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Mature trees and shrubs normally do not need to be fertilized unless a soil sampling shows a deficiency of a specific nutrient, or unless the plants are showing signs of stress. With the rains we receive west of the Cascades, the deficiency will usually be Nitrogen since it is water soluble.

The good news the Clackamas Master Gardeners are conducting free soil testing at the “Fall into Gardening” event October 8 from 9 am to noon at the Milwaukie Center, 5440 SE Kellogg Cr Dr, Milwaukie. Instructions for gathering soil samples can be found in the “Testing Soil pH” handout at www.cmastergardeners.org under the “10-Minute University” ™ tab. Soil testing is also done at the Spring Plant sale which is always the week before Mothers Day.

PH is important to know because if soil pH is too high, it will be difficult for plants to take in nutrients.

If your Arborvitae do need fertilizer, the time to apply would be spring. Applying fertilizer in Fall can encourage growth that will be susceptible to damage from frost. This can be confusing because the time to fertilize lawns if fall as they come out of summer dormancy. The key is to fertilize before the plant begins its period of growth. Fertilizing any other time is a waste of the fertilizer and the potential to contaminate waterways. Since nitrogen is often the only nutrient missing, blood meal works fine to replace what is missing.

Normally just adding a layer of compost will add nutrients to the soil and improve the soil texture. Compost can be anything from commercial compost to leaf mulches. Be aware that using wood chips or bark dust by itself will actually decrease the amount of nitrogen available for the plants because the microbes use the nitrogen to break down the bark, leaving little for the plants.