Predicting hazardous trees
Freak accidents happen, but it is often possible to predict which trees are at risk of falling. It is wise to hire a certified independent arborist every two years to examine any large trees that are within striking distance of your house and evaluate the risk and recommend solutions.
For an independent assessment from someone who will not be selling you other services, search this website for those whose services primarily include tree risk assessment, education, and witness.
Long term care
Limit wood mulch
The only things trees need under them is their fallen leaves. If you feel a need to mulch, use wood chip mulch, which will allow air to reach the ground, no more than 2-4 inches deep, and not near the base of the tree. Do not keep piling on more year after year.
Avoid soil compaction
Compacted soil will seriously reduce tree growth.
Plant shrubs and groundcover under trees to dissuade pedestrians from walking over the roots (and to prevent accidents with lawnmowers and string trimmers).
Where compaction has already occurred – such as in a construction area now covered by lawn – many trees that naturally occur in swamps and along streams are often surprisingly good choices. The ability to grow in low-oxygen conditions makes them well adapted to dry, compacted soils.
Add a three foot in radius ring of mulch (do not let it touch the tree trunk.) Organic material gradually leads to soil loosening.
Don't disturb more than a third of the root zone, at the most. For a 10 inch diameter tree, don't disturb the soil closer than 20 feet from the trunk.
Don't drive machinery over tree roots.
Rule of thumb: don't cut roots that are wider than your thumb! This is especially true for Tulip trees, which do not compartmentalize well when the roots are cut, so fungal disease can spread right up through the tree.
Be very careful about gardening under trees, and if you are installing perennials, try to choose 2 inch plugs rather than quart pots.
Watch for shallow roots. Generally speaking, trees that are adapted to grow in bottomlands will have shallow roots. Examples include Red Maples and Sycamores, but there are many others - just look to see if the tree can be grown in wet conditions. (An exception is Hackberries, which grow in wet areas but have deep roots.) Interestingly, those wet-tolerant plants often grow quite well in dry, compacted soil, because it is not that their roots like water so much as that they can tolerate low oxygen conditions.
Never pile more soil on top of the ground over tree roots within the drip line.
Planted too high, then victim of a mulch volcano
Some trees have an expected lifespan, which in the case of small trees tends to be quite short. Redbuds, for example, typically only live about 20 years. But many of the larger species will keep on growing, even hundreds of years, until something happens to shorten their lives: disease, an accident, drought, flood, etc. Humans add to that list in a number of ways, some of which can be avoided by proper tree care.
The first 2 or 3 years
Tree mortality is highest in the first months and years after transplanting. See the tips below to get your tree off to the right start.
When do trees need pruning?
For the health of the tree
Three year old trees may need limited structural pruning to direct growth.
Ragged tears of branches are pruned to create a wound more capable of healing.
"Health and maintenance" pruning (typically of 1.5-2" deadwood) does not improve tree health.
For human reasons
Some dead branches in some locations pose a hazard.
Branches sometimes get in our way. (They should be cut just beyond the branch cuff, not further out - the tree can't heal wounds in random places. This is one of the reasons why topping trees is a bad idea.)