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Elisabeth C. Miller Library

Archives: Plant Answer Line Q & A

Plant Answer Line questions and answers.

on pruning asters

Can tall asters be sheared or lopped off early in the growing season to control height and make them bushier? I have Aster novae-angliae ‘Wild Romance’. I love the color and bloom-time, but would like them shorter.

 

Yes, asters (now renamed Symphyotrichum) can be pruned. This is sometimes referred to in England as “the Chelsea chop,” and it is a technique that may be used for a number of different perennials, as this article by Bunny Guinness in the Telegraph describes. An excerpt appears here:

“Plants now commonly manicured by their snip happy owners are Campanula lactiflora, sedums, rudbeckias, echinaceas, asters and heleniums. These have their shoots chopped back by around a third in late May/June. The basic rule is that perennials which only flower once should not be chopped or you will lose the flowers; varieties such as peonies, irises and aquilegias.”

Here is similar information previously available from the Colorado State University Cooperative Extension website:

“…control the height and shape of an aster by pruning. Gardeners can pinch asters like mums, regularly removing little bits of new growth until the first of July. However, an easier approach is to cut the aster back by one half in mid-June. At this time, the aster can be shaped. Outer stems can be cut lower than inner ones to produce a nice mounded plant. This shaping tends to encourage bloom near the base of the aster and discourage ugly brown stems. Although this pruning may sound extreme, it tends to delay flowering by only a few days and produces a much prettier plant.”

on pruning abelia

I recently bought an Abelia ‘Edward Goucher.’ When I got it home, I noticed a lot of the stem tips were broken off. I figured it probably happened when the salesperson pulled it away from the other plants. I tried to be very careful when I planted it, but again, some of the tips bent and broke. Then after a rainstorm the other day, I found a couple more broken. I love the plant, but is it going to be that fragile? I planted it on the southwest side of my house. Will the stems grow more sturdy?

Also, what is the best way to prune it? I thought I read that you shouldn’t just trim branches but rather take some back to the ground. So if branches keep breaking, what will happen?

 

Woody shrubs purchased in nurseries often have the damage you describe, mainly from being packed into a truck for transport from the grower. Abelias are not particularly fragile when established – the branches thicken up and get stronger with time. You are right about not trimming (shearing) branches but cutting them to the ground or to a strong main branch.

The best guide to pruning abelias that I found is in Cass Turnbull’s Guide to Pruning (Cass Turnbull, 2004, Sasquatch Books). She recommends removing whole branches if they are dead or damaged. When the plant is older, she suggests removing some of the lower branches that grow along the ground, and some of the taller branches that grow straight upwards. As mentioned, prune them back to a main stem. You can remove up to a quarter of the branches at a time. Pruning is best done in the winter months; pruning an abelia during the growing season will encourage it to grow even more. (In your situation, though, you might want to do some pruning during the growing season to encourage this sort of quick growth.)

Of course, if all your branches are broken, you will have to wait a year before you can do this kind of pruning – don’t cut them all off. Old, overgrown plants can be cut to the ground for renewal, but a new plant probably will not survive this.

Finally, remember that this variety grows to 5 feet. Trying to keep it smaller by shearing it will lead to growth of water sprouts, and even more pruning…Cass explains all of this very well.

burnt leaves on Corylopsis

Our Spike Witchhazel has burnt leaves throughout the plant on the
tips. What could be the cause of this? This is its third year in the same spot with full sun.

 

Are you referring to Corylopsis (sometimes known as Spike winterhazel or Buttercup
winterhazel), or to a Hamamelis? (They are in the same plant family, but
look quite different). Here are some pictures for comparison:

Corylopsis

Hamamelis

Hamamelis family (Hamamelidaceae)

Corylopsis leaf edges do have a tendency to get sunburned in full sun and
windy sites. They prefer a partly shady spot, and organic, acid,
well-drained soil, although they will do well enough in full sun (except
for the burnt leaf edges). Again, though, I would be concerned about
fertilizer burn as well. I never use fertilizer of any kind, just the
occasional addition of organic compost, on my Corylopsis, and it does
nicely in a mostly sunny spot.

Campsis radicans and its invasive potential

I was given a Campsis radicans ‘Galen.’ I’ve heard this vine can be
invasive. Do I dare plant it? What do I need to be wary of?

 

Campsis radicans is certainly aggressive and potentially invasive in some areas, but to
what degree depends on where you live. It is listed as a weed in the
Northeast, the South, and Kentucky. The Missouri Botanical Garden has another profile of this plant.

I have seen it growing here in Seattle, and it seems to do well but
perhaps not so well that it is a major concern in this area. Since it
spreads by runners and seeds, keep it trimmed and do not let the flowers
go to seed. Wear gloves when handling it, as it can be a skin irritant.

control of cypress tip moth on cypress trees

I haven’t been able to find much about control of cypress tip moth on true cypress (Cupressus). I’m looking for a non-toxic control instead of the WSU recommendation of Orthene. Would Neem possibly work? Spinosad? Both are registered for leaf miners (fly larvae), but this is a moth larvae. Bt (Bacillus thuringiensis) won’t work because the larvae are inside of the foliage. What’s the best timing for a non-toxic? WSU recommends controlling the adults in July-August.

 

University of
British Columbia Botanical Garden’s forum has this to say:

“In the west, cypress tip moth sometimes infests cypress (Cupressus and
Cupressocyparis) and false cypress (Chamaecyparis); those are also
sometimes called cedars. In the east, cedar often refers to Juniperus
(red cedar), Thuja or Chamaecyparis (white cedar); all are subject to
bagworm infestations and various tip-miners. In the west, timely shearing
is the most effective way to control cypress tip moth, and this may also
be a tactic in other parts of North America.

“In many cases, infestations occur because there are few natural enemies
about to reduce pest levels. Sometimes, pests are attracted to plants
that are already weakened by stress. Healthy plants and diverse
plantings, together with a reduction in pesticide use, will over time,
increase beneficial organisms which will in turn reduce pest levels.
Spraying to reduce pests generally affects beneficials to a greater
degree than the actual target pest. This is because pest species often
have a greater capacity to rebound — they often reproduce faster, have a
greater tolerance for pesticide residues and have a greater capacity to
become resistant to pesticides.”

Oregon State University’s IPM site only mentions chemical controls.

From an online forum, ‘Horticulture Guy:’

Q. I have a row of 16 – three year old “Emerald Green” arborvitaes. I
suspect they have arborvitae leafminer (cypress tip moth). I have
noticed the moths before, but now there are more and I just recently
noticed brownish-yellow tips on a couple of the trees. All of them have
lots of needles falling from the interior. My problem is that I have
received conflicting reports about the proper time to spray for them, and
is there anything I can do in the meantime to lesson the damage? Thank
you! Linda Brieger – Tacoma, WA

A. The way to gain control over any pest population is to know its life
cycle. Spraying is geared toward eliminating the adult form of the
insect, which is a moth as the second of the two common names indicates.
The most likely reason you may see conflicting reports on when to spray
the moths is because of varying times the moth may emerge in different
regions where they are present. They are generally active in our area
from April to June with a peak of activity in May. The moths lay their
eggs during this period and the eggs hatch and then burrow into the
needles of the host plant. According to the WSU extension the adult
moths are silver-tan and approximately 1/4″ in length. External sprays
won’t have an effect on the larvae once they burrow so you need to spray
weekly during this period to catch the larvae as they hatch. Systemic
insecticides are able to kill the larvae once they are in the host. You
can limit systemic insecticide spraying to one application near the
beginning of the activity since they generally remain effective for some
time (see labels for instructions). As far as “in the meantime” a
sprayless solution is to prune out and destroy infected parts of the host
now so that there are less moths in the spring. You can also keep an eye
out in the spring for the white cocoons that form after the larvae exit
the host to become adult moths. You can remove these as well.

University of California Integrated Pest Management suggests that proper
cultural care and removal of susceptible plants is the answer. Excerpt:

Provide proper cultural care to keep plants vigorous. Prune out and
dispose of foliage infested with immature leafminers to restore the
plant’s aesthetic appearance and provide some control. Consider replacing
plants especially susceptible to the cypress tip miner. High populations
and damage can be reduced on established plantings by applying a
broad-spectrum, persistent insecticide such as acephate on susceptible
varieties when adult moths are active. Beginning in early spring, examine
foliage tips for the cocoons. When these appear, vigorously shake foliage
and watch to see if silvery tan, tiny moths fly up then settle back on
the foliage. One application to foliage can be made when a large number
of tip moths appear, between March and May in California. This reduces
browning next season.

You could try using the Neem oil (instead of the more toxic alternatives)
although I did not find any information specifically suggesting this as a
control for cypress tip moth. The WSU book, Pacific Northwest Landscape
IPM Manual (2002) suggests natural parasites which attack this species of
insect, but they do not specify the identity of these predators. They
state that there are no “biorational pesticide management options” for
this pest.

Powdery mildew and Euonymus

Have you heard about a problem with Burning Bush
(Euonymus alatus) getting a mildew this year? The
leaves have turned yellow green with small spots of
lighter yellow discoloration.

 

Powdery mildew is a common and usually not life-threatening
problem with Euonymus. Make sure the plant has good air
circulation, and be sure to clean up and destroy fallen
leaves which are infected. However, the symptoms of this
fungal problem would be whitish coating on the leaves, rather
than yellowed leaves. This makes me wonder if it is a
different problem such as scale (which is actually an
insect). Check and see if there are small bumps on the leaves
or stems. Scale can cause yellowed leaves. If your plant has
a small infestation, you can try scraping the scales off with
your fingernail, prune out the most infested parts of the
plant, and then apply dormant oil to the trunk and branches
before growth starts next spring, or apply superior oil
during the growing season. There are also other fungal and
bacterial problems that could be causing the spots.

See this fact sheet from Penn State for more on Euonymus scale.

Here is an archived link to additional information, which comes from
University of Illinois Extension. Excerpt:

Burning bush (also called Winged Euonymus): Euonymus alatus

Cold injury – Winter injury may be caused by very low
temperatures as well as drought stress. With excessively low
temperatures, the moisture in the cells freezes (due to
chemical compounds in plants, moisture freezes at various
degrees below freezing). Drought stress already has resulted
in limited moisture in the plant cells. Dry, freezing winds
during the winter reduces the moisture level even farther,
often resulting in dead plant tissue. Diseases can help
magnify or increase susceptibility to winter kill. Nectria
canker kills the sapwood tissue thus reducing or even cutting
off moisture to tissue further out on the plant. Winterkill
also makes plants more prone to infectious diseases and
insect problems.

Dieback/canker – See bridal wreath spirea. In addition
Botryosphaeria dothidea will infect and kill for similar
reasons.

Winged euonymus scale – Lepidosaphes yanagicola occasionally
occurs in the southern half of Illinois on burning bush. It
is an armored scale. And will attack several trees as well.
This scale can cause premature leaf drop, branch die back and
cause the plant to become more prone to winter injury. It is
found between the “wings” – the bark ridges. It does not move
to the plant’s leaves. The scale over winters as an adult and
lays its eggs in June. Eggs may be laid for up to a month.
Mating occurs before frost.

Euonymus scale – Unaspis euonymi – females are black and
males are white. The scale causes the foliage to develop
yellowish green spots. Heavy infestation results in early
foliage drop and often stems are killed. Eggs survive by over
wintering in the female body. The eggs hatch about early June
in Northern Illinois. Crawlers emerge and move onto new
growth or can be blown by wind to other plants.

Since I cannot diagnose the problem remotely, it makes sense
to take plant samples to a Master Gardener Clinic.

growing Allium from seed

Could you tell me how to grow Allium from seed?

 

I will assume you are propagating ornamental Allium. According to the American Horticultural Society’s Plant Propagation (edited by Alan Toogood; DK Publishing, 1999), Allium seeds may be sown any time from late summer to early spring. Seeds should be collected when the flower heads turn brown and before the seedpods open. If you tug gently on the flower stalk and it comes away easily from the base, the seed is ripe. Cover the spot where the stalk was removed with soil to prevent entry to pests. With smaller flowering Allium, you can shake seeds directly into a paper bag (without removing stalks). Sow the seeds fresh, or store them at 41 degrees F, and sow in the spring. Germination time is usually 12 weeks, but in some cases it will take up to a year.

The Royal Horticultural Society says that Allium cultivars may not come true from seed, so you may want to consider alternate methods of propagation, such as by offsets or aerial bulbils.

pruning and training Wisteria

Is it all right to cut off the hanging pods from the Japanese
Wisteria? Will cutting them have any adverse affects to blooming next
year? Some are hanging so long that we keep walking into them! Maybe I should cut them and bring them inside for decoration.

 

Cutting off the seedpods on your Wisteria is not a problem, just be
careful not to cut the stems back too far (unless you are intending to
prune, which you can certainly do if you need to control growth) as there
may be buds further up which will be next spring’s flowers.

Fine Gardening online has a helpful illustrated article on wisteria pruning which includes the following:
“Some seedpods may be left on the vine for winter interest, but just know that if you bring them inside, warm temperatures will cause them to explode.”

You may find the following links to general information on care and
pruning of Wisteria helpful:

 

Excerpt from an article, “Pruning Vines,” by Donald Hodel and Dennis Pittenger:

Pruning wisteria extensively during the dormant season may
encourage rampant vegetative growth the next spring.
Instead, in July prune out the long, straggly growth
except those branches needed for climbing. This is more
likely than anything else to induce flowering. Shoots
should be cut back one-third to one-half their length.
This will induce them to produce the short spurs that will
bear next season’s flower clusters.

diagnosing rhododendron problems

The edges of the leaves on one of my rhododendrons are turning brown and
crispy. This condition advances toward the midline until the leaf dies. My older, more established rhododendrons are looking fine. All have flowered this spring.

 

We can’t diagnose the problem via e-mail. However, what you describe could be a couple of things.

It might be marginal leaf necrosis which can be caused by cold damage, and made worse by wind and drought; by drought while the plant is in active growth; by high
amounts of salt in the soil, often due to overuse of soluble fertilizers,
and seen often if plant is located close to the house, where the eaves
prevent rainwater from reaching the soil. Other causes are poor drainage,
planting too deep, root damage, root weevils, nutrient deficiency, or
disease. (Source: How to Identify Rhododendron and Azalea Problems,
Washington State University Cooperative Extension, 1984).

There are descriptions and illustrations of rhododendron problems on the HortSense website of Washington State University Extension.

For comparison purposes, see the lists of diseases and problems affecting
Rhododendrons from University of California, Davis Integrated Pest Management Online. To get a definitive answer, it may be a good idea to take samples of the
affected leaves to a Master Gardener Clinic for diagnosis.