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Elisabeth C. Miller Library

Archives: Plant Answer Line Q & A

Plant Answer Line questions and answers.

Root weevil resistant Rhododendron varieties

My rhodies are being devastated by root weevils. They have stripped many of the branches clean of their vegetation, and have destroyed ~50% of the remaining leaves. My rhodies look like they will require years to recover, if they ever do.

If I replace them with resistant varieties or plants that are not susceptible to these pests, will this eliminate the weevils?

Root weevils are the most common pest attacking Rhododendrons in the Pacific Northwest so they can only be temporarily eliminated from any garden. If the environment is right and their food source returns, so will the root weevils.

If you want to keep your current Rhododendrons, the weevils can be controlled if you’re diligent (forever!?). An article by Caroline Cox in the summer 2005 issue of the Journal of Pesticide Reform discusses their control. However, it sounds as if you’re willing to remove them and start fresh. Some of the most susceptible (host plants) are Rhododendron and Azalea, Heather, Salal, Manzanita and Kinnikinnick, Pieris, Maples, Viburnum, most Conifers, Astilbe, Cyclamen, Helleborus, Hosta and Primrose.

(Source: Root Weevils in the Nursery and Landscape; Identification and Control, by J. DeAngelis and G. Garth, EC 1485, Oregon State University Extension Service).

The extension bulletin from the Washington State University Extension website has an excellent list of resistant Rhododendron varieties.

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Pruning hibiscus

I have a small hibiscus that I would like to train into a
tree with the twisted trunk and I have no idea how to go about that.
Please advise.

When you prune your hibiscus into a tree-like form with a single trunk,
it is called a standard. There are even braided topiary forms. To achieve
the twisted shape, you will probably need to create a support or frame.

The following general information on pruning comes from Tropical
Hibiscus:

“While the tropical hibiscus can be pruned any time, probably the ideal is
the earliest where the resulting tender new growth will be safe from cold
damage*. For shaping purposes, some growers will prune the longest third
of the branches and return in 4 to 6 weeks and prune the next longest
third. Only sharp, clean shears should be used. A clean cut should be
just above and angled down and away from an ‘eye’ or node. (A node is the
junction of a leaf and the stem. There is a small bud in this junction
that is activated after pruning.) Cutting above outward pointing “eyes”
will encourage growth in that direction. The new growth resulting from
pruning invigorates the plant and will provide a source for many new
blooms.”

The American Horticultural Society’s Pruning & Training (edited by Christopher Brickell and David Joyce, DK Publishing, 2011) describes the technique for creating a braided stem:
“Form a braided stem simply by braiding together three flexible young shoots. Select the strongest three on a multi-stemmed young plant, and remove the remainder. Single-stemmed plants can be cut back hard to produce multiple stems.”
The book also describes what is called a “barleysugar stem,” which may be more like the twist or spiral you envision: in this technique, “use a sturdy wooden pole with dowel pegs inserted in a spiral along its length. Train one or two stems around the pole, holding them in place by looping them beneath the dowels. Remove the pegs and the pole in sections when stem growth has hardened.”

Here is a link to Brooklyn Botanic Garden’s article on espalier forms, Special Cases: Pruning for Particular Purposes.

The Miller Library has a good selection of books on pruning and training,
and specifically on topiary. You can search the library’s catalog by clicking this link.

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Transplanting snowberry shrubs

I need to move some established snowberry shrubs forward about 8 feet to make room for a large 7′ propane tank. I’m hoping to salvage the snowberry shrubs which are currently in a mostly shady location and would continue to be in shade after I move them. (I’m hoping they will block the view of the propane tank. Do you have any tips on transplanting this type of shrub? Or is it too difficult to do once they have reached 5+’ tall? Am I better off starting with smaller snowberry that are only 3′ in height?

According to the following information from University of Connecticut’s Plant
Database, snowberry or Symphoricarpos albus, is easily transplanted.

From an Olympia nursery catalog, Sound Native Plants:

Symphoricarpos albus – Snowberry
Exposure: full sun to shade
Soil moisture: very moist to dry
Transplanting success: high
Growth rate: rapid
Form: deciduous shrub to 2-6 feet; fibrous, shallow root system, spreads
vigorously by suckers

Snowberry is an incredible survivor, flourishing in situations that would slay a
lesser plant. It transplants well, tolerates sun or shade, withstands drought
and/or occasional flooding, and spreads quickly even in poor soil or on steep
hillsides. Another plus for snowberry is that it is one of the few native shrubs
that stays small–it averages three or four feet tall–and thus is a good choice
for areas where view corridors are important. Hooray snowberry!

If it makes it easier for you to move the plants, you can prune them back (this
is usually done in spring). If individual plants have grown into a dense mass of
stems, you can also dig up each whole plant and only replant smaller pieces of
it.

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Use of horticultural oils

I have a dwarf Braeburn apple tree that gets spotted apples
every year. The leaves drop off and the apples are stunted and not
edible. I am spraying with dormant oil spray per the instructions and it
looks beautiful right now. I need to know how often to spray it and how
long into the season. The instructions aren’t clear on this. Also, does
the dormant oil spray make the apples unsafe to eat at all?

Here is what Michael Phillips says in his book, The Apple Grower: A Guide
for the Organic Orchardist (Chelsea Green, 1998):
“Oil sprays smother the overwintering eggs and emerging nymphs of a
number of foliar feeders. Use of a highly refined oil is tolerable in an
organic orchard, but generally not necessary.” He recommends encouraging
beneficial insects to control aphids. Aphids may be a sign of a deeper
imbalance that needs addressing.

Whether the dormant oil spray makes the fruit unsafe to eat depends
greatly on what the oil is made of: many such sprays are petroleum-based
and would therefore not be safe. See the following information formerly available online from BeyondPesticides.org:

Excerpt:

“Most horticulture oils used today are petroleum based (Grossman 1990),
yet a growing number of horticulture oils are being made with vegetable
oils, which are considered a least toxic pesticide. Carefully read the
label or ask your pest control service provider to determine if the
horticulture oil is vegetable or petroleum based.”

From Washington State University Extension agent Mary Robson:

“How Do I Use Dormant Sprays?”

“Neither the spray nor the applicator is dormant in a ‘dormant spray’: the
plants to which it’s applied are. The term refers to winter-applied
sprays for insect pests and diseases, put on before foliage begins to
leaf out.

“To use dormant sprays, first identify the reason for the spraying. They
are often used on fruit trees to control over-wintering insect pests such
as scale and aphids. (The aphids over-winter as eggs, and the spray
smothers the eggs, preventing spring hatching.) A dormant spray isn’t an
all-purpose winter splashing of pesticide around the garden: it’s a
specific spray chosen for a specific pest. The dormant spray used on
fruit trees is often horticultural oil (sold as superior-type oil), and
it may be mixed with lime-sulfur depending on the pest to be controlled.
It’s sprayed thoroughly to give good coverage on the trunk, branches,
small limbs and shoots.

“Because dormant sprays are generally applied early in the season, they
tend to be less disruptive to beneficial insect predators and parasites
which aren’t in active life stages in mid-winter. While generally used in
fruit tree maintenance, dormant oil sprays are helpful for landscape
plants with similar aphid or scale problems. Ornamental plums
(purple-leaf plums) often suffer from infestations of aphids or scale; if
that’s been the case, a dormant oil spray may help reduce the
populations.”

The following link is from the Colorado State University Cooperative Extension: Insect Control: Horticultural Oils. Excerpt from a version of this page no longer available online:

“Most horticultural oils contain naphthene and paraffin compounds.
Paraffins are valuable to gardeners because they’re more toxic to insects
and less toxic to plants than other oil compounds. In contrast, oils
containing naphthene are less pesticidal and more likely to injure plants
than paraffinic types. Oils high in naphthene also contain more
impurities such as phytotoxic aromatic and unsaturated hydrocarbons.
However, the newest horticultural oils contain only tiny amounts of those
compounds.”

Have you determined what the cause of the spotting on your apples is?
Might it be apple scab? In case that is what you have been seeing, here
is what Washington State University Extension says:

“Apple scab is caused by a fungus which also causes scab on crabapple and
hawthorn. The first infections occur during wet weather in the spring.
Initially, the disease causes tiny, pale, chlorotic, water-soaked spots
on the leaves. The spots enlarge and darken to a dark, velvety,
olive-green then to black. Leaves may become distorted, puckered, and
mottled. Leaves may drop, sometimes resulting in severe defoliation of
susceptible trees. Scab can also affect fruit. Fruits infected early in
development show olive-green to brown, roughened or corky spots which may
develop deep cracks. These apples are often misshapen. Fruits infected at
later stages develop small black “pinpoint” scab spots while in storage.
The disease is favored by cool, wet conditions and overwinters in
infected plant debris.

“Management Options:

“Select Non-chemical Management Options as Your First Choice!!

  • Avoid overhead irrigation.
  • Plant in full sun.
  • Plant scab-resistant varieties such as ‘Akane’, ‘Chehalis’, ‘Liberty’, ‘Paulared’, ‘Prima’, or ‘Tydeman Red’.
  • Rake and destroy (do not compost) fallen leaves, or cover them with
    soil.
  • Space plantings and prune to provide good air circulation and light penetration.
  • The application of nitrogen to the leaves in the fall will enhance
    the decomposition of the fallen leaves.”

The following website is for large-scale growers, but may have
information of interest to you:

Apples: Organic Production Guide by Tammi Hinman and Guy Ames, 2011

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Synonyms and invalidated plant names

What does it mean when a plant name is described as a synonym? And is a synonym “valid?”

It can get confusing when plant taxonomists decide to change the names of plants. According to the International Code of Nomenclature (Melbourne Code, 2011), a replacement name “is a new name based on a legitimate or illegitimate, previously published name, which is its replaced synonym. The replaced synonym, when legitimate, does not provide the final epithet, name, or stem of the replacement name.” To me, that sounds like the synonym is the name which is no longer current. This link from Stephen Saupe of the biology department at St. John’s University, Collegeville, MN is a little easier to understand. Here is an excerpt:

“The correct name for a plant is the oldest, validly published name. Although this sounds simple, in practice it can be challenging to sort through all of the names that have been published for a species and determine which is the correct one.

“Although it may seem that botanists change scientific names just to frustrate us, this is not the case. Names are changed because additional scientific study shows that the original name: (1) didn’t follow the rules (i.e., wasn’t the earliest, or is taxonomic or nomenclatural synonym) or (2) because our taxonomic ideas of the genus and species has changed since the original study (i.e., additional studies showed that two closely related species are actually one). Perhaps this is a good time to mention taxonomists who are ‘splitters’ (focus on the differences between taxa) and those who are ‘lumpers’ (focus on the similarities).”

As you may have noticed, people often continue to refer to plants by invalidated names. And sometimes the rejected names are made valid again later. Unless you are publishing a scholarly article and need to be precise and up-to-date about the plant names, aim to use names that people will recognize and understand.

Lilacs and failure to flower

I have a lilac bush given to me as a gift 13 years ago. I
don’t know the variety but the leaves look slightly different from the
common lilacs I see. This bush has healthy looking leaves and while it
has slowly put on growth over the 13 years it has never bloomed. I have
tried adding ashes to the soil to make it more alkaline but nothing seems
to work. What is the problem and how can I get this bush to bloom?

There are several reasons lilacs may fail to flower. Here is an excerpt (no longer available online) from North Dakota State University Extension horticulturist Ron Smith in
answer to a question similar to yours:


Lilacs fail to flower because of insufficient sunlight, planted too
deeply, too much nitrogen, improper pruning or winterkill of the flower
buds. You said the lilacs get plenty of sunlight, but unless you used a
lawn fertilizer to provide nutrients, it isn’t likely too much nitrogen
is the problem. If you planted too deeply, pull some of the soil back so
the top of the roots are slightly exposed. If you pruned in July, then
doing so removed the flower buds for the next growing season. If winter
killed the flower buds, then hope for milder winters or purchase hardier
lilacs.

Colorado State University Extension’s article, “Renewing Lilacs,” (no longer available online) offers other suggestions, such as late freezes, decreasing sunlight, and pest problems.

Sunset’s Western Garden Book (2001 ed.) says that annual pruning is
needed for optimal flower production. Most lilacs bloom on wood formed
the previous year, so they should be pruned just after flowering. Remove
the spent blooms and cut back to a pair of leaves. There are a few lilacs
which bloom on new growth, so it might be useful to know exactly what
type of lilac you have. You could send photos or bring in samples for identification.

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Defoliation on cedar trees

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 will lead you to their website.

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Lichen on trees

My Corylus contorta has greenish yellow lichen on the branches. Should I be concerned and if so what should I do?

The lichen will not harm your Corylus contorta. Lichen is actually a sign that the air is relatively unpolluted.

You may find this page of discussion from University of British Columbia
Botanical Gardens of interest.

Here is an excerpt from a 2007 entry formerly available on the site of Treelink, now part of the Alliance for Community Trees:

“No need for a preemptive strike against the lowly lichen. Lichens are
composite, symbiotic organisms made up from members of as many as three
kingdoms.
The dominant partner is a fungus. Fungi are incapable of making their own
food. They usually provide for themselves as parasites or decomposers.
“Lichens are fungi that have discovered agriculture”– lichenologist
Trevor Goward.

“The lichen fungi (kingdom Fungi) cultivate partners that manufacture food
by photosynthesis. Sometimes the partners are algae (kingdom Protista),
other times cyanobacteria (kingdom Monera), formerly called blue-green
algae. Some enterprising fungi exploit both at once.
Most lichens grow very, very slowly, often less than a millimeter per
year, and some lichens are thought to be among the oldest living things
on Earth.

“Lichens are important in many ways in the habitat. Some make
the nitrogen in the air usable to plants, They are homes for spiders,
mites, lice, and other insects. All are important in the nutrient cycle
in the places where they grow. Many lichens are very sensitive to
pollution in the air. When there are too many harmful things in the air,
lichens die. If you live where there are many lichens it probably means
the air is clean. But, if there are only a few lichens in your
neighborhood, the air you are breathing is probably clogged with
automobile fumes or industrial wastes. Some trees and shrubs can develop
a layer of fungi, algae, lichens or moss on their bark. These are
non-parasitic organisms and do not injure the plants on which they grow.”

Growing garbanzo beans in Seattle

I’m interested in growing garbanzo beans. Do they work in the Seattle climate?

The following information from University of Wisconsin’s Alternative Field Crops Manual indicates that garbanzo beans are grown as a crop in Eastern Washington.

According to Steve Solomon’s Growing Vegetables West of the Cascades (Sasquatch Books, 2000), Cicer arietinum should be sown here in early March to early April, the earlier the better, because the crop needs soil moisture. However, you should sow when the soil is warm enough for the seed to sprout. Plant an inch deep, 3 to 4 seeds per foot, in rows 3 feet apart. You can mix compost into the rows before planting, which should be sufficient nutrition for the plants. If you need to amend the soil further, you may add bone meal with the compost (5-10 lbs./per 100 row feet). Thin the seedlings once established to 8 inches apart. Keep the planting well weeded. Harvest in midsummer.

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Growing Magnolias in containers

I recently purchased a magnolia that had no tags on it. I have an extremely large pot that I would like to plant the magnolia in. My books at home lead me to believe that I should plant it in Azalea and Camellia potting mix. A local nursery has advised me that this would be fine, although another has said no. They also disagreed with my plan of putting rocks, bitumen, and old leaves in the bottom of the pot to help with drainage. They believe a quality potting mix and nothing else is the way to go. What are your suggestions?

Here is what the book, Magnolias: A Gardener’s Guide, by Jim Gardiner
(Timber Press, 2000) says about growing Magnolias in containers:

…considerable experience is needed to retain magnolias in a container
for any length of time. The roots are particularly sensitive to being hot
and dry during the summer months and frosted during the winter months…
Evergreen magnolias and clones of Magnolia grandiflora, in particular M.
grandiflora ‘Gallissonniere,’ can be grown in very large containers for
indoor use in atria.

I think if you take the matter of extreme heat and cold into
consideration, you should be able to grow your magnolia in a container. I
would be curious to know which species you have, because some get very
large, and for these a container might not be a good choice.
Magnolias prefer good, free-draining acidic soil that does not dry out,
according to Rosemary Bennett’s book, Magnolias (Firefly Books, 2002).
Since Azaleas also prefer acidic soil, the idea of using Azalea and
Camellia potting mix makes sense.

You may find the following information on growing trees in containers
helpful:

Virginia Cooperative Extension: Trees for Landscape Containers and Planters

University of Tennessee Extension: Trees to Plant in Containers or Wells

UBC Botanical Garden Forum: A discussion on requirements for magnolias in containers

UBC Botanical Garden Forum: A discussion on potting guidelines for a particular magnolia This discussion suggests that the container should be filled with soil-based compost which provides some nutrients to the plant.

As for container drainage, here is what Prof. Linda Chalker-Scott of Washington State University says. In short, she says that putting coarse material in the base of a pot for better drainage is a myth.

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