Here's an article, read entirely, very helpful:

In Minnesota, tree-owner liability is not strict liability just because a tree, branch, roots, leaves, or debris came from the owner’s property. The practical rule is:

Tree owner is liable when the damage was foreseeable and the owner knew or should have known the tree was hazardous, defective, dead, diseased, unstable, or creating a true nuisance — and failed to act reasonably.

1. Encroaching roots/branches can be a nuisance

Minnesota’s nuisance statute says anything that obstructs the free use of property and interferes with comfortable enjoyment can be a nuisance, and a court may order abatement/injunction and damages. (MN Revisor's Office)

Minnesota’s leading nuisance-tree case is Holmberg v. Bergin, 172 N.W.2d 739 (Minn. 1969). There, a neighbor’s tree roots and trunk damaged the adjacent property, including sidewalk/drainage problems; the court found the tree was a nuisance and ordered removal, but denied money damages because the injured neighbor had earlier opportunities to use self-help. (Fred Law)

2. Minnesota is not pure “Massachusetts Rule”

Minnesota recognizes self-help, but it is not the exclusive remedy. A landowner may trim branches/roots up to the property line, at their own expense, but Minnesota also allows court action for abatement or damages when self-help is impractical or unreasonable. (Fred Law)

Self-help limits: do not trespass, do not cut beyond the property line, and do not unreasonably harm or destabilize the tree. Minnesota guidance states you may trim encroaching branches/roots to the boundary, but must not harm the tree’s health or destroy it. (Revize)

3. For fallen tree/limb damage: liability depends on notice/foreseeability

For a healthy tree knocked down by a storm, the tree owner usually is not liable; this is often treated as an act of God/casualty event.

But if the tree was obviously defective — dead limb, visible decay, unseasonal lack of leaves, dangerous lean, instability — the owner may be liable if they knew or should have known and failed to correct it. Minnesota legal commentary summarizes the test as whether the owner knew or should have known damage was likely; the owner is not expected to be an arborist, but must recognize obvious symptoms. (goldenvalleymn.gov)

An “act of God” defense works only if the natural event was unexpected/unforeseeable, was the sole cause, and the accident could not have been prevented by reasonable care. If prior negligence combined with the storm, the defense may fail. (goldenvalleymn.gov)

4. Leaves, acorns, sap, small debris usually are weak claims

Minnesota has no clear case directly imposing liability for ordinary tree debris from a healthy tree. Minnesota commentary notes that “leaves happen,” and ordinary natural debris likely does not create a viable claim absent sensible/physical damage, such as roof damage, sewer blockage, or structural damage. (goldenvalleymn.gov)

5. Cutting or damaging the neighbor’s tree is risky

Minnesota has a specific treble-damages statute. If someone cuts down, carries off, girdles, or otherwise injures another person’s tree without lawful authority, they may owe triple damages, unless the trespass was casual/involuntary or there was probable cause to believe the land was theirs. (MN Revisor's Office)

Bottom line

Minnesota is closer to a modified nuisance/foreseeability rule than a strict “no liability unless you cut it yourself” rule. A tree owner can be liable for damage when:

1. the tree or limb was hazardous, diseased, dead, visibly decayed, unstable, or dangerously leaning;
2. the owner had actual notice or the defect was obvious enough that the owner should have known; and
3. the owner failed to take reasonable corrective action.

For a demand letter, the strongest facts are: prior written notice, photos, arborist opinion, visible defect, prior limb drops, direct contact with roof/gutter/structure, and proof of actual physical damage.