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Last week's column covered why trees throughout the valley still have their leaves. These, of course, are deciduous trees whose leaves should have dropped by now. This week, in contrast, we are concerned about why many of the needles on the conifer trees are suddenly dropping. I suspect some poetic musician should have written a song about the falling needles drifting past the window, but because of the shape of the needles they don't drift but simply fall beneath the tree.

Conifers include pines, spruces, firs and many other trees where the leaves have adapted into needles. Conifers are great additions to many landscapes because they are generally evergreens. The term evergreen gives the wrong connotation for many of these trees. Even though the needles are evergreen, they are not everlasting. All conifers shed their needles in varying life cycles ranging from one to 17 years. The average life span of most needles on the common landscape conifers is about three or four years.Each year about this time, the older needles turn brown and drop. This is of no concern because the tree is just going through its natural life cycle, providing it drops only the older needles.

Each spring the tree sends out a new bud called the candle. The needles develop on the candle and remain there for several years. As each year's growth develops, the tree forms an annual growth ring. This annual growth ring is the key to determining the age of the needles - count back from this year's growth to determine the needles' age. Needle drop is often confusing because in many years the needles turn brown gradually and drop almost unnoticed over a long period of time. During other years, they turn brown and drop quickly. This condition is most noticeable in Scotch and Austrian pine, the two most common long-needled pines in our landscapes.

It is extremely difficult to determine the exact cause of brown needles on evergreens. Needles that drop from age have occasional spots and blemishes. Old needles sometimes show mottled, brown discoloration from the effects of non-disease-causing fungi. Spots and blemishes on new needles are generally caused by insects or diseases.

Needle drop, confined to the older needles, requires no control. This natural drop provides some excellent mulch for the perennial garden or for the compost. Concern is warranted only when newer needles are affected. At that point, careful investigation is needed to determine the real cause.

Other reasons why needles will drop include mites, drought, improper planting, poor nutrition, herbicides, winter damage and wet or poorly drained soils. Do not confuse these with the natural seasonal drop of evergreens. Normal needle drop is confined to the interior of the plant.

Another common problem is improper planting techniques. Often evergreens are simply pulled out of the can and dropped in the ground without ever examining the root system. The root system should be examined carefully before the tree is planted. Cut girding roots to eliminate serious problems in the future. Roots that grow in a circular pattern around the trunk will eventually strangle the plant and cause its demise. Similar problems occur when wire or twine is left around the base of the plant.

One of the most common causes of brown needles on evergreens is winter drought. This problem was particularly serious in the past four years with long, warm autumns and no moisture. Most soils have received adequate moisture this year, but if the soil dries, give the trees a drink during the winter thaws. Needles continue to transpire water throughout the year, which causes brown needles if there is no moisture in the soil.

We are fortunate in this area to have very few insects and diseases that actually attack the needles of our evergreens. Occasionally pine- needle scale or needle miners will attack the plant. Scale causes uniform spots on the needles that can easily be scraped away. Control this pest with dormant oil sprays. Needle miners are rare and usually do not warrant control. Needle diseases are generally confined to the higher mountain elevations where snowfall and moisture are much higher. Conifers are not often sprayed for insect pests. Two common exceptions would be pine bark beetle (controlled in June) and spider mites (controlled in hot weather).

Beautiful evergreens depend upon good cultural care and freedom from stress. This controls brown, needle-drop problems. Normal needle drop is not a problem and can be ignored. Proper care will reward you with both summer and winter beauty from these trees for many seasons to come.