Finding the root of the problem

There are numerous diseases that can damage trees and shrubs.

Root weevils: Adults notch leaves and larvae eat the roots. Control throughout the summer with insecticides or in October with beneficial nematodes.

Phytophthora root rot: It is soil-borne and spreads in water and on tools. Can kill rhodies, some cedars, raspberries, and other plants. No good treatment. Improve drainage. Worst in clay soils. Don’t plant rhodies in a hole of heavy clay. Make a small berm and plant on that, covering the plant roots with good soil and organic matter.

Dogwood anthracnose: Much worse in wet springs. Disease spreads among new foliage creating dark blotches on leaves. Can girdle and kill twigs. Prune for air circulation and possibly try copper fungicides. Plant resistant varieties.

Aphids: Damage new growth on trees and shrubs, especially important to  newer plant trying to put on size. Rains “honeydew” on unsuspecting homeowners. Now largely controlled in trees by that new ladybeetle that so many curse when it decides to spend the winter with you. Wash them off and spray them off smaller shrubs and roses.

Tree borers: Usually a sign that something is stressing the tree. There are some chemical controls that aren’t always helpful, especially if the tree stress condition isn’t fixed.

Cherry diseases: Wet weather increases the problems with both bacterial canker and brown blossom fungal blight. Flower buds and twigs are killed. The bacterial canker can move into the water conducting tissues and cause tree death. Sprays (both spring and fall) can reduce the infection pace. Pruning can remove infected twigs and improve air circulation.

Scales: Soft of hard bodied insects feed on plant sap sucking through bark or leaves. Oil sprays and chemicals help control infestations.

Lawn mowers and string trimmers: It isn’t helpful when tree trunks are damaged or girdled by thoughtless landscape maintenance. Disease can start in the wound and it is often fatal.

Native trees for wet areas: We have quite a mix of soil and drainage types in Columbia County, often on the same property.  Successive cycles of Douglas fir harvests and re-plantings have left some root disease pockets that can kill both young and older Douglas fir. So tree planting choices are a bit complicated sometimes. One alternative tree for root disease pockets is western red cedar.

If there are no landscape considerations, plant the species type. If you want an outstanding landscape variety with a more compact, denser canopy than the standard type, look for a variety called Thuja plicata ‘Atrovirens.’ Other native trees that do reasonably well in dampish sites include Oregon ash, red alder, grand fir, and Sitka spruce. All will need some initial protection from deer.  

Compost, plants, and tough sites

Construction activity rarely leaves soils in good shape. At worst, most of the topsoil may have been removed. At best, there is major soil compaction. What are the best ways to improve degraded soils for your garden and landscape?

The answer (and no surprise for many gardeners) is compost. Compost improves water infiltration, loosens soils, stimulates biological activity, and adds nutrients. But what some interesting research done at OSU’s North Willamette Research and Extension Center in Aurora revealed is that it didn’t really matter whether the compost was tilled into the soil or applied to the surface with new plants installed through the compost.

The amount was important. Two inches of compost applied over 150 square feet was the best amount. That is equal to one cubic yard for that area. After planting, all plots (tilled in compost, tilled without compost, no-till surface compost, and plain without compost or tilling) were mulched with 3 inches of fine Douglas fir bark.

The research group tried both standard landscape plants and some drought tolerant ones. Since the planting area was not watered after the fall planting or the following years, the drought tolerant materials fared much better. Ceanothus gloriosus and Rosemary “Blue Spires” were very strong performers.

Bottom line: Right plant, right place, and compost!

For more information, see Improving Garden Soils with Organic Matter EC 1561 from the OSU Extension publications web site


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