Ask Mr. Smarty Plants

What's going on in Pierce County, Washington? This is the second question today from people who planted non-native trees there and now have yellowing leaves on them. So, let's address the problem with your Arbutus unedo, dwarf strawberry trees.

Because your trees are native to Ireland, southern Europe and the western Meditteranean, we have no information on it in our Native Plant Database. Instead, we went to this USDA Forest Service Arbutus unedo; these USDA sites are usually pretty impartial about discussions of plants and the locales where they can do well. The map of distribution of this plant showed a small area around Puget Sound where the strawberry tree can be found. Since that's where Tacoma is, we're assuming you're in the right place.

Yellowing leaves on a tree usually indicates chlorosis, or the loss of chlorophyll in the leaves, which makes them green. This happens most often when an acid soil-preferring tree is planted in an alkaline soil. The plant needs the trace element iron from the soil, and in alkaline soils the iron may be inaccessible to the plant roots.

In the West, many plants don't get enough iron because their roots are unable to obtain it from the soil. Roots may be damaged by a lack of oxygen in overwatered or poorly drained soils--typical after a rainy spring--or by extreme soil temperatures. High concentrations of limestone (calcium carbonate) make the soil more alkaline, which makes the iron less soluble. The information we found on Arbutus unedo indicates that it "tolerates" alkaline soil and clay, but "prefers" sandy, slightly acid soil. And, too much water around around the roots in a clay, poorly-draining soil may also harm the tree's capacity to utilize iron from the soil.

So, you asked if you were watering too much? Probably so. To help amend the soil to a more acidic nature and better draining texture, first, add some iron tablets to the soil. Then, trying not to damage the roots, get some organic material, cottonseed meal, etc., in the soil around the roots. Mulch heavily with an organic mulch like shredded hardwood. This mulch will hold in the moisture, help keep the roots cool, and decompose slowly, adding to the organic material in the soil, again helping to neutralize the effect of the clay soil. If, when you water, the water pools on the surface and remains for 30 minutes or so, you are definitely dealing with poorly draining clay soil. Taper off on the watering, and try to create a better-draining environment for the roots.