Will root damage from trenching kill our old ginkgo tree?

Cutting roots within 4-5 feet on a mature tree can not only impact the health of the tree but, more importantly, it can compromise the stability of the plant. Research has shown that when trenches occur within three times the diameter of the stem, stability is compromised. So if the stem of the tree is two feet across (at four feet above ground), cuts made within six feet of the base can cause the tree to fail especially in high winds. Ginkgo is very tolerant of stress and health may not be significantly impacted by the root cut. However you should contact a certified arborist to inspect the tree and advise you regarding the failure potential of the root system. In the meantime, mulch the root zone of the tree and irrigate during dry periods to avoid drought stress. For irrigation, it is best to use soaker hoses beneath the mulch and irrigate heavily (several hours) once per week when rainfall is not adequate.