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PAL Questions: 2 - Garden Tools:
We are looking for a good screening tree/shrub that is evergreen and interesting. The plant cannot grow over 6 feet high. We have very sandy soil, western exposure, and live in the Magnolia neighborhood. We would like it to be drought tolerant as well. I found Myrtus communis (Myrtle) and Rhus (Sumac)--I am not sure which variety of sumac would be best. I found the information on these plants in the Sunset Pacific Northwest Garden Book. I would love to get your advice on these, and if you have any other ideas as well.
Because of the height limitation of your site, I suggest primarily shrubs (rather than trees) that are evergreen and drought-tolerant.
Most of the Rhus I have seen growing in Seattle is of the deciduous type, but there are several evergreen varieties, such as Rhus virens and Rhus lancea. They are natives of Texas and Baja California. They will not be as hardy as the deciduous varieties.
Myrtus communis does well in seaside gardens although it can exceed your 6 foot height limit, reaching 10 feet or more (according to W. J. Bean, Trees and Shrubs Hardy in the British Isles, 8th ed., John Murray, 1973 and Top-Rated Evergreen Shrubs, Golden Press, 1983). The dwarf variety Myrtus communis 'Compacta,' would be too low-growing to act as a screen.
I would suggest Osmanthus delavayi, which has small, glossy dark green leaves, and very fragrant white flowers in March. It can eventually grow to 8 feet, but is easily maintained as a hedge or screen (see the website Great Plant Picks for pictures and information).
Other ideas would be Arbutus unedo 'Compacta' (Strawberry Tree). Or you could try Ceanothus concha, which has small dark green leaves and blue flowers. The California nursery Las Pilitas has information about this and other varieties of Ceanothus.
You may also wish to come to the Miller Library and browse the many illustrated books on shrubs and trees.
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I was told by a local arborist that Arbutus 'Marina' in the area are all dying of some disease. I have 5 that were adversely affected or died over last winter but at the time I thought this was due to a cold winter. Is there a disease going around, or is it safe to get some new plants of this species and try again?
I know that the City of Seattle has listed Arbutus 'Marina' as an alternative to the disease-prone Arbutus menziesii, because it is supposedly less susceptible to the fungal and bacterial problems affecting our native madronas. However I imagine it is not immune, and perhaps what you have observed indicates that conditions for disease development were just right this year. You might want to talk to the city arborist and ask how the city's plantings of this tree are doing. You could also talk to tree expert Arthur Lee Jacobson to see if he has any thoughts on this.
University of British Columbia Botanical Garden and University of Washington both have information about diseases that affect the genus Arbutus. A commercial operation, San Marcos Growers, comments on disease in Arbutus 'Marina:'
"In the initial release of Arbutus 'Marina' the Saratoga Horticultural Foundation noted that the tree was a fairly pest free and disease resistant; we in fact started calling it the garden tolerant Madrone. Through years of growing and gardening with Arbutus 'Marina' we have discovered that it in fact has a few pests that feed upon it and a few diseases that can cause it harm. The new growth is occasionally attacked by aphids, which will cause the associated sooty mold. Ant control seems to be the best preventative for this. On occasion we also see Greenhouse thrip and soft scale. For these pests, the pest preasure on our garden plants have never reached a threshold that required us to treat with a pesticide. We treat our nursery plants as necessary to assure that they are pest free.
For many years we thought that Arbutus 'Marina' was resistant to plant diseases but in conditions that promote the disease we have found that Arbutus 'Marina' is susceptible to at least 2 plant pathogens. Phytophthora root rot is the most serious of these diseases but when planted correctly in well drained soils and not over irrigated this disease has not appeared to be a problem. In most cases we have seen this problem when the tree is being overwatered or has been planted too deep or in compacted soil. Unfortunately this disease seems to eventually kill the infected plant.
More recently we have seen several established trees losing lower leaves and small twigs inside the canopy. A laboratory analysis has proved this to be caused by the fungus Botryosphaeria. This fungus is opportunist in nature and usually only attacks plants that are under environmental stress of some form. A tree infected with Botryosphaeria may appear vigorous and healthy at its growth tips yet have twig die out within the canopy. The information for controlling this disease on ornamentals is limited but the general consensus is to reduce the stress to the plant and avoid wounding the plant unnecessarily. When pruning infected branches, do so well below all discolored wood and dispose of dead plant material. Clean pruning tools between cuts with a dilute solution of household bleach (1 part bleach to 9 parts water)."
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January 13 2017 10:35:53