Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 76. Herbicides
The UK LOAC Manual (2004) states:
[The 1977] Additional Protocol I is concerned with the protection of the environment from the effects of hostilities … [The Protocol] relates to widespread, long-term and severe damage. The terms are not defined. … These provisions do not … automatically prevent the use of certain means of warfare such as herbicides or chemical agents. The effects of … using these means must be considered. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 5.29–5.29.3.
In 1969, during a debate in the First Committee of the UN General Assembly on the question of chemical and bacteriological (biological) weapons, the United Kingdom stated with respect to the then still draft Resolution 2603 (XXIV): “The evidence seems to us to be notably inadequate for the assertion that the use in war of chemical substances specifically toxic to plants is prohibited by international law.” 
United Kingdom, Statement before the First Committee of the UN General Assembly, UN Doc. A/C.1/PV.1717, 10 December 1969, § 51.