Which of the following methods is effective in killing established conifers and other woody species?

The most effective method for killing established conifers and other woody species is hacking or girdling followed by treatment. This technique involves physically damaging the tree by creating cuts or removing strips of bark around the trunk, which disrupts the flow of nutrients and water in the plant. Once the tree’s vascular system is compromised, it becomes vulnerable to herbicides that can be applied directly to the wound. This approach is especially effective because it targets the tree's ability to heal and recover, ensuring that the herbicide has a greater chance of penetrating the plant system and causing effective mortality.

The other methods may not be as effective for established woody species. Drenching soil with water may aid in seed germination or assist plants that are already struggling due to drought conditions, but it does not target the plants directly. Spraying with non-systemic herbicides typically does not kill established trees efficiently, as these herbicides do not translocate within the plant's system and thus leave the root structure intact. Finally, planting cover crops can help suppress younger weed species but is unlikely to outcompete fully established trees and woody plants effectively.