every day, thousands of gallons of water and might lower the soil water table so much that established trees cannot get enough water.

Heavy watering will prevent damage from this cause. Light sprinkling merely moistens the ground for only a few inches and does not benefit the trees. A thorough soaking of the ground for several hours once a week is much better than sprinkling each evening.

A COMMON CAUSE of the decline of established trees in residential areas is damage done to them when the houses were built and streets laid out.

Soil piled on the ground over the root area of a tree can kill it within a few weeks or after a score of years, depending upon the depth of the fill, the sensitivity of the species, and other factors. A few inches of earth fill over its roots can kill the American beech. The American elm will often withstand several feet of earth fill.

If the earth fill happens to be gravel or has a large amount of decomposed organic matter in it, the effects may not be evident until 15 to 30 years later. Gradual compaction of the filled-in soil and the complete decomposition of the organic material slowly reduces permeability of the soil to air and the roots die slowly from suffocation. Eventually the tops of trees with damaging earth fills over their roots begin to die back. Often they blow over in storms the stubs might have rotted below the soil line and for some distance above it; oxygen starvation of the roots combined with wood decay has finally killed the tree.

How can one tell whether an earth fill has been made over the root area of a tree? Normally the base of a tree at the ground line and just above it is greater in diameter than the main trunk a few feet above the ground. A buttress or flaring of the trunk just above the ground line indicates that it has grown normally at that soil level. However, if the trunk enters the ground without expanding, flaring, or buttressing, one should be suspicious that an earth fill has been placed over the roots. The soil around the base of the trunk should then be removed to determine if a fill has been made.

The bad effects of a fill can often be prevented or overcome by installing drain tiles in the soil. The tiles should be placed at the old soil level. They should open into a well built around the base of the tree. This well can either be left open or can be filled with coarse stones. The tiles may be installed either in a radiating pattern or be laid in parallel lines. In either case they will provide both aeration and a place to introduce water during drought.

MANY DISEASES of shade trees are caused by parasitic fungi and bacteria. Some virulent parasites will attack and disfigure or kill trees even though they are growing under the best possible conditions. Many weak parasitic organisms, however, that do practically no damage when trees are growing well can destroy trees that grow under poor conditions.

To avoid the attacks of many kinds of twig blights and trunk and branch cankers caused by weakly parasitic organisms, give your trees the best possible growing conditions keep them well fertilized, provide organic material in the soil, make certain that the soil is permeable to air, and water adequately during droughts.

Many of the virulent fungus parasites can be controlled by the application of fungicides, but for others no means of control are yet available. Some diseases can be prevented by controlling the insects that spread them.

Most leaf diseases (which do their damage by reducing the ability of the leaves to make sugars and other foods needed for growth and other normal functions) can be controlled by spraying with fungicides. Sycamore anthracnose, a destructive fungus disease, will respond to pruning of the cankered branches and spraying with a fungicide two or three times in summer.

Bordeaux mixture is the commonly recommended fungicide. For this work power sprayers are usually necessary.

Leaf spots of elm, maple, horsechestnut, ash, and many other kinds of trees can be controlled by spraying.

Many leaf diseases are not sufficiently serious to warrant spraying for them. They ordinarily will not kill a tree in one season, but if the attack is serious year after year, great damage may result.

The fungi that cause cankers of the limbs and branches can often be controlled by removing the diseased branches or by cutting out the cankered tissues. When cankers are excised, the wounds should be painted with an asphaltum paint containing 0.2 percent phenol-mercury nitrate, a powerful fungicide.