Parkland Press

Saturday, October 19, 2019

Diagnosing plant problems not easy

Wednesday, May 22, 2013 by The Press in Focus

Here are some questions to ask yourself when diagnosing plant problems in the landscape.

What is the name of the plant? It is important to know the normal appearance and growth habit of a plant before diagnosing problems. The perceived "problem" may be normal for this plant. Accurate plant identification is the first step in diagnosing plant problems.

Examples: Larch is a deciduous conifer and appears "dead" in winter. White pines shed inner needles profusely in autumn. Sweetgum has corky ridges on its stems. Variegated cultivars often appear abnormal in color.

What are the common pests that affect this plant? Most plants have a limited set of insect, mite and disease pests that affect them. When diagnosing problems, it helps to know the signs and symptoms of these common pest problems.

Check references such as "Penn State's Woody Ornamental Insect, Mite and Disease Management Guide" that outlines common pests of trees and shrubs.

Does the plant have specific site requirements or intolerances? While many plants tolerate a wide range of growing conditions, others do not. Know the requirements of each species.

Examples: Yews and Douglas fir are intolerant of wet sites. Rhododendron, pin oak, most hollies, and many other plants require acidic soils.

Some plants tolerate shade, others don't. Know the site preferences for your trees or shrubs. Many plant identification texts can help you determine site intolerances for specific plants.

How long has the plant been established on this site? Dead or dying plants that have been established less than two years may be affected by problems relating to the establishment of that plant in the landscape, beginning with the quality of the plant before it reached the site. Improper digging, handling, planting and post-transplant care are common plant killers.

Examples: Plants desiccated in transport; plants over or under watered in storage or after planting; plants dug with improper ball size; planting too deep in the nursery and/or the landscape.

Has there been recent construction, paving, grade change, or other disturbance on the site? Soil compaction and grade change reduces soil oxygen levels in the root zone. Tree roots can't function without adequate levels of oxygen. Trees decline and die. Decline may not occur until several years after the construction activity which makes diagnosis difficult.

What's happening where the plant stem meets the soil? This is where you'll find girdling twine, rodent injury, girdling roots, mechanical injury, plastic burlap, graft incompatibility, improper planting depth signs, plastic pots still around the root ball, etc.

Is the site extremely wet for any extended periods during the year? Some sites are inherently too wet for certain landscape plants. Be aware of changing soil-water relations around plants due to construction, water diversion, broken rain spouting, etc.

Were herbicides applied recently? Herbicides used in normal turf and landscape management rarely kill landscape plants. The ability of a herbicide to cause landscape plant injury will vary depending on its mode of action, the plants involved, and many other factors. Some herbicides cause distinct plant injury symptoms while others will cause a range of symptoms that vary with the plant species involved. Herbicides are unlikely to injure individual plants in the landscape, so if symptoms are confined to one plant and surrounding plants are healthy, consider another diagnosis.

Were there recent, unusual weather conditions? Low temperature, late spring freezes, high winds, hail, drought, and excessive rainfall all affect plant growth. Weather conditions such as these are a convenient explanation when no other explanation is obvious and are sometimes inaccurately used. On the other hand, don't underestimate the impact of extremes in weather, often a year or more after the stress occurred.

Does the landscape have adequate soil resources to support plant growth? New construction sites often leave just a veneer of soil over cinder blocks, rubble, mounds of tree stumps, rocks, asphalt, etc. Unfortunately, you may never determine that this is the problem unless your postmortem includes use of a backhoe to reveal what is going on underground.

Has there been any other unusual activity that may have affected tree, shrub and turf health? This list is endless. Keep your eyes peeled for the unusual. Knowledge about the site history is often essential.

Examples: fired up the gas grill under the lilac, piled de-icing salted snow in the shrub beds all winter, never took the dog more than 10 feet from the porch all winter, fertilizer in deteriorating bags left under trees in the landscape, etc.

Diagnosing plant problems is not easy. Trees and shrubs, like other living things, often succumb to an accumulation of stresses rather than a single factor. This makes diagnosis difficult and sometimes impossible.

Experience and judgment come into play. For instance, while cedar apple rust and aphids are frequently found on crabapples each year, neither are plant killers. You must be able to discriminate among your observations before drawing conclusions. Sometimes, we fail to look closely enough to gather the information needed to diagnose the problem.

"Growing Green" is contributed by Lehigh County Extension Office Staff and Master Gardeners. Lehigh County Extension Office, 610-391-9840; Northampton County Extension Office, 610-746-1970.