Spring has sprung and gardeners are digging in. Got questions as you carry on? Get answers from Ask an Expert, an online question-and-answer tool from Oregon State University’s Extension Service. OSU Extension faculty and Master Gardeners reply to queries within two business days, usually less. To ask a question, simply go to the OSU Extension website and type in a question and the county where you live. Here are some questions asked by other gardeners. What’s yours?
Q: I am wanting to plant shade tolerant shrubs and perennials in an area of very compacted soil with many exposed roots from mature Douglas fir and big leaf maples. Info on the internet is contradictory, i.e., don’t cover roots, cover with 4 to 6 inches of compost or bark chips twice a year, don’t plant near big roots, etc. Can you advise? – Washington County
A: To plant successfully under conifer trees recognize that the area will probably have slightly acidic soil from the needles, which are mildly acidic. Compacted soil will need to be gradually built up and drainage improved. The chosen plants need to be shade tolerant, and be able to withstand dry conditions. Trees take up an enormous amount of water, leaving the soil around them very dry. That said, many shade-tolerant, drought-tolerant plants do well under trees. This article has a good discussion and includes several plant options, Plants that grow well under pine trees.
Trees have two main kinds of roots, non-woody and woody. Non-woody, fibrous roots appear in the first few inches of the soil. Their purpose is to absorb water and nutrients. They are sometimes called “feeder roots”. Woody roots are large, lateral roots. They exist in the top 8 to 12 inches of the soil. Because they are perennial, growing with annual growth rings, they can become exposed. The purpose of woody roots is to support and anchor the tree, as well as transport water and minerals to the body of the tree. They also store carbohydrates. This article has additional information, Tree Root Systems.
To plant around exposed roots, use small plants at first that can develop in the spaces between the roots. Adding a large amount of soil to the area will suffocate the trees’ roots. Mulch the area after planting with a couple inches of loose compost or bark dust. This will retard soil moisture evaporation, add nutrients, improve drainage and allow the roots to become established. Make sure you keep the tree trunks mulch free for at least 12 inches from the tree, as the trunk needs to “breathe”. Water weekly when weather is dry, even more often when conditions are hot and dry. A soaker hose will provide long slow moisture. Avoid fertilizing for the first year, as it encourages top growth rather than the roots which need to develop. Mulch the area every spring with 2 to 3 inches of organic matter like compost, shredded leaves, well-rotted manure, etc. The exposed roots can be covered as well. Soil organisms will gradually bring the nutrients down into the soil.
There really is quite a selection of plants that will work in your situation: sweet woodruff, hostas, ferns, bleeding heart, rhododendron, hydrangea, wild geranium, violets, trillium, hardy cyclamen. Great Plant Picks, a site managed by Pacific Northwest nurserymen has great information, pictures, growing conditions, size, etc. and is specific for our area. Look at shade tolerant and drought tolerant lists for plants. The main goal is to use plants that are of the right size and type to be happy in that environment; and to ensure they have access to enough moisture to thrive. This is especially important during the first couple of years while the plant is young. – Anne Schmidt, OSU Extension Master Gardener
Q: Our coral bark maple tree was planted professionally. The following year we accidentally broke off a couple of major branches, leaving the tree looking lopsided. I am not sure when we noticed the dark streaks on the trunk. The third year we talked with an arborist about trimming the tree to make it more balanced. He diagnosed it with verticillium wilt, an air borne disease, and that there nothing we could do about it but it would not hurt the tree. He trimmed it lightly in March and said it would fill in with new growth. Three days ago (May 17) a small branch (about the size of my little finger) fell off and we thought nothing of it. A day later a large branch (size of my fore arm) fell off during the night. I am enclosing pictures of the tree and the open wound left by the broken branch. Is there any hope for the tree? What is the treatment for the tree? The soil? Should we take it out and replant it?
A: Although Verticillum wilt is very common in the Pacific Northwest on maples (like 80% of the time), I do not think this is that disease. My main worry is a large crack in the trunk. I see a crack in the trunk going from a discolored wounded area near the branches, that I assume the large limb fell from, all the way down past a weeping, darkly colored spot to near the ground. Weeping wounds of this nature tend to be due to winter injury followed by colonization of various microorganisms or Phytophthora canker. The limbs may have had tight angles to the trunk which can be weak points letting in the microorganisms. In time, they may fall off as you have experienced.
If irrigation water is hitting the trunk then if could be Phytophthora canker. I see micro sprinklers in one image so maybe it is not. You can read about that disease here and see an image.
As for winter injury, look at the fourth image down at this site.
In either case, I think you will continue to experience branch problems until the tree heals itself. Branches on the side of the crack will be especially problematic. That may take a few years. Or it may continue to decline and die. Make sure irrigation water does not hit the trunk and give it good care. Not sure there is much else you can do for it. – Jay Pscheidt, OSU Extension plant pathologist
Q: I have found many of these growths on one small oak tree. I cut one open and it’s very juicy. – Washington County
A: These are galls from one of a number of oak gall wasps. The likely species in this case is California gallfly (Andricus californicus), which is known for the large apple-like galls that it stimulates the oaks to grow. Gall wasps or their larvae produce chemicals (phytotoxins) that stimulate the host plant to grow the gall structures. Galls provide a relatively safe and nutritious place for the young wasps to develop. See these links below for more information: PNW Insect Management Handbook; Andricus californicus; and Gall Wasp. – Glen Ahrens, OSU Extension forestry specialist.
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