Q. I would like to plant some new trees in my yard, but am concerned about their roots invading the sewer lines. I believe that my community forbids planting weeping willows because of this problem. Can you suggest any trees that are less likely to cause a problem?

A. Tree roots cannot break their way into intact sewer lines -- not even weeping willows. The problem occurs when old terra cotta pipes crack. The roots of almost any tree will take advantage of the situation and could wind up clogging the lines. This is much less of a concern in newer developments because now builders use PVC pipe, which is much less likely to crack and invite an invasion of roots.

Most tree roots are confined to the top 12-15 inches of the soil, where there is adequate moisture, oxygen and nutrients to support the trees' growth. The fine, hair-like feeder roots that are responsible for absorbing water and nutrients from the soil extend out two to three times the reach of the branches (also known as the drip line).

We used to believe that tree roots were a mirror image of the top of the tree, but research has shown that tree root systems are much shallower and broader spreading than that.

Smaller trees such as crabapple (Malus spp.), Japanese maple (Acer palmatum) and 'Winter King' hawthorn (Crataegus viridis), or large shrubs such as bottlebrush buckeye (Aesculus parviflora), quince (Chaenomeles spp.), and blackhaw viburnum (Viburnum prunifolium) would have less extensive root systems than very large trees such as Norway, silver or sugar maples (Acer spp.), oaks (Quercus spp.) or willows (Salix spp.).

You should be more concerned about doing a thorough assessment of your site so that you can chose trees that will grow well in those conditions. The factors to consider include:

• How much sun or shade they will have to tolerate.

• Soil texture and structure. Is your soil compacted clay or clay subsoil, or is there some topsoil?

• How well the site drains. Does water pond on the surface after a rain?

• Does the site maintain adequate soil moisture or does it tend to dry out quickly?

• Will the trees be exposed to road salt?

• Are they near concrete sidewalks, driveways or buildings with concrete foundations or mortar joints? Calcium carbonate leaches out of concrete and may raise soil pH (the measure of acidity or alkalinity) to a range that limits your ability to grow trees that prefer an acid soil. These include maples, oaks, dogwoods, magnolias, conifers, and broad-leaved evergreens such as rhododendrons and hollies.

• Exposure to wind, especially in the winter. Wind is hardest on broad-leaved evergreens.

Once you have assessed the planting site, then you can narrow the list of possibilities to those tough, durable trees that do not have major insect or disease problems that would require your constant attention to keep your chosen plants healthy.

Here is a brief list of Web sites to help you choose plants that will tolerate the conditions in your yard:

• consumerhorticulture.psu.edu/pdfs/plants_wet_sites.pdf

• consumerhorticulture.psu.edu/pdfs/plants_dry_sites.pdf

• consumerhorticulture.psu.edu/pdfs/shade_tolerant_plants.pdf

• www.treeboss.net/salt-tolerant_trees.htm

• www.extension.umn.edu/projects/yardandgarden/ygbriefs/h408claysoil.html