Plant Answer Line questions and answers.
How far apart should I plant my shallot starts?
According to Steve Solomon’s Growing Vegetables West of the Cascades (Sasquatch Books, 2007), plant them 3 to 4 inches apart in rows 18 inches apart. Other sources suggest you can make the rows as close together as 12 inches, and the plants as far apart as 6 inches.
Ontario Ministry of Agriculture has a useful page about shallots which recommends 4 to 6 inches between plants.
Do you know which type of cottonwood tree is growing along shores of Lake Sammamish, and the normal life expectancy is for these trees? Someone told me they are “blackheart cottonwoods.” Is that true?
We also have Oregon swamp ash growing here. Are these trees really just indigenous to Oregon, and what is their life expectancy?
I could not find any reference to “blackheart cottonwood,” but here is information about black cottonwood (I cannot be sure these are the trees you have around Lake Sammamish). Populus balsamifera ssp. trichocarpa (its botanical name) is native from southern Alaska to northwestern Mexico, according to Arthur Lee Jacobson’s Trees of Seattle (2006). It has a relatively short life span (and probably shorter in urban and suburban settings than in the wild) of 100-250 years. These trees have brittle limbs, and sometimes fall prey to strong winds. Jacobson mentions several hybrid cottonwoods, one of which closely resembles the native species but is crossed with a northeast Asian species, P. maximowiczii.
Just to complicate things, there is another Populus whose common name is black poplar (Populus nigra) and it is native to northwest Africa, Europe, western Siberia, and the Caucasus. There are many varieties of this Populus as well.
The USDA and CalFlora have information about black cottonwood for you to compare with the trees around the lake. Compare and contrast with the USDA page on black poplar.
As for the longevity of these trees, see what Arthur Lee Jacobson says about old trees. Here is an excerpt:
“Frequently the oldest trees are not the largest, so it is not as simple as finding the biggest trunk. Cottonwoods grow gigantic in a hurry, then bust up, earning their nickname rottenwood.”
Arthur Lee Jacobson has led tours of the trees on the Sammamish River Trail, and he is the best-versed person locally on what trees are growing in our area, so you may want to contact him directly.
As for Oregon swamp ash, there are trees called Oregon ash, and trees called swamp ash, but I could not find a tree called Oregon swamp ash. Oregon ash is Fraxinus latifolia, and is a Northwest native whose range also extends to southern California. According to Jacobson’s book, it closely resembles green ash (Fraxinus pennsylvanica), except that the native species has bark which flakes easily from the trunk, leaflets of 7 (rather than 9) which are hairy, rounded and broad. Its seeds are broader than green ash.
The USDA Forest Service has detailed information as well. Below is an excerpt:
“Oregon ash has moderately rapid growth for 60 to 100 years and attains a height of 18 to 24 m (60 to 80 ft) and a d.b.h. of 40 to 75 cm (16 to 30 in) in 100 to 150 years on good sites. Individuals may grow twice as large and reach 200 to 250 years of age under favorable conditions, although they generally grow slowly after their first hundred years.”
The website SelecTree says Oregon ash trees usually live over 150 years.
Swamp ash is also known as black ash, or Fraxinus nigra, and is native to the northeastern swampy woodland areas. They are not mentioned in Jacobson’s book or in other local sources, so I suspect they are uncommon in our area.
Last year I collected several hundred seeds from our local native columbine (Aquilegia formosa) in Thurston County. I recently planted some in commercial seed-starting mix in my small greenhouse. Could you tell me the best methods for storing and germinating these seeds as well as some of the native tiger lily?
In general, seeds should be stored in a dark, dry, cool place. I have had good luck storing extra seeds in a sealed jar or bag in the refrigerator, but this is not always recommended, because it is damp. According to Seeds by Jekka McVicar, columbine seed is generally viable for five years. McVicar suggests placing flats outside in fall and leaving them out all winter, since the cold helps to prepare the seeds for germination in spring. Alternatively, “in late summer fill tray or pot with compost, smooth over, tap down, and water in well. Use fresh seed (…) sowing the seed thinly on the surface of the compost. Cover with perlite or vermiculite (…)Place the tray or pot in a warm place out of direct sunlight at an optimum temperature of 50 degrees. Keep watering to a minimum until germination has occurred, which takes 14-28 days with warmth. Prick out into pots. (…) Overwinter young plants with a bit of protection in a cold frame.”
Eileen Powell, author of From Seed to Bloom, suggests refrigerating the seedling trays for two to three weeks and then sinking the flats into the ground in a shady location, covered with glass, and transplanting seedlings as they appear. She also says “seedlings are delicate; keep out of strong sunlight and water gently.”
Neither of these authors discusses our native columbine. However, Propagation of Pacific Northwest Native Plants by Robin Rose et al. suggests an easier method for Aquilegia formosa: “Seeds can be stored for up to two years at a low temperature and humidity or longer in sealed containers in low moisture. Prechilling for three days is required for germination. Direct seed in spring or fall. (…) Plant in containers or scatter evenly over a seedbed (this is made easier by first mixing the seeds with fine sand). Cover with a very thin layer of soil or weed-free compost and keep moist. Seeds should germinate in two to four weeks.”
As for propagating the native tiger lily, Lilium columbianum, Plants for a Future database provides directions, excerpted here:
“Seed: autumnal hypogeal germination. Best sown as soon as ripe in a cold frame, it should germinate in spring. Stored seed will require a warm/cold/warm cycle of stratification, each period being about 2 months long. Grow on in cool shady conditions. Great care should be taken in pricking out the young seedlings, many people leave them in the seed pot until they die down at the end of their second years growth. This necessitates sowing the seed thinly and using a reasonably fertile sowing medium. The plants will also require regular feeding when in growth. Divide the young bulbs when they are dormant, putting 2 – 3 in each pot, and grow them on for at least another year before planting them out into their permanent positions when the plants are dormant. Division in autumn once the leaves have died down. Replant immediately. Bulb scales can be removed from the bulbs in early autumn. If they are kept in a warm dark place in a bag of moist peat, they will produce bulblets. These bulblets can be potted up and grown on in the greenhouse until they are large enough to plant out. Stem or leaf cuttings.”
Here is what Kathleen Robson et al. have to say in Encyclopedia of Northwest Native Plants for Gardens and Landscapes (Timber Press, 2007): “Propagation: collect seeds in the late summer after capsules have ripened. Plant them within a few weeks of harvest, either directly into the garden or into deep containers, and leave them outside in the cool, moist winter weather for germination the following spring. Seedlings will take several years to reach flowering size. Those that were sown in containers can be left in them for part of that time; it may be easier to protect them from slugs.”
I covered my iris bed with wood chips to keep the ground from drying out so fast. Since the rhizomes grow partly above ground, will being covered with chips harm the plants?
Your mulch is unlikely to harm your bearded (rhizomatous) irises, as long as it is less than one inch thick over the rhizome tops and allows air through. Here is some information from The Gardener’s Iris Book by William Shear: “How deep should the rhizome be set? That depends. In light-textured soils, it can be covered by as much as one inch of soil, but for average to heavier soils, the top of the rhizome is best left exposed to the healthful influences of sun and air. Remember that the rhizome is a stem, not a root, and needs to breathe!…If you do apply a mulch, it must be loose and airy, so it won’t pack down and get soggy–a sure ticket to rotting rhizomes in the spring.” Shear suggests pulling back the mulch in spring (since it is for freeze protection in his mind), but it seems to me that would defeat your purpose. You might meet both goals (moist soil and dry rhizome tops) by mulching around the irises but not right on them.
Can I move my rhododendrons now, in late winter to early spring?
According to A Plantsman’s Guide to Rhododendrons by Kenneth Cox (Ward Lock Ltd., 1989), “rhododendrons are generally quite easily moved, most even in full flower and at considerable age. […] Size is really no problem, provided you have the means to do the digging and the moving. Obviously, the more rootball you can take with the plant the better, but usually you can reduce it considerably without too much harm being done. If you end up with a disproportionately small rootball, you can reduce the size of the top somewhat to compensate. The roots of a rhododendron generally extend to about 50% of the plant’s foliage diameter […] it can be far more or much less. The roots are usually less than 18 inches deep, even on a very large plant. To move a large plant, start digging […] quite far out from the stem, and continue towards it until you meet roots. Then dig all round underneath the rootball […] gently rocking the plant to ease the rootball from the soil. Watch out when lifting a plant by its main stem; it may not be strong enough to carry the weight of the rootball. The root can best be reduced by prising soil from it with a fork. […] A rhododendron can remain out of the ground for considerable periods if you keep frost and sun from the roots, and ensure that it receives regular watering. Heeling it into the ground, or covering the roots […] usually gives adequate protection. Although rhododendrons can be moved during the growing season, they will require extra watering after transplanting.”
In addition, you may find the Royal Horticultural Society’s directions on moving a mature tree or shrub helpful.
I have a very large hydrangea that has been in the ground at least 15 years. I’d like to move it, and have heard that it can be divided into several bushes. Are there any special details I should consider when performing this task?
I found a reference to the technique you describe in Hydrangeas: A Gardener’s Guide by Toni Lawson-Hall and Brian Rothera: “DIVISION. Sometimes, when moving a large H. macrophylla cultivar, the plant falls apart during the operation. It has been found that, provided each section has good roots, planting the separate pieces is totally successful.” This is similar to the process of layering, where branches are nicked and then pinned down into the soil to allow roots to form, and then severed from the parent plant with a sharp shovel six months to a year later. The small plants will be genetically identical to the original plant.
How and when do I prune snowberry (Symphoricarpos albus)?
Symphoricarpos albus is a suckering plant, so you can literally prune it to the ground. It will probably come back healthier than ever (assuming it’s healthy at the time), especially if you do it in early spring. Snowberry can take over and will likely crowd out lower plants; keeping it in check can be done with a lawn mower, weed eater, or by hand pruning. I suppose you could shear it or selectively prune it, but to maintain its natural growth habit, pruning it to the ground is your best bet.
I have tried to grow fennel and cannot get it to bulb. I’ve tried growing it in several different locations, and I only get stems and tops. Do you have any suggestions?
You probably know that there are two kinds of fennel. Common or bronze fennel (Foeniculum
vulgare), which is 3-5 feet tall with a tough stalk and looks a
bit like dill, does not form bulbs. It is also listed as a non-regulated noxious weed in King County. The one you need is Florence, or bulbing, fennel Foeniculum vulgare
v. azoricum, which is less than 2 feet high with a white stalk that thickens
at the bottom to form a bulb-shaped vegetable.
Assuming you are planting the Florence fennel type, here is some basic information about growing it: it needs full sun and moderate water. According to Vegetables, Herbs, and Fruit by Matthew Biggs et al., it should be planted in the ground half an inch deep in late spring to late summer. It needs warm weather and will bolt (not form much bulb and get spindly) if the weather is too cold, too dry, or if it gets
transplanted. “Plants need a sunny, warm, sheltered position and well-drained, moisture-retentive, slightly alkaline soil. A light, sandy soil with well-rotted organic matter dug in the winter before planting is ideal. Stony soils and heavy clays should be avoided. Never allow the soil to dry out: mulch in spring and hand weed around bulbs to avoid damage. When the stem bases start to swell, earth up to half their height to blanch and sweeten the bulbs.”
What is the disease causing needle drop, and even killing in some cases, Cedrus trees in our area? Is there a fungicide recommended to help control the disease, and if so, what timing is recommended?
I can’t be absolutely certain what the cause might be, but taking a look at some Northwest integrated pest management resources could provide ideas on the likely culprits.
Oregon State University’s IPM site mentions Kabatina and Sirococcus conigenus.
Here is what the print companion to the website (2008 Pacific Northwest Plant Diseases Management Handbook) says about Cedar needle blight:
“The fungi Sirococcus conigenus and Kabatina sp. have been associated with blighted needles of Atlas and Deodara cedars in both Oregon and Washington; however, Sirococcus is found most often. More of a problem in years with prolonged wet, cool springs. Infection is on or adjacent to needle bases on new shoots. The disease cycle is completed in 1 year, although spore dispersal from dead parts may continue (…) 10 months. The fungus overwinters in dead shoots. Conidia are dispersed by splashing water in spring and summer. (…) Temperatures of 60 to 70 F are most favorable for disease development.(…) Cultural control: Remove and destroy blight plant material and debris that is found under trees or caught in limbs. Chemical control: No chemicals are specifically registered for this disease on cedar.” (The full information is available in the link above.)
Washington State University in Puyallup has a document indexing plant hosts of various pathogens, and for Cedrus they list Sirococcus species as well as Phomopsis.
There are discussions on the topic of Cedrus blights at University of British Columbia Botanical Garden’s online forum which mention tip blight sometimes appearing in tandem with borers or mites.
I have a 75-foot long Photinia x fraseri hedge that is losing leaves. The leaves have dark spots on them. I wonder if it is a mold or fungus. I have not removed trimmings in previous years and air circulation has not been optimum. Some plants have lost almost all their leaves but there is new growth there too. The hedge is about 4 feet tall and is about 20 years old. I was thinking it could use some sort of fertilizer and mulch after I clear out all the old prunings. Any suggestions?
There are a few possible reasons your hedge is dropping leaves. In my own experience with Photinia, they were constantly dropping leaves throughout the year. Some of the leaves had dark spots, but some were just older foliage, reflecting this plant’s tendency to shed older leaves. According to Washington State University’s Landscape Plant Problems (2000), spots on Photinia leaves can be caused by a fungus called Entomosporium mespili, but they can also occur in fall and winter with no disease present. The fungal spots tend to have purple black edges and ash grey centers. Virginia Cooperative Extension has descriptions and images of this fungal disease.
There is another leaf spot of unknown origin, simply called ‘physiological leaf spot,’ described by Washington State University Extension as follows:
“Physiological leaf spot occurs on Photinia in western Washington. The symptoms resemble those of early fungal leaf spot infections. Small red to purple spots appear on the leaves, but do not develop the dark centers characteristic of fungal leaf spot infections. This problem is more common than fungal leaf spot. It causes little damage to plants, although some leaf drop may occur. While the cause of this problem is unknown, leaf spotting appears to be more severe on plants in low-lying or shady areas. Cold temperatures appear to be involved.”
If you want to know for certain what the spots are, you should bring samples to your local county extension agent so that they can identify the cause.
Improving air circulation and removing and disposing of all fallen leaves are always good garden hygiene practices. Avoid getting the leaves wet when you are watering the garden, and avoid summer pruning. The American Horticultural Society book Pruning & Training (DK Publishing, 1996) recommends pruning in spring, though hedges will need to be clipped two or three times a year.
I don’t recommend adding fertilizer, as it could cause your hedge to grow more quickly than you want to prune it. Mulching lightly with compost would probably be beneficial. In general, Photinia is a tough and vigorous plant, and the leaf spots are unlikely to cause its demise.