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Summary: Growing grass in shade can be a major lawn problem for homeowners. Even with excellent grass seed germination, sowing grass seed does not solve this problem. In this article we will begin looking at the issue of grass in the shade.

Question: In parts of our yard where the trees shade much of the grass the lawn is very thin and does not do well. What is the best grass that grows in shade under trees and when should we sow grass seed for the lawn in those areas? Mike, Milford CT

Answer: Mike, the grass problems associated with growing grass in the shade can be the most exasperating of all lawn problems.

The causes of “grass failure” to get good turf growing under trees are wide ranging, most of which are interrelated.

So, it is not possible, to give an exact answer that this thing or that thing is the cause when grass does not thrive in shade. As with most plant, garden, landscape and lawn problems, there is almost always a chain of circumstances is responsible.

For our discussion of – grass that grows in the shade – the problem is divided the contributing difficulties into four groups listed in the order of their importance:

• Lack of sufficient plant nutrients
• Shortage of moisture during summer months
• Absence of sunlight
• Other unfavorable factors

Strangely enough what at first might be considered most important is almost the least important. Absence of sunlight is the factor most easily solved.

This can be fixed by supplying a mixture of grasses that are actually adapted to growth in shade. Unfortunately, because of lack of information, the cost of seed, or because it is a little extra trouble, the proper grass seed is seldom provided.

The other difficulties are not so easily solved. To overcome them it is necessary to understand the trouble, so we need to consider the various causes in detail and then prescribe what have been found the best corrective measures.

Lack of Plant Food

There is strong competition between trees and grass for the limited supply of plant food being lawn fertilizer for the grass or landscape fertilizer for the trees.

The nurseryman says grass robs the trees of nutrients, while the grass man says the tree, being more aggressive, gets all and little remains for the grass.

However that may be, we have it on good authority that a medium sized tree each year requires nutrients equivalent to 20 pounds of a high analysis complete fertilizer.

Sod under this same tree would need about the same quantity per year if it were to make a satisfactory growth, factors being favorable.

Thus a plot of ground 30 x 40 feet on which a tree is growing and which is in sod must supply each year plant nutrients in the amount contained in approximately 40 pounds of commercial fertilizer.

It is, of course, better for trees if they can find sufficient nutrients located fairly deep in the soil. But if it is not there they will send their feeding roots toward the surface and thereby enter into direct competition with the grass.

Usually the tree will emerge victor, take most of the available surface plant nutrients and leave nothing for the lawn.

A permanent remedy for such a condition requires:

• Regular and Correct Feeding of the Tree Roots
• Regular Feeding of the Grass

One of the best methods of feeding trees we discussed in “How To Fertilize a Tree”.

Feeding grass is not a difficult matter. To insure a constant and adequate supply of nutrients, shaded lawns should be fertilized three times yearly, namely in spring, early summer, and fall.

Moisture Conditions

As previously indicated, the supply of nutrient materials is only one of many problem factors affecting grass for shaded areas. In some ways the moisture condition, deficiency or over-abundance, exercises a greater influence because of its close relationship to mechanical and chemical soil composition.

Botanists tell us that trees as well as grass require much larger quantities of water than we might suspect. A medium sized tree might well transpire some 500 pounds of moisture over a period of twenty-four hours in a normal summer day.

From the soil under this tree we could reasonably expect an evaporation and transpiration loss of about 700 pounds of water which must be replaced if the tree and grass are to be kept from suffering.

This would total a monthly loss of about six inches. Compare this with the average summer rainfall in the north central states of about 35 inches per month.

In general it seems that for grass growing under trees an average of at least one inch of water per week should be supplied (to keep the turf growing vigorously). This would be equivalent to an application of about 5000 gallons on every 1000 square feet.

The image to the right gives some idea of the extent of a tree’s root system. Obviously grass roots are not a match for such roots and so if moisture is deficient, trees get the surface soil moisture leaving little or none for the lawn.

Another condition affecting the water supply available to tree shaded lawns is the action of the tree foliage in diverting water, so that such areas do not get even their share of the scant late spring and summer rainfall.

Many Other Problems

So far we have only considered a few of the many conditions affecting shaded lawns.

Next we will look at the matter of soils and particularly their relation to moisture supply. Other difficulties include damage by the heavy wash of rainfall down tree trunks and the dripping of large drops of water from tree branches and leaves.

The problems of acid soils and toxic soils, unfavorable bacterial environment, moss and grass smothering by leaves will also be look at. Stay tuned!