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Elisabeth C. Miller Library

Archives: Plant Answer Line Q & A

Plant Answer Line questions and answers.

controlling woolly aphids on apple trees

Is there something I can do to prevent my apple trees from getting woolly aphids? I’d rather not have to spray anything.

 

Encouraging beneficial insects is one step you can take. A 2013 study at Washington State University found that planting Alyssum flowers attracted syrphids which did a good job of reducing woolly aphid populations. Here are highlights of the paper that was published based on the study’s findings:

  • Sweet alyssum flowers had the highest attractiveness to syrphids.
  • Faster suppression of woolly apple aphid occurred on trees closer to alyssum flowers.
  • Higher densities of natural enemies were observed near sweet alyssum plantings.
  • Natural enemies were found to move between sweet alyssum and adjacent apple trees.

As Washington State University’s HortSense website (search under “tree fruit,” “apple,” then “aphids”) indicates, encouraging beneficial insects is a good practice for the control of all 3 main types of aphids affecting apples, be they woolly, rosy, or green:

  • Control honeydew-feeding ants, which may protect aphid colonies from predators.
  • Encourage natural predators including ladybird beetles, lacewings, syrphid (hover) fly larvae, and parasitic wasps. Avoid use of broad-spectrum insecticides which kill these beneficial insects.
  • Hand-wipe or prune to control small, localized infestations (when practical).
  • Provide proper nutrition. High levels of nitrogen encourage aphid reproduction. Switch to a slow-release or low-nitrogen fertilizer.
  • Wash aphids from tree with a strong stream of water before leaves curl.

Berberis species and their blooming time

A couple of questions: first, what’s the correct name these days, Mahonia or Berberis? Secondly, I’m trying to plant a succession of flowering Mahonia (or Berberis?) to attract hummingbirds to my Seattle garden. I have Mahonia X media ‘Charity.’ In what order do various species bloom?

 

The current accepted name for Mahonia is Berberis. I checked several local sources to compare flowering times for different species:

In approximate blooming order, here are several Berberis species:

  • Berberis x media ‘Charity’ and other cultivars (December-January)
  • Berberis repens (February-March)
  • Berberis aquifolium (as early as January but peaks March/April, into May)
  • Berberis bealei (“early spring,” according to Gossler)
  • Berberis nervosa (May)

pests affecting currant trees

We moved into a new house which has a large currant or gooseberry bush. Now that it has leafed out there are numerous caterpillars eating the leaves. I know they are not tent caterpillars, but I cannot identify them. They are whitish-green with yellow bands across the top and bottom, with many black dots or bumps. The head and first six legs are black. It would be nice to learn more about them.

 

I cannot make a conclusive pest identification remotely, but there is a possibility these caterpillars are currant sawfly, or imported currantworm. Here is some information about this pest from the Pacific Northwest Pest Management Handbooks.

If this pest is the culprit, the book, The Organic Gardener’s Handbook of Natural Insect and Disease Control edited by Barbara Ellis (Rodale Press, 1996) recommends using Pyrethrin spray, spraying into the center of the bush. Bees are severely affected by this pesticide, so follow the guidelines shown on this University of California, Davis Integrated Pest Management page. Also see this guide to reducing bee poisoning from pesticides.

For a definitive pest identification, you may want to bring a sample of the pest and its damage to a Master Gardener Clinic.
Using the following link, you can locate a Master Gardener Clinic in your part of Washington State.

Japanese Maples and Verticillium wilt

I want to test the roots of our Japanese Maple for Verticillium wilt. Are there places which could test for that?

 

There is information about Verticillium wilt and how to manage it on the Washington State University Extension’s HortSense website.

To have a sample from your Japanese maple diagnosed, you can take samples to a free Master Gardener Clinic, or you can send samples (for a fee) to
WSU Puyallup Research and Extension Center:

go to “How to Submit a Sample,” scroll down to “Plant Problem Diagnosis,” then you can download a form by clicking on “Form C1006.”

My personal experience with this disease is that the Japanese maple lived with it for quite a few years before totally succumbing, at which point we had it removed by an arborist. If you need to remove the tree, you may want to consult this list of plants that are susceptible and resistant to Verticillium.

deer-resistant plants

Do deer commonly eat Erythronium (dog-tooth lily)? I enjoy them (the plants) in a semi-protected area near the house, but would like to put them in with ferns in an area where we know deer have eaten other plants. I don’t want to spend the effort and money this fall if it is something they love.

 

There is probably no plant that deer won’t at least try once. Dog-tooth lily is not the common name I’ve heard most often for this plant; it is more commonly known as dog-tooth violet or trout lily. Unfortunately, it looks like Erythronium is considered a delicacy, according to this U.S. Forest Service page on Erythronium grandiflorum, a species called glacier lily, but the same genus as dog-tooth violet. Here is an excerpt:

IMPORTANCE TO LIVESTOCK AND WILDLIFE :
Glacier lily is an important forage for grizzly bears, which dig for the corms in spring. Ground squirrels will also feed on corms. Foliage is grazed by large ungulates such as sheep and cattle. In an Idaho study, glacier lily made up the bulk of mule deer diets during May.

PALATABILITY :
In Idaho, mule deer ate disproportionate amounts of glacier lily
compared to its availability, suggesting there was some preference for
the lily. Bears will stray from their normal course of travel along ridges to seek out glacier lily corms. Glacier lily provides fair to poor forage for cattle, sheep, and horses, and fair graze for small mammals, deer, and elk.

There are lists available of plants deer “won’t” eat (or won’t like, once having tried them). Washington Department of Fish and Wildlife’s Living with Wildlife website (based on the book of the same name by Russell Link) has additional information. If you scroll down, you will find a list of “Deer-Proof (or close to it) Plants for Washington Landscapes.”

 

on pruning combination fruit trees

I have a five-variety dwarf apple tree that is doing well, but seems to be developing a very strong central leader. (I planted it last year.) Is this going to be a problem? I seem to remember hearing that I shouldn’t let it do this, but I can’t find any information about how to prune this type of apple tree.

 

As I suspected, the answer to your pruning question was lurking in the pamphlet I received many years ago with my Raintree Nursery tree order. Here is what they say about “combo fruit trees” and their care:

“Combination fruit trees with several varieties on the same plant can be a fun way to grow lots of varieties in a limited area. They can be somewhat challenging too. Often one or more varieties (branches) will be much more vigorous than others. If this problem isn’t carefully addressed, then the tree can become more and more lopsided and the most vigorous varieties will overgrow the others and dominate the tree. Prune back the most vigorous branches upon arrival (if we haven’t already done so) to even out the branch lengths. Prune the most vigorous branches back again in the summer to maintain a balance. The most vigorous branches are the most upright. Spread the branches if they are supple enough to spread without breaking. If you keep any upright branches and they are too stiff to spread, cut them back, if possible to the lateral side branches. The combos should be grown as open center trees. On most combo trees, the varieties are named on the plastic label attached to the tree with the bottom budded variety listed first, the second from the bottom listed second and so on. Missing varieties are crossed out on the label.”

University of Minnesota Extension has information on open center pruning.

Oregon State University Extension has a good general guide by Jeff Olsen to Training and Pruning the Home Orchard.

on disbudding roses

I’ve been removing all the small, spiky buds and leaving the main developing bud on my rose (I think it’s a Floribunda, but I’m not sure). My husband told me I was doing the wrong thing. Is he right? I thought I’d heard that removing the ancillary buds would give me a better-looking single bloom, like the long-stemmed roses you see in displays.

 

I’m a lazy grower of roses, and so it would not occur to me to disbud in order to have one single larger rose. But you are correct that this is in fact done in some cases. I found a brief item in Rose Magazine online (now archived) which illustrates the practice (it seems to be used mainly for roses being used in displays, where a kind of perfection is desired). Note the following, about Floribunda roses, which describes exactly the opposite procedure:
“On roses that produce multiple blooms, like those of Floribundas or Grandifloras, it is the terminal bud that is removed. On these roses, the terminal bud will open first. By the time surrounding blooms form the terminal rose is almost fully blown. The result is a floral spray with a hole in the center. By removing this terminal bud early in its formation, the rose’s energy goes toward those that remain. The result is a floral spray that is full — without the hole in the center.”

The website of the Desert Rose Society also describes disbudding.

“The practice of disbudding applied to roses can produce some impressive results in the size and quality of the bloom. This is how you get those big lovely long-stemmed roses. When disbudding for one bloom to a stem roses, such as hybrid teas, you remove the side buds that develop at the leaf axils below the main bloom. This is done by rubbing the tiny buds out from of the angle created between the leaf and stem. I find my thumb works best for getting right in there. The earlier you do this in the development of the side buds the better, for you will leave less of a disbudding scar or black stub. How many buds do I remove? Enough that you will have the desired stem length with no side buds. In a rose show, a single bloom on a stem will be disqualified if it has side buds, with the exception of old garden roses and shrub roses.

When disbudding roses that bloom in clusters like the floribundas, it is a little different process. You have to look at the stem and see how many buds there are. If there is a central bud and only one or two side buds, remove the side buds and go for a one bloom stem. When presented with many buds and a central bud, remove the central bud and make this stem into a spray (or cluster) of blooms. The central bud would normally bloom first and be faded when the rest of the buds open. A spray, for show purposes, must contain two or more blossoms and three or more blossoms are best. If trying for a spray for show be sure that the multiple buds have at least three buds of about the same size so they will be open at the same time to give the desired blossom count.”

For the meticulous home gardener or professional rose exhibitor, you may find this information on the American Rose Society’s judging criteria of interest.

To sum up, the answer to your question about whether or not to disbud seems purely an aesthetic choice.

preventing veggies from bolting

I planted Broccoli raab seeds early this spring. They came up just fine, but they more or less bolted immediately. What happened to cause this?

 

Broccoli raab, Rapini, or Brassica rapa, Ruvo group, resembles turnip greens more than it does the more familiar head-forming Broccoli. It has a tendency to bolt (form flowers and go to seed) when temperatures are too low or too high. Sometimes cole crops bolt when they have been exposed to seesawing temperatures, but they may also bolt in reaction to other stresses, such as insufficient nutrients (like nitrogen), or competition from weeds. The following information from Clemson University Extension refers to Chinese vegetables, but it provides useful tips on growing cole crops, such as:
Bolting can be prevented by:

  • Maintaining a steady, moderate rate of growth.
  • Setting out young, healthy transplants that have not been stressed.
  • Watering well when transplanting to start root growth and remove air pockets from the soil.
  • Planting at the correct time for your area.
  • Growing slow-bolting varieties.

Although you were probably hoping for green unopened flower buds, the flower shoots, stems and leaves are also edible. Try sowing through early fall, and provide the optimum water and soil conditions. North Carolina State University Extension (link now archived) has additional information about this plant.

evergreen wind-resistant plants

I am looking for evergreen hedges that will tolerate a windy site. Do you have any suggestions?

 

Sunset Western Garden Book (2007 edition) has
a list of wind-resistant plants. From that list, there were a few plants
which meet some of your site’s needs (evergreen, fast-growing, about 7-10
feet tall). They are:

  • Arbutus unedo (Strawberry tree)
  • Arctostaphylos (Manzanita)
  • Ceanothus
  • Chamaecyparis
  • Cotoneaster
  • Escallonia
  • Morella californica
  • Pinus species (you would need a dwarf pine for your size limits)
  • Pittosporum (many of these grow taller than 10 feet over time, but P.
    tobira     might work)
  • Pyracantha

I don’t know if it is tolerant of winter winds, but Osmanthus delavayi
makes a nice, dense evergreen hedge with flowers, and reaches about 8
feet. It grows fairly quickly also.

Two good resources for finding more information on the plants above are
Oregon State University’s Landscape Plants and Great Plant Picks.

Also, I found an article (no longer available) on wind tolerance from Colorado State University Extension which may be of interest. Here is an excerpt about the physical characteristics of wind tolerant plants:

“When considering which trees and shrubs do well in windy conditions,
examine the shape and thickness of the leaves, stems and branches.
Wind-resistant trees usually have flexible, wide spreading, strong
branches and low centers of gravity. Wind tolerant shrubs often have
small, thick or waxy leaves or very narrow leaves (or needles), to help
control moisture loss. Plant species that have large, flat leaves “catch”
wind. These plants have a tendency for branch breakage when strong gusts
blow, or if laden with heavy, wet snow.
Evergreen (conifer) trees are an excellent choice, having needles and
being flexible in high winds.”

tree topping as a solution to landscape problems

We just moved into a new house that has a beautiful 20-foot-tall Colorado blue spruce planted too close to the house. Can it be topped and shaped so it could be left in that spot? If we wanted to remove it and plant it somewhere else could we do that? Or will it just die anyway? What to do?

 

I would not recommend topping the tree. Since this tree can reach mature heights of 30 to 60 feet or more, it may not be the right tree for the site. Here is a page about Colorado blue spruce (Picea pungens) from University of Illinois Extension’s Selecting Trees For Your Home.

Here is information (now archived) from a local organization, Plant Amnesty, on why topping trees is not a good solution to your landscaping problem.

This organization has an “Adopt-a-Plant” service, if you think you would like to give the tree away. There are also referral services available from Plant Amnesty if you need the tree removed or moved to another location. You could also contact a certified arborist through the
International Society for Arboriculture.

I also suggest looking at resources like Great Plant Picks, which lists trees and shrubs which will do well in our area, and includes information on their growth habits and ultimate size at maturity.