Elisabeth C. Miller Library

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Plant Answer Line Question

Keywords: Control of wildlife pests, Thuja plicata

I have a medium size cedar in my yard. Squirrels have been stripping the bark off - it is reddish and seems to come off fairly easily in flexible strips a few to several inches long. I wonder if I should be concerned about this affecting the health of the tree and if so what I should do to protect it.


It is possible that the bark-stripping may cause lasting damage to your cedar tree. Here is a document on managing squirrel damage from the British Columbia Ministry of Agriculture and Lands. Here is an excerpt:
"The location of bark damage on a tree varies among tree species and is probably related to the ease of bark removal and bark thickness, and hence to the growth characteristics of different species. Basal damage (within 1 m of the ground)is the most common type of damage in beech (Fagus sylvatica). Crown damage frequently occurs in the main canopy of oaks and many conifers, while stem damage usually occurs between the base and canopy in, for example, sycamore (Acer pseudoplatanus), beech, birch (Betula spp.), larch (Larix spp.), and lodgepole pine (Pinus contorta). [...] Crown damage affects the growth and appearance of the tree; severe crown damage kills it. Damage to the base and stem is cumulative occurring over a number of years. Wounds tend to callous over, hiding the damage until the tree is felled. Trees girdled by excessive stem or basal damage will die."

There are various methods of discouraging squirrels, but nothing is a fail-safe approach. The book, Outwitting Critters, by Bill Adler, Jr. (HarperPerennial, 1992) suggests dried blood fertilizer, ultrasonic devices, or live-trapping with peanut butter and small fruit as bait.

Washington State Department of Fish and Wildlife's website has a page which includes suggestions on how to protect trees from squirrels.

Date 2019-07-17
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Plant Answer Line Question

Keywords: Throughfall, Thuja plicata

I am looking for some hard data on the ability of Western red cedar to intercept rain water and the timing of release. Got numbers?


If I am understanding your question correctly, you are asking about the extent to which Thuja plicata (Western red cedar) allows rainwater throughfall. I discovered that there was some research of possible relevance to your topic being done at Evergreen State University as well as at University of Waterloo.

The Miller Library has a book entitled Water in Environmental Planning by Luna Bergere Leopold (1978), which might also be relevant to your search.

Date 2019-02-06
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Plant Answer Line Question

Keywords: Thuja, Thuja plicata

I have a golden cedar about 6 feet high. This winter many sparrows sat on the top portion while waiting their turn at the feeder. I don't know if they ate the leaves or if their little feet knocked them off, but many of the branches are stripped and brown. Will they come back? Should I cut them out and hope that new branches will fill in the spaces? It is approximately half of the front top of the tree. Or is it time to take it out? I would prefer not to, unless it can't be saved.


I am assuming that your golden cedar is a form of Thuja occidentalis or Thuja plicata. It is possible the sparrows caused the damage, but there could be other factors involved. It is difficult to tell without seeing the plant. Below are general comments on the liabilities of Thuja occidentalis as a landscape plant, previously available from the Ohio State University Extension website.

  • Winter evergreen foliage color is often an unattractive yellow-brown
  • very prone to bagworms and their feeding damage
  • very prone to branch separation under snow and ice loads
  • widens at its base with age, or separates into several leaning but divergent canopies with age (this applies to both upright and rounded cultivars)
  • does not recover from severe pruning (where the bare stems are exposed, although side branches may slowly envelope the dead stems)
  • interior foliage noticeably sheds in Autumn

The defoliation you describe might also be the work of bagworms. If the tips of the branches are dying back, that could be a result of winter injury, drought stress, or a fungal disease. Since I cannot diagnose the problem remotely, I think it would be best if you brought a sample of the affected plant to a Master Gardener Clinic. If you are in King County, this link to their website will lead you to their website.

Date 2019-04-06
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Plant Answer Line Question

Keywords: Trees--Wounds and injuries, Thuja plicata

I have a mature Western red cedar with an inverted-V gap in the bark, right at ground level. The point of the V is about 2 ft. off the ground; the base of the gap is perhaps 9-10" across. What's the current thinking on protecting this exposed area from diseases and critters? Paint with some sort of goop? Leave it alone? Or something else?


Here is a link to information on managing bark injuries, from Cornell University's Plant Disease Diagnostic Clinic, which includes illustrations. Excerpt:

"When a split occurs on a tree, what should you do? In recent years, quite a bit of research has been done on closure of tree wounds. These investigations have indicated that tree wound paints are of little value in helping a tree to callus over. For this reason, do not paint or try to seal a split with paint or tar. Tracing the bark around the split can be very helpful in aiding wound healing (Fig. 2). With a sharp knife, starting from one end of the split, trace around one side of the wound, about 1/2 to 1 inch back from the split bark. Stop at the other end and do the same procedure on the opposite side of the split. Knives should be sterilized between cuts by dipping them for several minutes in a 1:10 bleach:water solution or a 70% alcohol solution to avoid contaminating the cuts. Carefully remove the bark from inside the traced area. You should now have a bare area resembling the diagram in Fig. 2. Remember to leave this untreated. A tree growing with good vigor usually calluses over quickest. Encourage vigor in the tree with yearly spring fertilizer applications -- and be sure to provide adequate irrigation in hot, dry weather. Bark splits will often close over completely leaving a slight ridge in the trunk where callus tissue has been produced."

The book Practical Tree Management: An Arborist's Handbook by T. Lawrence et al. (Inkata Press, 1993) confirms the method described above. Trim back the bark to healthy tissue around the wound using tools such as a chisel, gouge, hammer, and sharp knife. Wound margins should be rounded, and damaged wood within the wound should be smoothed with a chisel or gouge, but only to the most minimal level (don't go deep).

If in doubt, I would recommend contacting a certified arborist for assistance. You can obtain a referral from Plant Amnesty or the Pacific Northwest chapter of the International Society of Arboriculture.

Date 2019-04-27
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Plant Answer Line Question

Keywords: Thuja plicata, Trees--Diseases and pests--Washington

I have noticed that my old cedar has a very large number of cones on it this year. Several areas of foliage have turned reddish brown. This has all appeared in the past month or less. Some of the other cedars in this area appear to have these characteristics. Is something different going on this year? I am concerned that there may be a disease that is affecting them?


Is your tree a true cedar (Cedrus) or a species of Thuja? The Thuja plicata (Western red cedar) in my garden also had a huge number of cones this year, and just like yours, it has some foliage turning reddish brown. This is probably cedar flagging, as described in this Washington State University Extension page. Flagging--the browning of older leaves and twigs--is a common occurrence on western red-cedar and related trees, such as arborvitae. It usually develops in late summer to early fall. Often, very hot, dry weather, followed by rain, will stimulate the sudden dropping of this older foliage.

If the browning were to be widespread, that might be more of a cause for concern. Additional links:

Cedar Flagging from Private Forest Landowners in British Columbia

Date 2019-04-18
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Plant Answer Line Question

Keywords: Shade-tolerant plants, Thuja plicata, Drought-tolerant plants, Ground cover plants

I have a large Western Red Cedar (Thuja plicata, I think!) under which grow a few weeds but not much else. I would like to find a low maintenance solution for the ground that won't do any harm to it. Ground cover? Pea gravel? I worry that the shallow root system can be easily smothered so that adding soil and plants is not a good idea, plus the roots are a dense mat and difficult to dig through. Do you have any ideas or suggestions? What native plants might grow under the tree and how can I get them established?


As you've realized, Western red cedars have a dense mat of roots close to the surface. It's not a good idea to add soil on top of the roots of trees because their root flare should remain above the soil -- and even if you did, the roots of cedars would spread into that soil in a short period of time. It's also important to keep in mind that under natural conditions the ground beneath Thuja plicata is usually bare of other plants.

If the area beneath your tree isn't in deep shade and has at least some sun, you could plant spring ephemerals, including bulbs. They emerge in spring when the soil has plenty of moisture, then most die back before our summer droughts. They're not difficult to plant under large trees because you don't need to dig a large hole for seeds, bulbs, or small bare-root perennials. Good choices are Anemone blanda, Aquilegia (Columbine), Corydalis lutea, Crocus, Galanthus (snowdrops), Iris reticulata, and various kinds of Narcissus, including daffodils. Most tulips are not long-lived in our area. Hardy Cyclamen emerge and flower at other times of the year, but they're also an excellent choice. Unfortunately, most of these plants are summer-dormant, when you'll probably be out in your garden, and some self-seed prolifically under ideal conditions. A valuable resource, available for checkout at the Miller Library, is Planting the Dry Shade Garden, by Graham Rice (2011). Some of the plants he recommends will require regular watering under cedars.

Another option, also feasible if your area has some sun, is to plant our native Western sword fern (Polystichum munitum). It's evergreen, so has a presence all year, and is the most sun and drought tolerant of our native ferns. I have a 60 foot red cedar in my garden, and have successfully maintained sword ferns beneath it in partly sunny areas, but not in deep shade, where they've died out. However, because they won't survive our summer droughts in nature under these conditions, I've needed to water them about every 3 weeks in order to keep them alive. I've planted fairly small plants and watered them more often than that during their first year. If your soil is very sandy, sword ferns might not do well.

If you require a reliably showy solution, staging large planters planted with annuals or perennials, shrubs and/or trees might be best. The plants you choose will depend upon how much sun the area receives. Of course, they'll need to be watered regularly, but large planters don't need watering as often as small ones. If your hose-bib is not too far away, installing drip irrigation on a timer will ensure that your plants survive when you're away from home.

Date 2019-02-16
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