cascading plants for retaining walls

I’m looking to plant in a narrow strip on our retaining walls some “spiller” plants which will overhang the walls (which face north).

I’d prefer evergreen plants which would fill in fairly quickly,
but I could also mix in slower-growing and deciduous plants. There’s great drainage since I have gravel reservoirs
behind each wall, and the part of the plant above the wall will get part to
full sun, though I could overplant them if necessary for a plant that couldn’t
handle full sun.

I would like plants with interesting foliage and form to soften the look of the walls, and so would prefer a furry look to a spiny one. Flowers and fragrance are less
important though always nice, and I’m hoping to have at least 2 or 3 different
plant types with different colored foliage (shades of green are fine).

Some of the plants that occur to me, based on the description of your site, are
Brunnera macrophylla, Epimedium, Geranium phaeum, Stachys byzantina, Lamium
maculatum, and Liriope. Of these, the Geranium and Lamium will trail somewhat, while the others are essentially upright.

Graham Rice’s article on the Royal Horticultural Society site features a selection of recommended trailing (or spilling) plants. Here is another good list of trailing plants for walls.

You could also try entering your site requirements into the plant-finding and
plant selection web pages below:

Great Plant Picks (a local site)

King County’s native plant guide

Missouri Botanic Garden Plant Finder

Royal Horticultural Society Plant Finder

The Miller Library has many books on gardening in the shade, so you may wish to
come in and do some research to help you in your plant selection. Here is a booklist that may be of interest.

encouraging rhododendron blooms

My two rhododendrons did not produce any blooms this year- they are healthy otherwise. Why?

I had the same problem with one of my rhodies this spring (all the others were fine), as did many other people in the Pacific Northwest. Following are the most likely causes:

NO FLOWERS, BUDS DO NOT OPEN. This is most likely to be caused by frost, either in mid-winter by the hardest frosts of the year, or in spring when the buds are swelling and about to open. Certain varieties have very frost-vulnerable swelling buds, while many species have buds which are easily destroyed even by quite mild winter frosts.

NO FLOWERS, NO FLOWER BUDS. There are several possibilities why rhododendrons may not flower freely:

Too much shade. This is very common in North America where, in order to regulate sun and soil temperature, plants are placed in deep shade. This allows healthy, if straggly growth, but can inhibit flowering. The more light you can give a plant, the more likely it is to flower, so there is a trade-off between the need for shade and the need for light.
The variety takes many years to flower (it does not sound like this is your situation).
Kindness. Rhododendrons flower in order to reproduce. A contented, well-fed, well-watered well-shaded plant may not feel any need to reproduce, as it perceives no threat to its survival. Do not feed after mid-summer, as this encourages growth at the expense of flowers. Nurserymen cut down watering in late summer to stress plants into flowering the following year.

(Source: Rhododendrons: A Care Manual, by K. Cox, 1998, p. 73).

The above is corroborated in other sources, e.g. Success with Rhododendrons and Azaleas, by H.E. Reiley, 1992, p. 132-133.

Miscanthus growing conditions and its invasiveness status

I am looking for an ornamental grass that doesn’t get over 5 feet tall
and am wondering what are the growing conditions for Miscanthus sinensis
(Gracillimus)? How much sun does it need, will it spread and invade my
other plants, is it invasive in our area (Seattle)?

I found several cultivars of Miscanthus listed on the local web site Great Plant
Picks. Miscanthus sinensis ‘Morning Light’ will reach about 5 feet tall
by 4 feet wide. This Colorado State University Extension article on from ornamental grasses may give
you additional ideas on grasses for your garden.
Although the following link is for southwest Washington gardens, this
Washington State University list of ornamental grasses may be of use.
It includes Miscanthus sinensis ‘Gracillimus,’ and indicates it does not
have a problem with self-seeding.

Excerpt:

Miscanthus

BE CAREFUL! Many are self-seeding.

M. sinensis ‘Gracillimus’ Maiden grass 4.5′ FS Most popular. Seldom
self-seeds.

M. sinensis var. purpurascens Purple maiden grass, Flame grass 3 – 5′ FS
Gorgeous red-orange fall foliage. One of the earliest flowering varieties
of maiden grass.

M. sinensis ‘Silberpfeil’ Eulalia 4 – 5′ FS One of the hardiest varieties
of maiden grass.

M. s. ‘Morning Light’ Dwarf maiden grass 4 – 5′ S, LSh Arguably best
all-around plant of the Miscanthus group. Blooms late with reddish
flowers.

M. s. ‘Adagio’ Japanese silver grass to 3.5′ S, LSh Compact with
silver-gray foliage. Two- to three-feet long panicles emerge pink, fade
to white.

M. s. ‘Flamingo’ Japanese silver grass to 6′ Large, loosely open,
pink-tinted inflorescences. Slightly pendant blooms appear late summer.

M. s. ‘Sarabande’ to 6′ Similar to Gracillimus, but finer textured.
Golden copper colored inflorescences in August.

M. s. ‘Strictus’ Porcupine grass 4 – 6′ FS One of the hardier Miscanthus
cultivars. Tolerates wet soils.

M. s. ‘Variegatus’ Variegated silver grass 4 – 6′ S, PSh Prefers
moist, fertile, well-drained soil.

M. s. ‘Zebrinus’ Zebra grass 4 – 8′ S, PSh

Update from 2012 on the invasive potential of Miscanthus cultivars:

Wendy DesCamp of the Washington State Noxious Weed Control Board reports the following:
“There is now a record of the plant growing in eastern Washington by the Columbia River in Benton County. [described as follows:] Shallow backwater on N shore of Columbia River . . . below McNary Dam, elev. 85 m, 45 degrees, 55.9 minutes N, 119 degrees 21.4 minutes west. Collected by Peter Zika, 17 June 2011.

From what I can find, this is the first collection of naturalized Miscanthus sinensis collected in Washington.”

The State Noxious Weed Board is considering whether it should be added to the monitor list or not. The monitor list is a list of plants the Board is keeping track of to collect information and to see if the plants are occurring or spreading in Washington.

UW Botanic Gardens Director, Professor Sarah Reichard had this to say about Miscanthus sinensis:
“We have had it in the Soest Garden for years and I have not seen it invade and I am looking for seedlings. However, not invading in the artificial environment of a garden, with water and nutrient inputs means little for invasion in the wild. I have not heard of it being invasive here, and I have been paying attention to both this species and Imperata cylindrica.It might be a good addition to the [noxious weed] monitor list.”

on growing Colorado blue spruce in the PNW

I am interested in planting a dwarf Colorado blue spruce in my yard (I live in Kirkland), but my landscaper said that blue spruce does not do well here. Is she correct? It is true that there aren’t any on the Great Plant Picks list.

According to local gardener and author Arthur Lee Jacobson, Picea pungens
f. glauca (Colorado blue spruce) is quite common in gardens here, but in
his opinion it does not have many assets beyond its striking color and
hardiness (North American Landscape Trees, Ten Speed Press, 1996). I
wonder if your landscaper has observed that it is susceptible to spruce
spider mites, aphids, and other pests and diseases. I see that it is
susceptible to Phytophthora, which is often a problem in our area. The
book Trees & Shrubs for Pacific Northwest Gardens by John and Carol
Grant (Timber Press, 1990) concurs that Colorado spruce is not the best
choice for our region because of pests and diseases. Newly planted trees
will also need regular watering for their first year, and if they do not
receive it, they are likely to suffer dieback or death. Here are links to
additional information from the Washington State University and the Urban Forests Ecosystems Institute SelecTree site.

Excerpt:

Pest & Disease: Susceptible to Aphids, Scales and Spider Mites, Oak Root
Rot, Phytophthora, Root Rot, Rust and Sooty Mold.

If you love the look of this tree, you could try growing it despite the
drawbacks, or you could look for other blue foliage conifers which may be
easier to grow here. Great Plant Picks mentions Abies pinsapo ‘Glauca,’
though this is not a dwarf tree.

You might want to search the Oregon State University Landscape Plants web
pages for additional information on any other trees you are considering
for your garden.

growing Gaura in the Northwest

I have planted at least two Gaura plants (I don’t recall which varieties) in the past couple of years, but they only lasted one season. One was planted in a sunny location with excellent drainage. What could I be doing wrong?

I have had bad luck keeping Gaura going for more than a year also. Mine were planted in the sunniest, driest part of the garden, but they failed to thrive. Gaura can succumb to root or crown rot, drought, snails, slugs, or even winter damage (it’s a good idea to mulch around it for protection against cold–keep mulch away from crown). Did you notice how they developed after you planted them? If there is any plant matter left, you could bring it to a Master Gardener Clinic for diagnosis.

If you want to try this plant again, you could try obtaining it from another
source (just to rule out preexisting disease as a cause), and continue to provide good drainage, full sun, and space around it for air circulation. According to Perennials: The Gardener’s Reference by Carter, Becker, and Lilly (Timber Press, 2007), Gaura is “short-lived in heavy soil in the maritime Northwest. […] We often use it as an annual; it is worth replanting on a regular basis.”

alkaline and saline soils

Can you tell me how alkaline soils are formed and if and how they are related to saline soils?

Saline soil may have a high pH, a characteristic shared with soils which are alkaline. You may find this information on soils and pH useful:

Diagnosing Saline and Sodic Soil Problems

Plant Materials for Saline-Alkaline Soils

Soil pH: What it Means

Alkaline soil: A soil whose reaction is greater than pH 7.
Acid soil: A soil whose reaction is less than pH 7.
Saline soil has a pH less than 8.5.

Saline soil, as defined in Soils: An Introduction, by Michael Singer and Donald Munns (Prentice Hall, 1999): “Saline soils …contain large amounts of soluble salts, appreciably more soluble than calcium sulfate. Most commonly, these are salts of Na, Ca, and Mg, with chloride, sulfate, and bicarbonate…soils are considered saline if their electrical conductivity exceeds 4 deciSiemens meter-1. Many plants suffer at this level.”

To answer your question about how alkaline soils are formed, here is what Clemson University Extension says:
“The pH value of a soil is influenced by the kinds of parent materials from which the soil was formed. Soils developed from basic rocks generally have higher pH values than those formed from acid rocks.”

Rainfall also affects soil pH. Water passing through the soil leaches basic nutrients such as calcium and magnesium from the soil. They are replaced by acidic elements such as aluminum and iron. For this reason, soils formed under high rainfall conditions are more acidic than those formed under arid (dry) conditions.

Application of fertilizers containing ammonium or urea speeds up the rate at which acidity develops. The decomposition of organic matter also adds to soil acidity.

growing and caring for Dracaena fragrans ‘Massangeana’

I recently repotted my Dracaena fragrans ‘Massangeana.’ Some of the leaves have turned brown. Can you tell me what might be causing this?

We cannot diagnose plant problems via email. However, we may be able to give you some ideas of what might be happening. The browning leaves could be the result of too much or too little water. The soil should be kept lightly moist, but avoid overwatering. Avoid giving fluoridated water, as this plant is especially sensitive to it. It is also sensitive to temperature changes (up or down) and should not be near heating or air-conditioning vents. Repotting may have caused some stress to the roots. According to The Complete Houseplant Survival Manual by Barbara Pleasant (Storey Publishing, 2005), brown spots on leaves can be the result of sun scorch. This plant prefers moderate to bright indirect light.

Here is a link to discussions from a gardening forum sponsored by the University of British Columbia. Here is another from the same site.

Here is a link to information about diagnosing problems with houseplants from Penn State Extension.

identifying horsegrass

What does “horsegrass” look like? According to Ciscoe, it can’t be gotten rid of and I want to see if this is what I have.

I wonder if you are referring to horsetail, or Equisetum, which is a very persistent weed.

Wikipedia has a picture, and here is another from CalPhotos

Here is an article on Field Horsetail and Related Species from Oregon State University Extension.

Here is what Ciscoe Morris said about this plant in the Seattle P-I (April 29, 2006):

“Hands down, horsetail (Equisetum arvense) is the worst weed you can get in your garden. If you’ve got it, just be glad you weren’t gardening in prehistoric times. Back then, horsetail grew to 90 feet tall and you were in danger of being stepped on by a brontosaurus while weeding.

The worst thing about horsetail is the speed with which it returns to make your life miserable after you weed it. No matter how great a weeding job you do, it will be back, practically to full size, within a week!

Do what we did at Seattle University. Plant a mix of shrubs, ground covers and fast-growing perennials that are thick and tall enough to hide the horsetail. Shrubs that hide horsetail include Cistus (rockrose) Lonicera pileata (privet honeysuckle) Lonicera nitida (Box honeysuckle) and rosemary. My favorite perennial to hide horsetail is the prolific hardy Geranium oxonianum ‘Claridge Druce.’ It will seed all over your garden, but new seedlings are easy to remove in spring. These drought-tolerant plants look great in their own right and because they are so thick and tall, no one will see the hoards of horsetail growing within.”

Washington Toxics Coalition recommends controlling it by persistently hand-pulling or hoeing the above-ground growth as soon as it appears. This will weaken the plant over time. It does die back over winter, when you could cover the affected area with black plastic (for a duration of 2 years), but even this may not be entirely successful.

An article by Irene Mills in the Fall 2008 issue of the Northwest Perennial Alliance’s Perennial Post says that pulling, digging, and covering with black plastic are a waste of time. The author recommends keeping an eye out in April for emerging spore-bearing stalks, and cutting these off and disposing of them in the garbage. She suggests improving the soil texture (improve drainage, add organic matter, increase soil fertility, and in some cases increase soil pH). She recommends this guide called “Controlling Horsetail” from Swanson’s Nursery, originally published in Gardens West by Carol Hall.(note: guide may not open correctly in Firefox browser)

native orange spotted lily identification

I am trying to discover the common and scientific name for the orange spotted wild lily that looks like an orange tiger lily. It blooms in the forests of the Pacific Northwest in June and early July.

You must be thinking of Tiger Lily, Lilium columbianum (also known as Columbia lily and Oregon lily).
Source: Plants of the Pacific Northwest Coast by Pojar and Mackinnon (Lone Pine, 1994).

Click here to see images of Lilium columbianum.

pruning hardy fuchsias

I have several hardy Fuchsias that are quite large, about 4 – 5 feet tall, that bloomed profusely this year. The leaves are gone now, but when should I prune them? How far back should I prune? Is there anything special I need to know about pruning Fuchsias?

Fuchsias are pretty tough shrubs, especially once established. You can prune just to tidy them up, but it is best to wait until the leaves begin to grow (between March and June).

Here are two links to more detailed information:

From Rainy Side Gardeners,
“Some years when we have an exceptionally cold winter, Fuchsias will die down to the ground. Before pruning in spring, wait until leaf buds swell, then prune out dead twigs, or prune down to the ground if winter knocked it completely back.”

Another site is the Hardy Fuchsia List from the Northwest Fuchsia Society.