lack of flowering on Magnolia grandiflora

I have a
very tall, well-established Magnolia grandiflora. The tree is located on
the southeast corner of the lot. Every summer it has produced large white
flowers, but last summer that there weren’t as many blooms, almost
none. It’s left alone and watered spring through summer by a sprinkler system. I have also noticed yellowing of the leaves at the ends of the branches. Usually the leaves
have been dark and green. I’m wondering if it is a lack of nitrogen or food of some sort.

There are a number of reasons that plants may fail to flower, and it
would be difficult to pinpoint precisely why the Magnolia made such a
weak show this past summer. Sometimes, cold temperatures kill off flower
buds (and there were some cold snaps last winter). The tree is not
immature, and it sounds as if it is not pruned improperly, so those
potential causes can be excluded. You also indicate that it is not
fertilized, so it is probably not receiving excessive nitrogen which can
lead to lots of leafy growth at the expense of flowers. I wonder if
anything else in its environment has changed: has the amount of light
changed (any new construction obstructing sun?), or has anything happened
to the soil where it is planted? You might wait and see if flowering
returns to normal this year.

As for the yellowed leaves, that might be a result of winter injury
(desiccation) or drought stress. However, yellow leaves can also be a
symptom of sunburn or lack of light, or nutrient deficiency. See the
link here to University of California, Davis’s page on Magnolia problems. Excerpt:

Mineral deficiencies:
Certain nutrients, in relatively small amounts, are required for healthy
plant growth. Deficiencies can cause tip chlorosis or necrosis or cause
foliage to discolor, fade, distort, or become spotted, sometimes in a
characteristic pattern that can be recognized to identify the cause.
Fewer leaves, flowers, and fruit may be produced, and these can develop
later than normal and remain undersized. More severely deficient plants
become stunted and exhibit dieback. Commercial laboratories can conduct
foliage tests or soil analysis to verify deficiencies.

Identification/Solutions:

Nitrogen and iron are the only nutrients in which woody landscape plants
are commonly deficient. Poor root growth caused by water-logged soil,
root diseases, and nematodes can also cause iron deficiency symptoms.
Fertilize only as needed and only if other problems have been eliminated
as the cause of poor growth. Avoid overfertilization, especially with
high-nitrogen fertilizers. Slow-release formulations of nitrogen or
organic fertilizers reduce some risk of overfertilization. Correcting
deficiencies of minerals is tricky. Apply only the mineral found to be
deficient. In some cases, soil characteristics may exacerbate
deficiencies. Alkaline soil (high pH) often makes iron or manganese less
available; reducing alkalinity with sulfur or organic amendments (peat
moss) may be all that is needed. Some minerals such as iron, manganese,
and zinc are absorbed more rapidly as a foliar spray than a soil
application.

If the environmental causes don’t ring true with your tree’s situation,
you may want to do a soil test to see if there are nutrients which need
to be supplemented.

on the “Burning Bush” plant

I am looking for more information regarding Rubus sanctus, also known
as the Burning Bush at Saint Catherine’s Monastery in the Sinai. I am interested in this plant because my church group is just finishing up our study of the Book of Exodus. And I thought this plant might make a really nice and symbolic gift.

I am beginning to understand that this plant may be rare, or possibly known by
another name?

The problem with English common names for plants of the Bible is that you are at several removes from knowing which plant the original Hebrew text describes. There are some sources which state that “burning bush” refers to Rubus sanctus, but it is more likely that it refers to Senna alexandrina. The Hebrew word in Exodus is sneh, which is the same as the Arabic word for the Senna plant.

Plants of the Bible by Michael Zohary (Cambridge University Press, 1982) says that “the plant in question, specifically named ‘sneh,’ might well have been a real plant in the local flora. As there is no hint in the text that the sneh was a thorny bush, and there are no plants in Sinai or anywhere else that are not consumed when burnt, sneh may be identified linguistically only.” He also suggests that the plant may have been Cassia senna, now renamed Senna alexandrina. There is no native Rubus in Sinai, Egypt, or southern Israel, and the bramble in the monastery garden at Santa Caterina is a cultivated specimen, planted by the monks “to strengthen the belief that the ‘burning bush’ has grown there since the revelation, so completely is sneh equated with brambles in the minds of scholars and Bible lovers.”

While Senna alexandrina may be a bit difficult to obtain, there are other species of Senna more widely available. However, if you wish to grow the Rubus you saw (also referred to as Rubus ulmifolius ssp. sanctus and Rubus sanguineus) as a keepsake from your trip to the monastery, you should go ahead. It is not easy to obtain except as seeds, and it prefers a moist environment.

managing lilacs with bacterial blight

I have a ‘Palibin’ lilac that appears to have a bit of bacterial
blight. I have pruned out the diseased branches. Is it too late to
spray to control the disease? I didn’t do a dormant spray this year,
and haven’t done any preventive spraying to this point, either. If it
isn’t too late, what spray product would you recommend? What else can I do to keep the blight under control?

There are cultural methods of dealing with bacterial blight you should try before using any spray. The information below should help.

Washington State University Extension’s HortSense website recommends:

Avoid injuring plants to reduce possibility of infection.
Avoid overhead irrigation.
Maintain proper plant nutrition. Healthy plants resist disease better.
Plant disease-resistant species such as Syringa perkinensis, S. microphylla, or S. vulgaris vars. ‘Alphonse Lavallec’, ‘Crepuscule’, ‘Floreal’, ‘Guinevere’, ‘Jeanne d’Art’, ‘Lutece’, ‘Maud Notcutt’, ‘Mrs. W.W. Marshall’, ‘Rutilant’, or ‘William Robinson’.
Prune and destroy infected tissues as soon as they are noticed.
Space plants properly and prune to provide good air circulation. This will slow down spread of the disease.

Here is more information from University of California, Davis’s
Integrated Pest Management site. Excerpt:

“Bacterial blight is promoted by prolonged rainy springs. Symptoms may be
more extensive in wetter areas. Prune branches showing dieback and severe
blight. Space plants to provide good air circulation. Prune during the
dry season when infection is less likely to occur. Do not wet foliage
with overhead irrigation; do not overfertilize. Small plants can be
protected to some degree by keeping them covered by plastic (or moved
under plastic). Plant resistant species if available. If the disease is
systemic or cankers appear on the trunk, the tree will probably die and
should be removed. If the disease is confined to leaves, damage is not
usually serious and trees normally recover. Sprays do not give reliable
control.”

observations on earthworms

So, here is a weird one for you. I was out in my garden this weekend and I noticed that the dirt paths that I had raked of leaves (but still had a few leaves around) all had small piles of decaying leaves. There were lots of these piles, measuring about 3-5 inches in diameter. When I pushed the leaves aside, I found a small half-inch hole under every single pile. When I dug into the hole, the only thing I found was an earthworm, which occurred in every one. I did not see this on the paths that I had not raked that still had lots of leaves, nor in the garden beds, most of which are heavily mulched with cedar chips.

I know earthworms are major decomposers of decaying leaves and that they mix the organic matter down into the soil. But do they gather the leaves into little bunches? How the heck do they do that?

I know earthworms go out onto the surface at night, grab a leaf or two and bring them back to their hole. I have witnessed this personally in my own garden (usually pathways or nearby).

But why and how?!

The book you need to read is The Earth Moved: On the Remarkable Achievements of Earthworms by Amy Stewart (Algonquin Books, 2004). It is one of those nonfiction books that is written with attention to prose, so I could not find a quick answer, but I am sure it is in there.

Other staff have also observed this, so it is not rare. It seems to be the nightcrawlers that do it. Go out at night with a flashlight to see for yourself.

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.