While there are no lists of shrubs/small trees best viewed from below, there is a list of trees with weeping habits in The Pacific Northwest Gardener's Book of Lists (Ray and Jan McNeilan, 1997). Many genera of conifers - Cedrus (cedar), Chamaecyparis (cypress), Larix (larch), Picea (spruce), Pinus (pine), and Tsuga (hemlock) - have weeping forms, often indicated by a variety name 'Pendula' or 'Pendulum'. There are weeping birches (Betula), beeches (Fagus), and cherries (Prunus), too.

You are correct about Styrax and Halesia. Additionally, I ran across a few individual species that may be of interest to you as I researched this question:
--Davidia involucrata
--Laburnum anagyroides
--flowering currants, Ribes spp.
--flowering cherry trees, particularly Prunus padus
--various plants in the Ericaceae family have bell-shaped flowers that hang on the underside of the stem.

Season All Season
Date 2007-05-21
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For several excellent images of weeping birch (Betula pendula), go to Oregon State University's landscape identification site at http://oregonstate.edu/dept/ldplants/ and click on Betula in the bright orange box.
Betula pendula is toward the bottom of the page.

Also try
http://www.cnr.vt.edu/dendro/dendrology/syllabus/bpendula.htm

Here are some other online tree identification guides:
http://www.oplin.org/tree/
http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm
http://selectree.calpoly.edu/

Season All Season
Date 2008-01-17
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The Seattle Department of Transportation, in cooperation with the City Arborist’s Office, has created lists of recommended trees for planting in Seattle. While the DOT is more concerned with street trees (trees planted in the strip of ground between the sidewalk and the street), their recommendations may help you. In addition, since the DOT deals with buckled sidewalks on a regular basis, these lists may account for a problem like yours.

Here is the web address:
http://www.seattle.gov/transportation/treeplanting.htm#recommend

The City Arborist’s Office also has a page of trees that they recommend *with reservations*. (White birch and weeping white birch are on the list.) At the bottom of this list are Prohibited Trees.

Here is that web address:
http://www.seattle.gov/transportation/treeswithreservations.htm

Season All Season
Date 2008-01-17
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Birches are commonly afflicted with aphids, and the aphids suck sap and secrete honeydew, which can be a nuisance, and is usually why homeowners contact us. Unfortunately, if your birches are overhanging a sidewalk, it is probably getting sticky from the honeydew. Otherwise, you could probably ignore the problem (except in the most severe infestations).

You can try spraying the aphids off the leaves with a strong jet of water. You can also encourage natural predators. Avoid over-fertilizing, or exposing the trees to lawn fertilizer, for example, as this will lead to succulent new growth which attracts aphids. Make sure the trees are not under any stress, as aphids are more likely to feed on a weakened tree. You may be able to avoid using the insecticidal soap as a control. If you do use it, you are correct that you need to reach all leaf surfaces, which is labor-intensive. Some of these soaps can cause damage, so it is always a good idea to test any such spray on a small area before coating the whole plant. An article by Colorado State University Extension provides information on insecticidal soaps. Aphids go through many generations in a year, and their eggs can overwinter.

Washington Toxics Coalition has created a document on managing aphids in the landscape.

Here are additional links on aphid control: Aphids from University of California at Davis
Managing Aphid Problems without Pesticides from the Northwest Coalition for Alternatives to Pesticides

Season All Season
Date 2007-08-01
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River birch (Betula nigra) is rated as having moderate root damage potential by the Urban Forest Ecosystems Institute.

The following, from University of Saskatchewan, supports what you have heard about planting this tree near a structure, but takes the approach that it is not good for the tree, rather than a danger to the foundation. Excerpt:

On a healthy birch, the roots will spread to a distance of at least twice the tree's height. This means that the roots of a mature tree may cover an area about one third the size of a football field. To permit proper root spread, trees should be planted as far as possible from any obstruction that may interfere with root development. Sidewalks, driveways, patios and building foundations will all limit root development. Where a tree is growing on heavy soils, aeration holes will help maintain root vitality.

Tree roots are not likely to infiltrate a solid foundation, but if there are cracks, it is certainly possible, and if tree roots expand sufficiently over time, they may begin to exert pressure on the foundation. However, birch roots are shallow, as indicated in this USDA Forest Service guide to growing birch trees.

It is not a good idea to plant any tree right next to a house, if only because the tree will undoubtedly require pruning to keep it out of the way of windows, doors, and so forth. If you like the tree and would like to move it to a better spot, you should consider contacting a certified arborist. Here are links to referral services.

Plant Amnesty

Pacific Northwest Chapter of the International Society of Arboriculture

Season All Season
Date 2007-09-21
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