control methods for Lamium galeobdolon or Yellow archangel

Is it safe to use a product like RoundUp to get rid of invasive plants such as Yellow archangel in my yard? What do you suggest?

Do you have a large area covered with Lamium galeobdolon (Yellow archangel, formerly known as Lamiastrum)? I successfully eradicated this plant from my garden by hand-pulling persistently over a few months. This method is certainly safer than using herbicide, but if you have a vast area to tend, it may be harder to achieve. King County Noxious Weed Control has factsheets on this plant which mention various methods of control, including chemical. Because we are librarians and not licensed pesticide handlers, by law we can’t actually recommend use of a particular pesticide. Note that the information linked here does say that RoundUp (glyphosate) isn’t as effective as some other products. You may also want to take into account the costs of using pesticides in a home garden (where they may affect other non-target plants as well as wildlife, pets, and human inhabitants) against the perceived benefits (perhaps faster and easier than manual control).

The following information about eradicating Lamium galeobdolon is from the Washington State Noxious Weed Control Board:
“Response to Mechanical Methods: Viny plants are easily pulled out by hand during the fall through early spring; however, great care must be taken to remove all parts of plant, as rooted fragments will regenerate (Graham, 2003). It should be noted that L. galeobdolon is highly susceptible to trampling (Packham, 1983).”

If your question is about the safety of glyphosate (RoundUp active ingredient), you may want to read the Northwest Center for Alternatives to Pesticides page about it.

managing rose slug and rose mosaic

My roses were diagnosed with both rose slug and rose mosaic. I would like to know your thoughts about treatment of these conditions, as it was suggested that I just remove affected leaves, and I am looking for a more effective solution.

I will summarize what Christine Allen’s Roses for the Pacific Northwest (Steller Press, 1999) says about these two problems:

Mosaic virus:
This disease infected the roots of your rose when the plant was grafted; the symptoms do no show up for a year or two. The problem is widespread anywhere that rootstocks are developed from cuttings (rather than seed). (In Canada, apparently, most rootstocks are grown from seed, so they have far less of a problem with the disease.) The disease is incurable, and affected plants will have yellow patterning on their foliage. Other plants in the garden cannot “catch” the disease. Sometimes the symptoms disappear by midsummer, but recur the following spring.

Rose slug:
The greenish-white worms are actually sawfly larvae, and they can skeletonize leaves. They aren’t caterpillars, so controls that are used for caterpillars (such as Bt) won’t help. Insecticidal soaps can kill them, but only by making contact, so this means repeated spraying. It is best to do this in early evening when the larvae are most active, and may be seen on the top surfaces of the leaves. Pyrethrins are effective, but they also are acutely toxic to aquatic life, moderately toxic to birds, and may kill beneficial insects such as honeybees [my comments, not the author’s], so they should be a choice of last resort. The Environmental Protection Agency has additional information on pyrethroids and pyrethrin.

To prevent or mitigate rose slugs, clean up leaf litter and other debris several times a season to eliminate pupae and interrupt the life cycle. Hoe the soil gently and not deeply, and apply annual mulch early in the year.

factors causing corn stalks to turn purple

I grew some corn plants from seed that is suitable for the Pacific Northwest. They germinated well, and some are forming ears, but I’m concerned that the stalks and tassels are turning purple. These aren’t purple corn varieties. Could it be a nutrient deficiency?

Since your plants are not naturally meant to produce purple corn, there is a good chance that the cause of the coloration is cultural or environmental. The Cooperative Extension website describes a number of factors that can cause the stalks to turn purple. Here is an excerpt:
“[Purple coloration may be caused by] cool temperatures, soil that is too wet or too dry, compacted soil, herbicide damage, insect damage, and root pruning by side-dressing knives or cultivators. Acidic soil can also intensify phosphorus deficiency symptoms. Low soil pH severely limits phosphorus availability to plants, which may cause deficiency symptoms even where high soil test phosphorus levels exist. Soil pH less than 5.5 typically reduces the availability of phosphorus in the soil solution by 30 percent or more. Acidic soil also reduces root growth, which is critical to phosphorus uptake.”

Before amending your soil in any way (such as adding phosphorus) you should have the soil tested.

common rhododendron problems and diseases

The leaves of 3 of my rhododendrons are covered with a fine yellow powder that can be rubbed off, but spraying with a hose does nothing. The plants, which are scattered throughout a large garden, look healthy otherwise. They receive regular water, have good exposure and are spaced well from other plants. There are other rhododendronss in the yard that are not affected. Any ideas?

We can’t diagnose the problem remotely, but I can tell you a few common rhododendron problems or conditions which sound a bit like what you describe. I consulted a Washington State University Cooperative Extension booklet entitled How to Identify Rhododendron and Azalea Problems, WSU 1984.

If the yellow coating is more like small speckles on the top leaf surfaces, look for small, tarry black spots on the undersides which could be a sign of lacebugs (this may not be a serious enough problem to require control).

If the leaves have light green to yellow diffuse spots randomly distributed around the leaf, and the underside has yellow-orange powdery spore masses, this might be rust (caused by fungus associated with Sitka spruce).

What seems most likely, based on your description, is algae, which coats the leaves with a yellowy greenish powder, and is easily rubbed or scraped off the leaves. Algae does not harm the plant, but may offend one’s sense of aesthetics. The Royal Horticultural Society describes algae on rhododendron leaves.

Excerpt:
Shaded, dense foliage is where algae builds up most on plants, particularly at the base of evergreen hedges, such as holly and yew, or on congested shrubs such as Camellia and laurel. These are areas where water does not quickly dry, creating the damp, shady conditions favoured by algae.
Control:

Clipping evergreen hedges so that they taper outwards towards the bottom can help improve light access to the base.
Congested evergreen shrubs should be thinned out at the appropriate time of year to increase air circulation around the leaves. This will allow rain to dry more quickly on wetted foliage and reduce the opportunity for algae to build up.
Feeding straggly plants in the spring to encourage vigour should also help.

problems affecting Bruns weeping Serbian spruce

I am thinking of planting a Bruns Weeping Serbian Spruce in a mostly sunny spot in my garden. From what I have learned, it appears to be rather small and narrow, suitable for a smaller yard–and very attractive.
Are there any issues that I should be aware of, besides the fact that it is pricey? The write-up I found online says it was voted 2007 Collectors Conifer of the Year by the American Conifer Society, and is more resistant to insects than most spruces.

When you are at the stage of seeking a source for Picea omorika ‘Pendula Bruns,’ you may want to know that there is sometimes a bit of confusion between ‘Bruns’ and ‘Pendula Bruns’. Make sure the nursery source has distinguished between these two cultivars.

There is a Michigan State University Department of Horticulture publication entitled “Conifer Corner.” The April 2008 issue focuses on narrow conifers, and describes the tree in which you are interested. The author of the article, Prof. Bert Cregg, wrote about this tree in another issue on weeping conifers. According to Cregg, “it’s always hard to go wrong with a Serbian spruce. ‘Pendula Bruns’ is a little slower growing and has a little tighter form than ‘Pendula’ and ‘Berliners Weeper.'”

In Gardening with Conifers by Adrian and Richard Bloom (Firefly Books, 2002), the authors list ‘Pendula Bruns’ as a cultivar to seek out: “Its leading shoot needs training for a few years, then its branches cascade down the trunk, making an arresting narrow specimen.” Richard Bitner’s Conifers for Gardens (Timber Press, 2007) says of Picea omorika that it “should be planted in full sun on well-drained soil. It tolerates heat, humidity, and wind, and is not damaged by snow. Many consider it a good choice for urban landscapes because it will tolerate atmosphere pollution better than most spruces and is also forbearing of limestone soil. It is resistant to the pests and diseases that plague many other spruces.” Of ‘Pendula Bruns’ the author says: “Very narrow selection with strongly pendulous side branches. Stunning!”

Nothing I found suggested specific pest and disease problems associated with this cultivar. Penn State University (now archived) cites Serbian spruce as being generally less prone to problems, keeping in mind that they are writing about the East Coast:

“Few diseases appear to bother Serbian spruce in the mid-Atlantic region. Some sources list aphids, mites, scale and budworm as potential insect problems, however so far there are no reports of these pests significantly affecting the tree in Pennsylvania. The notable exception is White Pine weevil. This pest will destroy the central leader and can seriously disfigure Serbian spruce if not controlled.”

For information on problems affecting spruce trees generally, see Oregon State University’s Online Guide to Plant Disease Control website. (Search by plant name.)

Considering the information from the sources above, this sounds like a fantastic tree choice for your garden.

on the Cinnabar moth caterpillar

I was just walking through a wild area in Seattle with lots of weeds, and came across some strange caterpillars. They are mostly hairless (to the naked eye, anyway), and are striped black over orange with black legs. Can you tell me what they are?

I believe you may have seen the Cinnabar moth caterpillar, Tyria jacobaeae, which was introduced to the United States to control a noxious weed, tansy ragwort (Senecio jacobaea). Here are pictures to compare, and more information:
Wikipedia
Flickr image

Both Washington State’s noxious weed control board and King County Noxious Weeds have information on weed control with Cinnabar caterpillars. (See page 5 of the document). Tansy ragwort is a Class B noxious weed, and control is required in King County.

Dutch elm disease and the use of fungicide injections

On the Master Gardener phone clinic today, a caller was concerned about wilted and dried leaves on his tree. This seemed to be throughout the tree, as opposed to being only at the top which might have indicated damage from the bark beetle causing Dutch elm disease.

He asked about ELMguard, which is apparently an injectable substance that increases the immune system of Camperdown elms. Do you know anything about this?

I know that there are fungicide injections that are supposed to stave off the demise of elms which are already suffering from Dutch elm disease, but if this is not what the caller’s tree suffers from, it would do little good, and might actually cause harm. He should bring samples to the Master Gardener Clinic for diagnosis as a first step.

From the description of the problem, it might be phloem necrosis (also known as elm yellows), which shows up as wilted yellow or brown leaves which drop early, and affects the whole crown of the tree. It is spread by leafhoppers.

Bacterial leaf scorch usually starts with the oldest leaves and progresses through the entire crown. Leaves are brown at the edges and have a yellow halo.

Dutch elm disease does affect Ulmus glabra ‘Camperdownii.’ This USDA Forest Service page describes it in detail, and compares it with the two problems I mentioned above. It also talks about the use of fungicide injections.

Excerpts:
“Foliage symptoms: Symptoms of DED begin as wilting of leaves and proceed to yellowing and browning. The pattern of symptom progression within the crown varies depending on where the fungus is introduced to the tree. If the fungus enters the tree through roots grafted to infected trees (see disease cycle section), the symptoms may begin in the lower crown on the side nearest the graft and the entire crown may be affected very rapidly. If infection begins in the upper crown, symptoms often first appear at the end of an individual branch (called “flagging”) and progress downward in the crown.”

“Multiple branches may be individually infected, resulting in symptom development at several locations in the crown. Symptoms begin in late spring or any time later during the growing season. However, if the tree was infected the previous year (and not detected), symptoms may first be observed in early spring. Symptoms may progress throughout the whole tree in a single season, or may take two or more years.”

“Injecting elms with fungicide. Certain fungicides, when properly injected, are effective in protecting elm trees from infection via beetle transmission. This treatment is expensive and must be repeated every one to three seasons, thus it is appropriate only for high value or historically important trees. The treatment itself also may pose risks to the health of the tree.”

“Eradicating Dutch elm disease from newly infected trees. If a new crown infection of DED is detected early enough, there is opportunity to save a tree through pruning, fungicide injection, or both. Eradicative treatment is not possible on trees that have become infected via root graft transmission. Pruning, which can literally eradicate the fungus from the tree by removing it, has a high probability of “saving” a newly infected tree that has less than 5% of its crown affected.”

To answer your question about ELMguard, there are serious doubts about its efficacy as an elm immune booster. I did find an article, dated July 2000, from South Dakota State University Extension which states:

“Dutch elm disease is showing a very large increase in the last week or two. Several cities are now reporting a four- to eight-fold increase in the incidence of the disease. Generally when we have a wet May the problem increases for the year. According to Dutch elm disease researchers the wetter years result in the creation of a vascular system that is more susceptible to the development of the disease. We have had several calls from people wanting to use ELMguard to save their trees, particularly those already marked by the city as having the disease. ELMguard is not a fungicide but a product that is designed to increase the tree’s ability to fight infection (according to the ELMguard company). It may be an excellent product and we hope to have more information on it later in the year. However the company does not recommend treatments with ELMguard at this time of year or into fall, thus homeowners should not be expect this to save their infected trees. If the city has marked a tree for Dutch elm disease the tree should be removed as soon as possible to reduce the chance of infecting nearby elm.”

cotton woods and slope stabilization

When a cottonwood tree is cut down, does the stump die, or does it send out shoots that grow into more trees?

And, if a cottonwood tree located on a hillside is cut down, what is the risk of erosion?

As it turns out, some poplars and cottonwoods sucker from the roots and some do not. Determining what kind of cottonwood you have is the key to answering this question.

Identifying tree varieties can be tricky. The best way to get a positive ID is to take a sample to the Hyde Herbarium at the Center for Urban Horticulture (near the University of Washington). It is definitely worth a visit, as it is the only herbarium on the West Coast that serves the public.

Hours, driving directions, how to collect specimens, etc. are on the Hyde Herbarium page.

As for your second question, here is what the Washington State Department of Ecology’s Vegetation Management: A Guide for Puget Sound Bluff Property Owners has to say (p.25):
Given the importance of tree cover on potentially unstable slopes and the advisability of retaining them for erosion control purposes, a landowner should explore alternative options to tree removal or topping…[if a tree must be cut] stumps and root systems should be left undisturbed…[to reduce the risk of erosion].

A companion website from the Washington State Dept. of Ecology contains a great list of groundcovers, shrubs and trees that will help keep your slope intact if you decide to remove the cottonwood. The website includes a Plant Selection
guide.

grass seed selection and planting for PNW gardens

I want to put in a wee bit of lawn, an area of 240 square feet. I have two questions: 1) what seed would you recommend for an area that is mostly dappled shade? 2) how do I prepare the area correctly?

on growing buffalo grass in the PNW

I was interested in trying out buffalo grass (Buchloe dactyloides) ‘Legacy’ from High Country Gardens. I was wondering if you knew anyone who has tried growing it in the PNW (esp. Whidbey Island) and what they thought of its performance.

According to the Sunset Western Garden Book, buffalo grass is best suited to Sunset zones 1-3, 10, and 11. Whidbey Island is Sunset zone 5. While the Sunset book does not address ‘Legacy’ in particular, you may find that this grass is not the best choice for your location.

The Washington State Extension in Puyallup created a ranked list of good turf cultivars for Western Washington.

Additional information, from the Extension, about lawns. Note that there is an article about buffalo grass, but it is focused on whether that grass will do well in Central Washington, not Western Washington.