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Elisabeth C. Miller Library

Archives: Plant Answer Line Q & A

Plant Answer Line questions and answers.

magnolia tree root systems and damage potential

Does a magnolia tree root system cause cracking on the house foundation if planted too close, especially on clay soil?

What kind of magnolia are you growing? Is it deciduous or evergreen? Is it large species or one of the smaller varieties?

According to SelecTree, a website of the Urban Forest Ecosystems Institute, most Magnolias have a low root damage potential. A few, such as Magnolia delavayi, are rated as moderate. I am not an arborist, so I cannot conclusively tell you that a Magnolia will not be problematic for the house foundation. However, it is always a good idea to plant trees far enough away from structures so that you do not have to do a lot of pruning to keep them from conflicting with windows, entryways, the roofline, etc. From what I have read, most tree roots that harm foundations are not themselves causing the cracks, but are exploiting preexisting weak points in the foundation. Roots do expand in size as the tree matures, and they do seek out water to some extent, depending on the type of tree.

Below are some links to information that may be useful to you.

North Dakota University Extension (no longer available online)
Excerpt:
“How close to the home should trees be planted?

Large trees such as ash, hackberries, maples, lindens and oaks should be kept at least 20′ from the foundation. Medium sized trees such as buckeyes, honeylocust and little leaf linden – 15-20 feet away and small trees such as flowering crabs, mountain ash and Canada red cherry – 10 feet from the foundation.

Do tree roots actually crack the foundation?

No, the wetting and drying of the clay soil causes the initial cracks. After these have developed, tree roots will grow into the cracks for moisture.”

A personal essay on tree roots and foundations, from Renegade Gardener,a garden blogger

International Association of Home Inspectors
Excerpt:
“Tree Roots and Foundations

Contrary to popular belief, InterNACHI has found that tree roots cannot normally pierce through a building’s foundation. They can, however, damage a foundation in the following ways:

  • Roots can sometimes penetrate a building’s foundation through pre-existing cracks.
  • Large root systems that extend beneath a house can cause foundation uplift.
  • Roots can leech water from the soil beneath foundations, causing the structures to settle and sink unevenly.”

on low hazard pesticides

I heard somewhere that the fungicide Rose Pride was less toxic to beneficial insects than plain baking soda. Is this true? I’d like to continue to use it in my garden.

Rose Pride is the chemical Triforine. I was not able to find any articles which suggest it is safer for beneficial insects than baking soda. Pesticides Action Network’s Pesticides Database indicates it is toxic in varying degrees to some forms of aquatic life. It is on the PAN List of “Bad Actor” pesticides, which means it belongs to a group of pesticides classified as most toxic (because they are known or probable carcinogens, reproductive or developmental toxicants, etc.). The Extension Toxicology Network also has a profile for this pesticide. Here are excerpts:

“In the United States, triforine is marketed for use on almonds, apples, asparagus, blueberries, cherries, hops, ornamentals, peaches and roses. Triforine is a ‘restricted use’ pesticide (RUP) with an EPA toxicity classification of I (highly toxic). Check with specific state regulations for local restrictions which may apply. Products containing triforine must bear the Signal Word ‘Danger’ on their label.

Triforine and the formulated product Saprol are considered of low hazard to honeybees and to the predatory mite Typhlodromus pyrii. It is also of low hazard to earthworms at recommended dose rates.”

My comment would be that “low hazard” is not the same as no hazard, and since there are many other areas of concern with this highly toxic product, it would be best to find an alternative. Locally, the Woodland Park Rose Garden converted a pesticide-dependent landscape to an organic one, and the roses look better than ever. (See an article about the garden from the Seattle Times.) Many gardeners are learning to live with a bit of black spot on their roses, and manage the disease by maintaining good garden hygiene. Don’t leave fallen leaves on the ground. Give your roses good air circulation, and keep the leaves dry when you water your plants. Mulching with wood chips can help, too, since they may prevent water from splashing up onto the leaves.

When deciding whether to treat a garden problem with pesticides, the “Precautionary Principle” provides an important perspective:
“When an activity raises threats of harm to human health or the environment, precautionary measures should be taken even if some cause and effect relationships are not fully established scientifically.”

fungal growth on trees

I have a 50-year-old Italian plum tree. The limbs have oyster-shaped growths on them. These growths will not come off–they are hard. Is it a fungus or disease? Or is the tree just getting old?

Hard fungal growths on trees are called conks, a type of bracket fungus, and they are not a good sign. According to The Sunset Western Garden Problem Solver, these growths “indicate a hazardous condition from decay inside.” Usually by the time they are seen, decay inside the tree is substantial and you may need to get an arborist’s help to keep the tree from breaking and dropping branches, and to assess whether it is a danger to structures or people. The Pacific Northwest Chapter of the International Society of Arboriculture provides a listing of local certified arborists. You can also get an arborist referral from Plant Amnesty.

on Cape fuchsia and tree fuchsia

I live in the Western Cape of South Africa. I’d like to plant a Cape fuchsia near a wall in a small garden. Can I do this without concern that the roots will lift the wall?

What we call Cape fuchsia over here is a shrubby perennial, not more than 4 feet tall in most cases. Its Latin name is Phygelius. It does tend to spread around the garden, and it is not easy to dig up its root system, but perhaps you may be talking about a different plant in the same family, Halleria lucida. Here it is sometimes called “tree fuchsia.” According to Pitta Joffe’s book Creative Gardening with Indigenous Plants: A South African Guide, Halleria lucida “has a non-aggressive root system,” so it is unlikely to damage your wall. Keep in mind that the mature size is about 6 meters in height and up to 5 meters in breadth, so planting it too close to a wall may not be ideal in the long run. Pitta Joffe highly recommends the tree because it attracts sunbirds and canaries with its nectar and many other birds with its fruit.

dwarf tree fern plant culture and care

I recently purchased a plant which the grower referred to as Blechnum gibbum, or dwarf tree fern. I have searched many databases including the BBC, IPNI, and fern societies. Ultimately, I would like to discern the accurate, current name for the plant and plant culture and care information for in-home.

I confirmed the name Blechnum gibbum on the Royal Horticultural Society’s website, as well as in Sue Olsen’s Encyclopedia of Garden Ferns (Timber Press, 2007). For now you can safely assume that Blechnum gibbum is the correct, current name for the plant commonly called dwarf tree fern. It is a member of the family Blechnaceae, native to Fiji, and commonly grown as a houseplant in North America.

According to The New Houseplant Expert by D.G. Hessayon (Sterling, 1991), tree ferns require a temperature range between 60 and 70 degrees Fahrenheit (over 75 is a problem), an east or north facing window (or some shade in a hotter exposure), moist air (misting ferns can help with this), regular watering (particularly in spring and summer), and repotting so that the crown is not buried and the pot is not overly full of roots. The House Plant Encyclopedia by Ingrid Jantra (Firefly Press, 1997) says much the same thing, adding that temperatures below 54 are also a problem and that a light feeding once a month is helpful.

cedar wood chips and allelopathy

We’ve taken down some big cedars and chipped the branches. I’ve heard that cedar mulch can damage plants. What is your take on this? I already put it around some choice pines and some viburnums, but I could move it if need be.

Washington State University Extension horticulturist Linda Chalker-Scott has written about this very issue, and her scientific research on the subject says that cedar (both Thuja and true Cedrus) wood chips are not allelopathic (toxic) to plant tissue. Here is the article.

This author has further information on the general benefits of wood chips as mulch. Here is a newer Washington State University factsheet on the subject.

on the Nearing Frame

I’m wondering what information you can give me about G. Guy Nearing, the Nearing Frame, and its use.

We don’t have any titles or subject headings in the catalog that refer to G. Guy Nearing or the Nearing Frame, but some of the books on propagation may describe it. Ken Druse mentions it briefly in Making More Plants (2000). Here is a link to information about the frame which I found by browsing Ken Druse’s blog. It refers to a book that we do have in the library:

“For detailed plans, see David Leach’s Rhododendrons of the World. For Guy Nearing’s original drawings and two-page patent application (1931), see the Summer, 2002 issue of the Journal of the American Rhododendron Society.”

 

invasiveness status of butterfly weed

Is butterfly weed invasive in the Northwest?

First, I have a plant name question for you: did you mean the shrub Buddleia davidii which is also called butterfly bush, or the herbaceous perennial Asclepias tuberosa, the plant most often referred to by the common names butterfly weed or butterfly milkweed?

In King County, Buddleia davidii is a Class B noxious weed, which means that its control is recommended but not required by law. Many gardeners grow this shrub as well as other less aggressive species. If you absolutely cannot live without this plant in your garden, you can go a long way toward preventing its spread by keeping the blooms deadheaded as soon as they fade (before they set seed).

Asclepias tuberosa can seed itself around, but it is classified as an endangered plant in New Hampshire, a threatened plant in Vermont, exploitably vulnerable in New York, of special concern in Rhode Island, and possibly extirpated in Maine. It attracts butterflies and hummingbirds. Here is more information so you can determine which plant you have in your garden:
Lady Bird Johnson Wildflower Center
USDA Plants Database

notes on artificial turf

I have a client who is interested in replacing a lawn with artificial turf, but I’d like to know of examples out there, and any other thoughts you might have on the subject.

I am guessing your client is interested in a low-maintenance ground cover that gives the appearance of lush lawn without the attending needs to water, weed, fertilize, and so on. On the surface, this makes artificial turf sound like a good alternative. If your client wants a place to sit and relax in the garden, a chair will be required, as plastic is not a welcoming seat. There are other considerations as well.

There are examples of artificial turf throughout City of Seattle Parks and Recreation, and Seattle Public Schools. There are a number of fields which use artificial turf, also referred to as synthetic turf. You could contact people at these departments for their thoughts on the subject. Here is the contact person for Woodland Park field:
Ted Holden at 206-684-7021 and ted.holden@seattle.gov
Seattle Public School has a grounds maintenance number, 206-252-0645.

I have some personal observations about the artificial turf installed at Magnuson Park and at Eckstein Middle School. The field at Magnuson is in the middle of a wetland. This turf area sheds tiny crumbs of plastic or rubber which are tracked indoors on shoes (and paws!). In the rainy season, the particles wash over the pathways and into the drainage areas of the remaining wetland. This and the aforementioned school field which was replaced by artificial turf used to have some value as a habitat for birds and other creatures but both are now ecological dead zones. When we have hot weather, these expanses of synthetic turf emit an odor like singed rubber.

Below are various links about the downside of artificial turf:

A related subject is the use of recycled rubber tires (also used in synthetic turf infill) as mulch in gardens. Washington State University Extension horticulturist Linda Chalker-Scott has written about the myth of this recycled product’s supposed ecological benefits.

Perhaps your client might consider more environmentally friendly, low maintenance ground covers. You can search the Miller Library’s database for ideas about ground cover plants and lawn alternatives.

saving Zantedeschia bulbs

I recently received a flowering Zantedeschia hybrid lavender lily whose leaves are turning yellow. I would like to save the bulbs to plant outside. What steps should I follow?

Your message doesn’t indicate where you are gardening, and the answer depends on your climate. It also depends on which species of Zantedeschia you have. Based on the color you mention (lavender), I would guess you have a more tender hybrid.

Just in case you decide to keep your plant indoors, I am including information from The Complete Houseplant Survival Manual by Barbara Pleasant (Storey, 2005), which says that Zantedeschia hybrids grown indoors can be brought back into bloom (inside) in future seasons if you give them a rest period in late summer: “Allow the plants to dry until the leaves wither, clip off the old foliage, and keep the pots very lightly moist in a cool, shady spot. In winter, a fresh pot of soil, along with warmth, moisture, and bright light, will quickly bring them back to life.”

However, you say that you would like to grow your plant outdoors. If the variety you have is especially tender, it may not work, but you can certainly try.

According to Sunset Western Garden Book (2007), in its hardiness zone Zantedeschia can be planted fall through early spring, setting the rhizomes 4 inches deep, and 1 foot apart. If your garden is outside the plant’s cold hardiness zone, you would store the rhizomes when the leaves die back, and keep them over the winter in a cool, dry place, planting them out in spring.

Here is information from the Royal Horticultural Society.

Excerpt:
The tender forms of Zantedeschia are mainly cultivars of Z. elliotiana and Z. rehmannii (also called Elliottiana hybrids and Rehmannii hybrids), but may also include Z. albomaculata and Z. jucunda. These are often referred to as calla lilies.

These tender varieties can be displayed either as houseplants or seasonal outdoor bedding displays. They will flower in summer. If used outdoors, you will need to lift the rhizomes before the first frosts and store them over winter before replanting them next spring when the risk of frost is passed. Store them in trays of compost in a cool, dark, frost-free place such as a garage or shed. (…)

Tender forms: Require a minimum temperature of 10 degrees C (50 degrees F) so need overwintering indoors or in a warm greenhouse or conservatory. Plants that have flowered at Easter can be kept growing over the summer and repotted in spring. Alternatively, they can be allowed to dry out in late June and rested for a couple of months in a dry and dormant state before repotting in late August or early September for an earlier show of flowers.