A big dead limb can let go on a still day, so the question is what sits under it.

The USDA Forest Service guide calls dead trees and branches "unpredictable." A dead limb hangs on only until the wood holding it gives out, and a calm afternoon does nothing to stop that. Why dead wood snaps where living wood would bend comes up on dead and rotten trunks. A live limb falling from a healthy tree is a separate question.

For trees that drop their leaves, a dead limb is easier to pick out in summer. NC State Extension's pruning guide names the growing season as the time when "it is easier to see what is dead."

Out of leaf, the Forest Service guide's own lists name defects that still show from the ground: a broken branch or top lodged in the crown, a crack, and mushrooms, conks or brackets on a branch, which it reads as indicators of advanced decay. It advises looking from every side, after leaf drop in fall and after leaf-out in spring, with binoculars for high branches. An upstairs window helps too.

Twigs on the lawn are another matter. The sign here is one dead limb heavy enough to hurt someone.

The USDA Forest Service guide's definition of a hazard tree pairs a defect with a target, the frame the hazard tree field guide is built on. Here the dead limb is the defect, and the target is whatever it would hit: a bedroom roof, the driveway where a car sits overnight, a play set, a walk.

The guide's urgency carries its own scope. Under dead wood it says "dead trees and large dead branches must be removed immediately," and its "Take immediate action if..." list includes a branch that "is dead and of sufficient size to cause injury (this will vary with height and size of branch)."

NC State Extension's gardener handbook puts the target in its own words: "Pay special attention to any plants that have damaged branches that may fall on people or property." Targets move, too, and moving what sits underneath is the first of the guide's corrective actions.

The guide singles out dead branches and tops "that are already broken off," which it calls hangers or widow makers and describes as "especially dangerous." A hanger rests where it caught, no longer attached to the tree, and the guide's take-action list for dead wood opens with that case: "A broken branch or top is lodged in a tree."

Limbs a storm snaps and leaves hanging in the canopy are the storm version, which what not to touch after a storm takes up alongside a storm's other hazards.

On the home page, the fence-line check reads big dead limbs as Worth a call. Watching a dead limb does not change it: dead wood does not recover, and the fix for one bad branch is work done in the tree.

The guide's remedy is narrow when the problem is: "A hazardous situation may be caused by a defective branch or branches, even though the rest of the tree is sound. In this case, pruning the branch solves the problem." The guide's list of when to prune starts with "A branch is dead." The work is tree trimming and pruning, and what drives a tree trimming quote explains when dead wood comes out.

On the walk-through the job gets walked with you and the plan explained in plain words, so know which limb you mean and what sits under it.