A defect turns a tree into a hazard only when it has something to hit.

The USDA Forest Service guide "How to Recognize Hazardous Defects in Trees" builds on that pairing. Its targets are concrete: "A target can be a vehicle, building, or a place where people gather such as a park bench, picnic table, street, or backyard." Its defects come in seven main types: dead wood, cracks, weak branch unions, decay, cankers, root problems and poor tree architecture.

The pairing cuts both ways: "A tree with defects is not hazardous, however, unless some portion of it is within striking distance of a target." Some targets move and some stay put, which is why the first question about any tree is what sits under it.

The check's first tier starts with a lean that is new, read as Call now: a trunk that stood straight and now tilts has moved, and it still stands over whatever it leans toward. An old lean the tree grew into reads differently, and a tree starting to lean compares the two.

Roots work out of sight, so ground that rises, splits or mounds on one side of a trunk is one way a root failure shows, and the check reads it as Call now too. Lifted or cracked soil at the base of a tree separates that from clay that simply dries and cracks.

Both are views of one event: a tree shifting where it stands.

The guide reads mushrooms, conks and brackets growing on root flares, stems or branches as indicators of advanced decay; mushrooms at the base of a tree tells those apart from the ones that come up in a lawn.

Dead trees and branches, in the guide's words, can "break and fall at any time," and of the large pieces it says "dead trees and large dead branches must be removed immediately." Big dead limbs weighs a dead limb by its size, its height and what lies below.

Two similar stems growing close together can trap bark in the joint, which the guide calls a weak branch union. It calls cracks "extremely dangerous because they indicate that the tree is already failing," a crack being "a deep split through the bark, extending into the wood of the tree." A crack at the fork reads that joint from the ground.

Bark leaving a trunk that held it is what the guide calls a canker, "a localized area on the stem or branch of a tree, where the bark is sunken or missing." Bark peeling off a tree sorts that from the bark that trees such as sycamore shed as they grow.

All four sit in the middle tier, Worth a call: each is trouble in the wood itself.

Sawdust gathering at the base of a tree comes from insects working in the wood it fell from, the debris NC State Extension says carpenter ants throw out as they dig. Ants on a trunk may only be foraging, and sawdust at the base of a tree traces a pile back to whatever made it.

A limb resting on the roof scratches at the shingles and can offer squirrels and carpenter ants a way to the house, and a limb across the wire to the house is a matter for the electric utility first; tree limbs touching the roof or the line to the house deals with both.

Neither is a defect in the wood on its own, which is why they sit at Worth watching, and either moves up when another sign joins it.

In the USDA Forest Service guide's advice, any time of year works, in leaf or bare; to be thorough, look after the leaves drop in fall, again after they come out in spring, and after severe storms. Take in every part, from the roots and the root flare, the swell where trunk meets roots, up the main stem to the branches and their joints, from every side, with binoculars for high limbs. Older trees, the guide says, may have collected several defects and extensive decay.

The method is eyes and the windows of the house; none of it needs a ladder, a pry bar or a shovel. After a storm, the storm's own hazards come first, as what not to touch after a storm lays them out, then the trees still standing, which the days after a storm works through.

Decay inside a trunk can sit behind wood that looks sound, which dead and rotten trunks shows.

Defects that stack are another limit. The guide says "Multiple defects that are touching or are close to one another should be carefully examined," and "If more than one defect occurs on the tree's main stem, you should assume that the tree is extremely hazardous." A crack running into a canker reads as more than either alone.

The guide is plain about itself as well: it "presents guidelines, not absolute rules for recognizing and correcting hazardous defects," and says "Evaluating and treating hazard trees is complicated, requiring a certain knowledge and expertise."

Corrective actions, in the guide, begin with a thorough evaluation, and if a hazardous situation exists, "there are three recommended options for correcting the problem: move the target, prune the tree, or remove the tree." Moving the target is the one a homeowner can do today: "Easily moved items like play sets and swings, RV's, and picnic tables can be placed out of the reach of the hazardous tree." Where the target cannot be moved and a serious hazard exists, the guide says to "consider blocking access to the target area until the hazard can be properly eliminated."

The other two are tree work, and two of the crew's five services. Pruning fits a problem confined to a defective branch or branches, "even though the rest of the tree is sound," and is the work of tree trimming and pruning. Removal the guide calls "the final option," to be used "only when the other two corrective actions will not work," and "inherently dangerous and is even more serious when homes and other targets are involved": the work tree removal describes.

Whether a particular tree stays is settled by someone standing at it. The guide's own advice is "When in doubt, consult an arborist," and NC State Extension's routes for the close calls are given on emergency or Monday job triage.