Skip to content
Elisabeth C. Miller Library

Archives: Plant Answer Line Q & A

Plant Answer Line questions and answers.

eradicating Yucca plants

Our home in Seattle came with a large number of established Yucca plants, and we would like to get rid of them. However, they are quite stubborn. We’ve tried a few things, including digging them up, but the root system seems quite deep and extensive and they always come back, and quickly! Any suggestions? I’ve thought they were non-native, but I guess they could be the sort that are found in eastern WA. Are there invasive species here in western Washington?

As you have observed, Yucca is very difficult to eradicate completely. Most of the literature on the subject suggests using herbicide, but even this may be ineffective, which makes the risk of using harmful chemicals to control the plant seem even less worthwhile. There are quite a few informal discussions on how to get rid of this plant on various online gardening forums, and one mentions local gardening expert Ciscoe Morris’s method for getting rid of unwanted Yucca:

“…he cut it back to ground level and put a couple of squares of heavy cardboard over it, piled on some compost/bark to hide the cardboard. I’m not growing yucca, but he said it really worked for killing it without
breaking your back. Leave in place for a year.”

The technique described here is called sheet mulching. This involves laying down overlapping layers of cardboard and then covering thickly with leaves, compost, and other materials. Agroforestry.net offers information on how to do this. StopWaste.org provides additional helpful information.

You may want to try a different approach, because current thinking is that sheet mulching with cardboard is not effective or a good idea. Freely available arborist wood chips make a good mulch, and if laid thickly enough over the area, they should help suppress regrowth of the Yucca.

Yucca is not (yet) considered invasive in our state. Here are links to information on locally noxious weeds.
Washington Noxious Weed Control Board.
King County’s Noxious Weed Control Program.

 

origins and spread of Phytophthora

Will the heavier than normal rainfall this fall and winter create an increased problem with Phytophthora this year? Is there anything we can do now or in the Spring to prevent a Phytophthora problem? Is there a chemical we should spray, and when?

We have numerous Rhododendrons, Azaleas, Heather, Camellias, etc. There are areas of our gardens where puddles have formed during, and remain for the day after, heavy rains. Some areas probably have a clay lens of soil underneath that is preventing good drainage, and others may have a mass of roots preventing the good drainage.

We’ve had Phytophthora before in two groupings of Rhododendrons (2 different varieties). We removed those plants, drenched the soil with the recommended chemical, waited a year, then replanted the same varieties as before. It seemed that things have been fine for at least the past five years, but now, seeing puddles, I’m concerned. Suggestions?

Phytophthora is a fungus which favors our cool, wet conditions and also tolerates heat and drought, so you may be correct that the heavy rainfall will intensify the problem. Here is a link to a September 2011 article from the journal Digger by Niklaus Grunwald, which discusses the origins and spread of the disease.

Here are some links to PDF documents with further information:

Oregon State University information about Phytophthora ramorum (sudden oak death) for forest managers and nursery operators

Toxic-Free Future (formerly Washington Toxics Coalition) has some information on Phytophthora root disease

You can ask when buying plants from local nurseries if their stock has any known problems with this fungus, and you can avoid purchasing affected stock, or planting especially susceptible host plants. If you have walked in an area where the disease is present, clean your shoes before walking in unaffected areas.

The Pacific Northwest Landscape Integrated Pest Management Manual published by Washington State University Extension (3rd ed., 2002) recommends the following methods of managing the root-rot (rather than leaf damage) manifestation of Phytophthora in Rhododendrons and Azaleas (your message does not indicate how your plants were affected by Phytophthora).

1. Plant resistant species and hybrids.
2. Avoid drought stress or flooding, high salinity, or poor drainage. The drainage issue sounds like one you should address in your garden.
3. Remove and destroy diseased plants.
4. Use clean water low in salts, and avoid splashing soil on plants when watering.

More information, from University of California, Davis’s Integrated Pest Management web site, indicates that the best way to prevent the disease is to provide good drainage and practice good water management. Here is more information from U.C. Davis.

public speakers on environmental issues

I am interested in finding a speaker in the Seattle area who can talk about environmental issues, such as conservation, ecology, sustainable gardening, and so on. Can you help?

You can try contacting Seattle Tilth at 206-633-0451, or email them at tilth@seattletilth.org.

Plant Amnesty may be able to help you (206-783-9813), or email them at info@plantamnesty.org.

Other resources include:

King County (Washington) Master Gardeners maintain a speakers list. The speakers address a wide variety of gardening topics.

The Arboretum Foundation has also had a speakers list. I’m not sure whether it is maintained now, but you could ask at info@arboretumfoundation.org.

on the use of Casoron and Preen for weed control

What is the best time of year to use Casoron and/or Preen for weed control on ornamental beds?

Both of these herbicides are registered pesticides, and the law requires that they be used in strict accordance with the directions (and only on the weeds/pests for which they are registered). It is safer for you and the environment if you manage weed problems without the use of pesticides.

You may wish to know more about these particular pesticides. Both Casoron and Preen are pre-emergents, meaning that they work to kill seedlings before they sprout. This means they will not eliminate weeds that have already broken through the soil surface and are growing above ground.

Casoron is persistent in both soil and water (i.e., it hangs around). Its active ingredient is dichlobenil. There are numerous environmental and health concerns associated with this chemical. Dichlobenil will kill any plants which are exposed to it, and will harm beneficial soil microorganisms. Below is a fact sheet about dichlobenil from Northwest Coalition for Alternatives to Pesticides.

The active ingredient in Preen is trifluralin. It is a suspected carcinogen, and is toxic to fish and aquatic life, and earthworms. Here is more information from Cornell University(now archived) and Extension Toxicology Network UK.

The links below provide information about alternatives to chemicals for weed control. Northwest Coalition for Alternatives to Pesticides has a page of factsheets about specific weeds and ways to manage them. Here is their page on managing weeds in garden beds.

Toxic-Free Future (formerly known as Washington Toxics Coalition) also has information on ways to handle weeds in the garden. Here is more information in a PDF file.

Before reaching for chemical weed control, it makes sense to adopt gardening practices which will help keep the weed population low. Mulch is an excellent way to control garden weeds. After you manually remove weeds from an area of your garden, apply a layer of mulch. This should suppress weed growth and help retain soil moisture. Here is what garden expert Cass Turnbull says about mulch:

“Not only does mulch retain water, smother tiny weeds and weed seeds, and make it easy to pull new weeds, it is also harder for new wind-borne weed seeds to get a foothold.

“Mulch can be spread anywhere from 1 inch to 4 inches thick. The thicker it is, the more effective and longer lasting. Spread it thick in big empty spaces. Spread it thin around the root zones of shrubs to allow for sufficient air exchange, especially around shallow-rooted plants like azaleas and rhododendrons. And never let mulch stay mounded up in the base or the “crown” of a plant. It can cause crown rot on some shrubs and can kill them, even a year or more later.”

Source: The Complete Guide to Landscape Design, Renovation, and Maintenance, Betterway Publications,1991.

controlling powdery mildew

What can I do about powdery mildew on my dahlias? Should I throw the bulbs away, or does it only contaminate the plant above the ground? I have heard both too much water and not enough water cause this problem. Is either true?

The main thing you will need to do is destroy all the foliage affected by the mildew. The mildew can survive the winter on infected foliage, and then spread to new foliage.

Powdery mildew thrives where plants are crowded and there isn’t enough air circulation, so give your plants space, a sunny site, and try watering in the morning, and watering from beneath the plants (not over the leaves) so they are able to dry off during the course of the day. As you indicated, too little water can also be a problem.

Here are two websites with additional information:
Univ. of California IPM Online Guide
Washington State University Extension

I did not come across any information specifically saying that powdery mildew will affect bulbs or tubers. I spoke to an experienced dahlia and begonia grower here who said that it should be all right to store and replant your tubers, as long as you thoroughly get rid of all the diseased foliage aboveground.

Some sources (such as The Organic Gardener’s Handbook of Natural Insect and Disease Control, edited by Barbara Ellis, Rodale Press, 1996) suggest that a baking soda spray (1 tsp. per 1 quart of warm water, with a bit of dish soap) is protective or preventive, but Washington State University Extension professor Linda Chalker Scott disputes the efficacy of this method. She says that other methods work better:
“Other treatments have been more successful in powdery mildew control, including horticultural oils, potassium bicarbonate, potassium phosphate, sulfur, milk, and even water sprays. Probably the most field success has been found in combining SBC [sodium bicarbonate] with horticultural oils, including mineral and vegetable oils (see the Fall 2008 MasterGardener magazine). The mixtures are so effective that they’ve been successful even on serious powdery mildew epidemics.”

care and maintenance for Phormium

What is the maintenance routine for Phormium?

According to Sunset Western Garden Book (2007), Phormium require well-drained soil, and may be prone to crown rot if the site is poorly drained. They are susceptible to damage from cold (below 20 degrees Fahrenheit). The Plant Care Manual by Stefan Buczacki (Crown, 1993) suggests that you provide them with full sun and shelter from cold winds, especially if your plants are variegated cultivars, which are less hardy. Mulch in spring and fall, and provide a balanced fertilizer mid-spring and midsummer. In fall, wear heavy gloves and cut or pull away dead foliage. You may need to divide every 5-6 years.

controlling dandelions without pesticides

I have a smaller lawn, in a slightly shady area and have been having problems with dandelion and white clover. I don’t mind a few weeds, but it is getting to be too many. Children and pets play on this lawn so I don’t want to put anything that would be toxic to them on the lawn. What would you suggest I do?

Given that you are concerned for your children and pets, it makes sense to hand-weed your lawn. A little pocket knife is a great tool for doing this quickly and tidily. If you just spend a little bit of time at it a few days a week, it will go faster than you might imagine. Try to live with the clover; it is extremely difficult to eradicate, and it is great for honeybees.

Small dandelions are easier to pull out. The City of Seattle has excellent information about caring for lawns without pesticides, including hints about controlling dandelions. Look in the right-hand menu for additional links to lawn care information.

This article on Dealing with Dandelions from the Journal of Pesticide Reform (Fall 2001) describes several types of dandelion weeding tools.

insect pests in the garden

Is it normal to have many different insect pests in one garden? Is it a sign that I am not taking care of my plants well? Everything looks fine except for one infestation after another!

As far as I am concerned it is perfectly normal to have all of these pests (because I also have many)! But some gardeners are more ruthless than I am. They would rip out the plants. Others would assault their garden with chemicals. I prefer the middle ground of tolerance of some damage and using low-toxic controls.

The mantra these days is RIGHT PLANT RIGHT PLACE and HEALTHY SOIL = HEALTHY PLANTS. For further information (and support) go to the Seattle Public Utilities website, which has a number of great publications on Natural Lawn and Garden Care.
There are lots of links to browse.

There is also information about Natural Pest, Weed, and Disease Control.

Other good resources are Toxic-Free Future (formerly known as Washington Toxics Coalition) and Northwest Center for Alternatives to Pesticides.

 

using treated wood and other alternatives

We are thinking about putting in a retaining wall and a fence on our property, which is near a lake. Should we avoid using pressure treated wood? If so, what are some alternatives?

There are many reasons not to use treated wood for your fences and/or retaining walls. The chemicals most used to preserve wood—creosote (on railroad ties, among other things) and penta—were banned by the EPA in 1986 for indoor use and for many outdoor uses. The chemical used to pre-treat wood (CCA, a mixture of copper, chromium, and arsenic called chromated copper arsenate) has been shown to leach into the soil and to transfer to human skin through contact.

There are safe paints and preservatives for coating wood; there are safe types of pre-treated wood; some people use stone, cement blocks, or other materials instead of wood.

Below is lots of info about treated wood and alternatives.

Start with the page on the EPA site, which is full of information on treated wood. It includes a section on alternatives and some questions and answers about studies.

If you find this too technical, try the next two links below.
The Natural Handyman website has good information.

Toxic-Free Future (formerly known as Washington Toxics Coalition) has a fact sheet about safe and unsafe paints and wood preservatives. Lots of background information on the toxicity of treated wood is included as well.

khat tree intoxication

Recently I met an Ethiopian couple who were picking the reddened leaves on an otherwise green bush/tree in my front yard. The man explained this was a “cat” or “chat” tree, the leaves produce a drugged like state when ingested. He asked me if he could harvest the tree, and asked me not to tell any other Somalis, Ethiopians, or Eritreans in the neighborhood about my tree. He also told me that if I lived in Mogadishu, this tree would make me a wealthy man! He ate some leaves in front of me, and I tried a couple, but they were bitter and unpalatable to my palate. I experienced a feeling of empowerment, strength, and mental alertness. Obviously the “Chat Tree” has some relationship to the “Bongo” young Somalis chew.

During the worst of the anarchy in the late 1990s in Mogadishu there was a lot of news footage of the street gangs, high on the plant they were chewing, and armed with machine guns and machetes, creating havoc.

Do you know the history of this tree?

What are the properties that cause the intoxication?

What is the tree’s botanical name?

Should I report the tree’s existence to the authorities?

Can you tell me what I have here?

p.s.-These trees are common front garden bushes that were widely planted in Perth, Western Australia. Next time I see someone hanging out under one of them, I think I will know why!

The chat, or khat tree, is Catha edulis (Celastrus edulis), and the leaves and branchlets have properties that stimulate the central nervous system. In addition to the euphoric or inebriating properties, chewing the leaves can cause irritability, decreased appetite, gastric upset, constipation, and inflammation of the mouth. Habitual use can lead to periodontal disease, and increased risk of esophageal cancer. The active compounds are Alkaloid D-norpseudoephedrine, as well as other alkaloids, and tannins. (Source: Medical Botany: Plants Affecting Human Health by Walter H. Lewis; John Wiley & Sons, 2003, 2nd ed.)

The Handbook of Medicinal Herbs by James A. Duke (CRC Press, 2002, 2nd ed.) indicates that Catha edulis has been used medicinally to treat a great number of ailments, including asthma, depression, diarrhea, glaucoma, and low blood pressure. Use of khat is an ancient, socially acceptable tradition in the Afro-Arabian culture (and became known as a recreational drug in the USA after American soldiers were exposed to its use in Somalia. Khat is subject to legal restrictions in many countries. (Medicinal Plants of the World by Ben-Erik van Wyk; Timber Press, 2004).

As for whether to report the harvesting of leaves from your tree, that would depend on whether khat use is specifically prohibited by law in Australia.